Abstract: — A striking increase in the shoulder of the u.v. survival curve but no change in the limiting slope is obtained when cultures of Escherichia coli strain TAU complete the DNA replication cycle in the absence of concommitant protein synthesis prior to irradiation. The u.v. sensitivity of protein synthesis or RNA synthesis is not altered significantly by this treatment. In contrast to the result for strain TAU, there is no significant change in the u.v. survival curve for the u.v. sensitive E. coli Bs-1 when its DNA replication cycle is completed under similar conditions. Following a period of inhibited protein synthesis there is a delay in the reinitiation of the normal DNA replication cycle when protein synthesis resumes. This delay would allow time for an intracellular repair system to operate before the attempted resumption of normal replication. Strain Bs-1, which is deficient in this repair system, would not be expected to benefit from such a delay, as consistent with the observed results. A model is presented to account for lethality due to attempted DNA replication during a period of repair synthesis. The maximum survival for a given u.v. dose would be predicted for a culture which has completed the normal DNA replication cycle prior to irradiation and which is not permitted to reinitiate the cycle until all possible repair synthesis is completed.

Abstract: — The formation of thymine dimers in the DNA of L-strain mammalian cells after irradiation with ultraviolet light has been demonstrated. The amount of dimer formed rises with the dose of u.v. light. In the course of post-irradiation incubation the thymine dimers remain in the TCA insoluble fraction and diminish as did the other thymidine-H3 derivatives with increasing incubation time. The dimer is not found in the soluble fraction. Thus, dimer excision (i.e. its liberation into the soluble fraction) as an expression of repair of radiation damage analogous to dark repair in E. coli was not found in these experiments.

Abstract: — Phytochrome-induced photomorphogenesis in the mustard seedling (Sinapis alba L.) which can be regarded as being representative of the dicotyledonous seedlings has been analysed. In the present paper a number of arguments are presented, including data on RNA and protein synthesis and on the effects of actinomycin D and puromycin, which support the hypothesis that the ‘positive’ photoresponses of the seedling can be explained by a differential gene activation through P,30. ‘Positive’ photoresponses are those which are characterized by an initiation or an increase of biosynthetic or growth processes (e.g. biosynthesis of anthocyanin; growth of cotyledons). The lag-phase of this type of photoresponse is rather long, ‘Negative’ photoresponses are those which are characterized by an inhibition of growth processes or other physiological processes like translocation. Here the lag-phase is short. Inhibition of hypocotyl lengthening is a typical response of this sort. The concept of differential gene repression through P730 may serve as a working hypothesis to approach the causal analysis of phytochrome-induced ‘negative’ photoresponses.

Abstract: — Light-induced changes in the yield of bacteriochlorophyll fluorescence have been measured in cells and chromatophores of photosynthetic bacteria, and coordinated with light-induced absorbancy changes. Comparisons were drawn during transitions between dark and light steady states and also between steady states established at different light intensities. Aerobic cell suspensions of Rhodospirillum rubrum, Rhodopseudomonas spheroides, Chromatium and Rhodopseudomonas sp. NHTC 133 showed a strict correspondence between changes in the fluorescence yield and the bleaching of P870 (P985 in Rps. sp. NHTC 133), as reported by Vredenberg and Duysens for R. rubrum cells. The relationship shows that singlet excitation energy in bacteriochlorophyll is quenched by P870 at a rate proportional to the concentration of unbleached P870. This implies that the photosynthetic units are not independent with respect to energy transfer. In anaerobic cell suspensions the change in fluorescence did not follow the bleaching of P870 in the manner described by Vredenberg and Duysens. Here a change in fluorescence may have resulted from the reduction of a primary photochemical electron acceptor as well as from the oxidation (bleaching) of P870. In chromatophore preparations there were further deviations from the Vredenberg and Duysens relationship which could be attributed to changes in the rate constants for quenching of singlet excitation energy. Finally there was a light-induced increase in the fluorescence yield which was related to a band shift of bacteriochlorophyll and not to the bleaching of P870. Aerobic cell suspensions presented a limiting case in which these complications were absent. No change in the fluorescence was associated uniquely with the oxidation of cytochrome or band shifts of carotenoid pigments. These results, when coordinated with earlier findings about the fluorescence of bacteriochlorophyll and P870, indicate that the singlet excitation quantum is the only energy carrier linking the absorption of light with the initiation of photochemistry in bacterial photosynthesis.

Abstract: — For some years the mechanism of the photosensitizing effects displayed by some furocoumarins on various biological substrates (human and guinea-pig skin, bacteria cultures, mammalian cells adapted to in vitro growth, viruses) have been studied. Recently it has been pointed out that a photoreaction occurs between the photosensitizing furocoumarins and DNA after irradiation at 3655 A. By use of a labeled furocoumarin, i.e.—O14CH3 bergapten or 5-methoxy-psoralen, this has been confirmed and more extensively studied. During the irradiation a stable combination of the furocoumarin with native DNA takes place with a quantum yield of 5·2 × 10-3. It is probable that the reactive sites of DNA are the pyrimidine bases. Yeast-RNA and the same DNA after heat-denaturation or in the presence of high NaCl concentration photoreact at a much reduced rate. This photoreaction may explain some various biological photosensitizing effects produced by furocoumarins.

Abstract: — The experiments reported give evidence that liquid-holding recovery (LHR) of u.v. irradiated E. coli cells involves basically the same type of dark repair which causes reactivation of phage and which results in much increased survival of the cells themselves [host-cell reactivation (HCR)]. LHR is very small in the two HCR(-) strains B syn- and Bs-1, but occurs to larger but different extents in the three HCR(+) strains B, B/r, and B/r (Λ). LHR is inhibited if the liquid contains caffeine or acriflavine, both of which are known to inhibit HCR. The results indicate that most of the LHR effect, if not all, occurs during the liquid holding, rather than under growth conditions after liquid holding. It is assumed that the holding itself allows a prolonged time for, and therefore an enhancement of, HCR. It is thus implicit that LHR can be observed only where otherwise HCR of repairable u.v. damage would be incomplete, and that different extents of LHR, as observed in the three HCR(+) strains, reflect different extents of incompleteness of HCR. It is concluded that the repairable u.v. hits which are not fully repaired by HCR are predominantly those concerned with the extra u.v. sensitivity of the strains B and B/r (Λ), relative to B/r.

Abstract: — Kinetic studies with the mustard seedling (Sinapis alba L.) support the hypothesis that the so-called ‘high energy reaction’ of photomorphogenesis can be understood solely on the basis of phytochrome. Light-induced anthocyanin synthesis (a typical ‘positive’ photoresponse(1) and light dependent inhibition of hypocotyl lengthening (a typical ‘negative’ photoresponse(1)) have been investigated. In order to explain the experimental data we have to assume that there are two different types of phytochrome 730 which differ greatly as far as their resistance to irreversible destruction is concerned. The existence of these two different types of phytochrome 730 has already been postulated on the basis of spectrophotometric measurements in vivo.(2)

Abstract: — The chief photoproduct of thymine produced in u.v. irradiated (2537A) vegetative cells of B. subtilis is the cyclobutane-type dimer while in spores very little of this dimer is produced (maximum yield 2·6 per cent of thymine) but a new photoproduct is produced in high yield (maximum of 28·4 per cent of thymine). This difference in photochemical response appears to be due, at least in part, to a difference in uydration of the DNA. The photochemistry of thymine in isolated DNA irradiated in solution is similar to that of DNA in irradiated vegetative cells, but differs markedly from that of isolated DNA irradiated dry. The yield of cyclobutane-type thymine dimer is much reduced in isolated DNA irradiated dry but a new photoproduct of thymine. is produced which is chromatographically similar to the spore photoproduct. The yield of this photoproduct, however, is never as great as that obtained in irradiated spores. The photochemistry of the DNA thymine of spores germinated in the presence of chloramphenicol is very similar to that of normal vegetative cells. Except for hydration, the physical state of the DNA is probably not otherwise altered by germination in the presence of chloramphenicol since DNA replication is prevented by the presence of chloramphenicol. These results are also consistent with the hypothesis that the unique photochemistry of spores is due, at least in part, to the hydration state of the DNA. The acid stability of the spore photoproduct is indicated by the fact that it is isolated from irradiated spores after hydrolysis in trifluoroacetic acid at 155°C for 60 min. It still contains the methyl group of thymine as judged by the fact that for a given dose of u.v. the same yield of photoproduct was obtained whether the spores were labeled with thymine-2–C-14 or -methyl-C-14. This photoproduct is stable to reirradiation (2537A) in solution under condiditions where thymine dimers of the cyclobutane-type are completely converted back to monomeric thymine. On a column of molecular sieve material (Sephadex-G10), the spore photoproduct elutes in a region intermediate between the cyclobutanetype thymine dimers and monomeric thymine. Of the numerous compounds tested by paper chromatography, the spore photoproduct is most similar (but not identical) in several solvents to 5–hydroxyuracil and 5–hydroxymethyluracil. Our data do not allow us to decide if the product is a monomer or a dimer. Although the photochemistry of thymine in the DNA of spores differs markedly from that for vegetative cells, several lines of evidence make it seem doubtful that the enhanced resistance of spores to u.v. relative to that of vegetative cells can be explained solely on the basis of this difference in the photochemistry of DNA thymine.

Abstract: — Ultraviolet-sensitive strains of E. coli, Hs30 (uvr- phr- arg-), lacking the ability to repair ultraviolet damage in the dark (uvr-), lacking photoreactivating-enzyme activity (phr-), and lacking the ability to synthesize arginine (arg-), and Hs30–R (uvr- phr+ arg-), having the same auxotrophic marker and the same high u.v.-sensitivity, were derived, respectively, from resistant strain H/r30 (uvr+ phr- urg-) and its revertant H/r30–R (uvr+ phr+ urg-). Hs30 and Hs30–R have about 25 and 35 times higher sensitivity than H/r30 and H/r30–R for u.v.-induced killing and mutation to prototrophy, respectively. Efficiency of photoreactivation (PR) of mutation in strain Hs30–R is about ten times higher than the PR efficiency in the parent strain H/r30–R at wavelengths between 3341 and 4358 A but not that much higher at 3132 A. On the other hand, Hs30 shows no PR at any of these wavelengths, although the parent strain H/r30 does show PR of mutation around 3341 A., These results support the. previously described model that PR in H/r30 is an indirect effect of PR light which enhances dark repair. It is concluded that Hs30 and Hs30–R lack dark-repair capacity for u.v.-induced mutational damage and that the same type of photoreactivable DNA lesions as those responsible for U.V. killing, i.e. pyrimidine dimers, are the primary cause of u.v.-induced mutation in the uvr-strains. PR spectra for killing in Hs30–R and mutation in H/r30–R and H/r30 were obtained and compared, and the quantitative conclusions reached are discussed in connection with the hypothesis that the same kind of lesion (pyrimidine dimer) is the primary cause of mutation in the uvr+ strains as well.

Abstract: — Studies of purine absorption and emission in seven solvents differing greatly in dielectric constant and hydrogen bonding potential, reveal a variety of solvent effects. For example, the resolution of structure in the absorption spectrum, the position and/or intensity of the X2 absorption band, the intensity of fluorescence, the magnitude of the long wave-lenth tail, and the position of the X1 absorption band are differentially affected—in the order listed—by the solvents tested. Even though it is possible to correlate the extent of decrease in the n-π* tail with increasing solvent dielectric constant, probably alterations in all of these spectroscopic parameters depend most critically upon the ability of the various solvents to form hydrogen bonds with the hydrogen on N9 and/for with the non-bonding electrons on the purine nitrogens: it is tentatively concluded that the probability of hydrogen bonding is directly correlated with the electronegativity of the aza nitrogens (N7 > N3 > N1). In solvents like isopropanol not all of the non-bonding electrons must be solvated maximally in most purine molecules since there is appreciable fluorescence under conditions where a long wavelength tail is readily observed in the absorption spectrum (alternatively some noa-bonding electrons may not te relevant to fluorescence quenching.) Decreases in fluorescence yield are associated with red shifts in the fluorescence maximum, and in the solvents of highest polarity the fluorescence yield is again small indicating that glycerol and water can enhance radiationless tunneling—presumably by altering Franck-Condon configurations and/or improving electronic-vibrational coupling between solute and solvent. The quantum yield is uniform throughout the atsorption band for a given solvent, but studies in aqueous buffers varying from pH 1 to 11 show that the fluorescence yield is greater for charged than for neutral molecules. Further, the fluorescence excitation peak is red shifted in powders. Since phosphorescence is the predominant emission at 777deg;K and increases in fluorescence can be correlated with the presumed solvation of non-bonding electrons, the singlet excited state of lowest energy in ‘unperturbed’ purine must be n-π* in nature. The shape of the phosphorescence band and the decay lifetime of ˜ 1 sec at 77°K lead to the conclusion that the emitting triplet is a π-π* state. The eight vibrational structures in phosphorescence emission can be readily grouped into two progressions: there is an average separation of about 1300 cm-1 between peaks within a given progression, and the two sets are mutually displaced by about 500 cm-l. Individual vibrational peaks are favoured in different solvents and the whole band may be shifted up to 500 cm-l. Even larger shifts are observed in charged purine molecules and in powders (up to 3000 cm-l) and the presumed 0–0 band is not observed.

Abstract: — Eleven strains of microorganisms were examined for their susceptibility to death by irradiation with 21–30 mW of light at 6328 A from a continuous-wave gas laser. Washed cells of seven species were rapidly killed when irradiated aerobically in aqueous solutions of toluidine blue: Sarcina lutea (two strains), Escherichia coli, Chromobacterium violaceum, Arthrobacter atrocyanus, Pseudomonas aeruginosa, Saccharomyces cerevisiae, and Rhodotorula glutinis. Cells remained viable when irradiated anaerobically in toluidine blue. This is not proof that oxygen is required for death since toluidine blue is subject to reduction to the leuco form under anaerobic conditions. None of the microbes tested, including cells of Bacillus cereus, Rhodospirillum rubrum, and Euglena gracilis, were killed in aerated suspensions without toluidine blue. Thus, it is unlikely that any strain has an endogenous photosensitizer capable of acting at 6328 A. However, more than 95 per cent of the cells of R. rubrum lost their motility. A colorless mutant of S. lutea began dying 10 min faster than the wild strain, which is known to be protected from photosensitized death by carotenoids for at least 2 hr when irradiated in toluidine blue with 1000 ft candles of tungsten light. Since toluidine blue is bleached in irradiated cell suspensions, logarithmic death ceases within an hour with 2·5 × 10=6 M toluidine blue. Yeasts are less susceptible than bacteria, exhibiting longer lags before death begins. The size of colonies produced by irradiated yeast cells decreases with irradiation time. This suggests that multiple hits are required for death.

Abstract: — Action spectra for light induced sporulation were determined for conidia of Alternaria dauci, and conidia and perithecia of Pleospora herbarum (Imperfect stage is Stemphylium botryosum) Only radiation less than 370 mμ induced formation of conidia, and less than 390 mμ formation of perithecia The action spectra showed increased effectiveness in the 230 and 290 mμ regions, and possibly in the 260–270 mμ regions Below 280 mμ these action spectra may not be representative of the “true” absorption spectra of the photoreceptors involved because of possible superimposed effects caused by nucleic acid absorption Though the action spectra were not identical, the number of characters in common were suggestive of a similar photoreceptor Similarities between action spectra for sexual and asexual reproduction of P herbarum indicate the possibility of a single photoreceptor for both processes

Abstract: — In the photosynthetic bacteria Chromatium, Rhoahpirillum rubrum, and Rhodopseudomonus spheroides the fluorescence of bacteriochlorophyll is probably free of contamination by a “fast” component of delayed emission, judging from the characteristics of the delayed light measured 3 msec after excitation. In Rps. spheroides the pigment P870, associated with photochemical reaction centers, is non-fluorescent in its photochemically active state. Fluorescence of P870 can be induced by either of two agencies that suppress its photochemical activity: exposure to Na2S2O4 and (in a dry chromatophore film) dessication. The yield of fluorescence from the major (light harvesting) component of bacteriochlorophyll in vivo is brought to a common maximum value by conditions that suppress the photochemical activity of P870. In addition to dessication and exposure to Na2S2O4 these conditions include saturating illumination and exposure to K3Fe(CN)6. Of these four treatments only the last two bleach the long wave absorption band of P870. These experiments support the following assertions: (1) P870 traps singlet excitation energy absorbed by the light harvesting BChl; the trapping function of P870 depends on its ability to initiate and participate in photochemistry. (2) Both dessication and exposure to Na2S2O4 suppress the photochemical activity of P870 by blocking an event that proceeds directly from the excited singlet state in P870. (3) The fluoresecence of BChl in vivo is emitted almost entirely by a major (light harvesting) component.

Abstract: — In frozen aqueous solutions at 777deg;K, UV irradiation (Λ>250 mμ) photoionises purine, adenosine and guanosine molecules, respectively, In alcoholic solutions, at 777deg;K, photosensitization of alcohol occurs A biphotonic absorption process involving the lowest triplet state is suggested The formation of radicals by these processes leads to a decrease of the phosphorescent triplet-state population of the molecules tested Some consequences for the photochemistry of nucleic acids are discussed

Abstract: — Purple photosynthetic bacteria contain a component, absorbing near 805 mμ, distinct from the major light harvesting bacteriochlorophyll component. The minor component, designated P800, resembles P870 in that it resists oxidative treatments that destroy the light harvesting bacteriochlorophyll. Light induces a reversible blue-shift of P800 together with the reversible bleaching of P870. The ratio of P800 to P870 in Rhodopseudomonas spheroides is constant. Both pigments are absent in phenotypes that cannot grow photosynthetically; they reappear together in revertants to photosynthetic competence. Action spectra for light-induced bleaching of P870 and for bacteriochlorophyll fluorescence show that P800 transfers energy more efficiently to P870 than to the bulk bacteriochlorophyll. It is concluded provisionally that P800 is a specialized bacteriochlorophyll molecule in close proximity to the reaction center component P870.

Abstract: — Substitution of an amino group at the C2 carbon of purine causes the two main components of the X absorption band to be separated by 200–500 A: primarily this reflects the Occurrence of the lowest-lying transition at much longer wavelengths than in purine since, except in deoxyguanosine, there is an appreciable absorption component at ˜ 2420 A in these compounds. Unlike purine, the wavelength shifts produced by different solvents are not simply related to the dielectric constant or hydrogen bonding capacity of the solvents. In agreement with previous predictions thatπ-π*states should be the lowest-lying excited singlets, no phosphorescence is observed from 2–aminopurine—the fluorescence quantum yield is close to unity. Besides strong fluorescence very weak phosphoresence can be detected when another amino group is added at C6 and when an oxygen is attached to the sixth carbon (guanine) fluorescence and phosphorescence of comparable intensities are observed: the sum of the quantum yields for both types of emission is of the order of unity. Thus, the lowest-lying singlet transition must be a π-π*state. At room temperature themal quenching is the most important parameter in determining the fluorescence intensity of these molecules, however, at lower temperatures the intensity—similar to the wavelength—f this emission apparently also depends upon the ability of a given solvent to reorient and/or perturb Franck-Condon surfaces. Discrepancies observed between absorption and excitation spectra in deoxyguanosine and azaguanine appear to reflect emission caused respectively by aggregation of molecules and by keto-enol shifts.

Abstract: — The irradiation of TMV with u.v. light of 2537 A wavelength results in the binding of protein subunits to the RNA. These bound subunits are stable towards warm sodium dodecyl-sulfate; however, the binding is not covalent since the subunits are removed by 66% acetic acid, guanidine hydrochloride, or phenol. Approximately one protein subunit is bound per lethal biological ‘hit’. The virus and the nucleic acid extracted from irradiated virus show virtually identical rates of inactivation.

Abstract: — For use with very intense light source, e.g. flash lamps and high-pressure arcs, some new stable transmission filters for the U.V. were developed. All filters were composed to give maximum transmittance in the desired bands together with optimum absorbance of the visible light, at an optical path of 1 cm. The filters described are easy to prepare and have considerably better properties than the well known NiSO4--CoSO4 filter and other filter combinations. During initial irradiation with high-intensity light sources the transmittance of the freshly prepared filter solutions increases slowly till maximum transmittance is attained. The position of the transmission bands, however, does not change. Based on the fact that the 6700 A absorption band of some non-aqueous solutions of CoCl2 has an extinction coefficient about a hundred times larger than the 5100 A absorption band of the aqueous solution with the same concentration, filter solutions were developed for the following transmission bands: 2600–3900 A, 2900–3900 A and 3100–3900 A with transmission maxima of 90, 90 and 75 per cent respectively, and 2600–4600 A, 2900–4500 A and 3100–4600 A with transmission maxima of 95, 95 and 90 per cent respectively, at an optical path of 1 cm.

Abstract: — Young sporangiophores of the fungus, Pilobolus kleinii, respond to unilateral illumination by bending or by growing toward light of wavelengths between 312 and 530 mμ, with peaks of sensitivity near 360 and 450 mμ. Young sporangiophores exhibit a negative phototropic response to wavelengths shorter than 300 mμ, with a strong negative response at 280 mμ. Since the action spectrum did not correspond to the absorption spectrum of the pigmented zone as measured in vivo, and since colorless sporangiophores formed on media containing diphenylamine were capable of phototropic response, it is unlikely that the conspicuous orange-yellow pigment in young sporangiophores is the photoreceptor for phototropism. The results of probing with small beams of light and the behavior of sporangiophores submerged in mineral oil, together with measurements of the refractive index of the tip and base indicate that the photosensitive region is located in the tip of the young sporangiophore.

Abstract: — It is known that thymine forms dimers when aqueous solutions are irradiated with ultraviolet light while in the frozen state, but does not form dimers when solutions are irradiated in the liquid state. The eutectic point of aqueous thymine solutions was found to be. —0.02°C. Since the irradiation of frozen solutions is always carried out at lower temperatures, the dimerization must be occurring in the solid state. Activation energies and quantum yields for dimer formation were determined by irradiating 1–mm layers of thymine solution at —5°C to — 707deg;C for various lengths of time. As expected, the activation energy was zero. After measuring the amount of radiation scattered by samples of ice, the extreme values for the quantum yield were found to be 0.73 and 4.08. The lower limit assumed that all the scattered light was absorbed by thymine; the upper limit assumed that none was absorbed. Since the theoretical maximum quantum yield is 2, the best estimate of the quantum yield is considered to be between 1 and 2.

Abstract: — A novel approach to the measurement of phototaxis is described. It is based on recording the change in optical density which results when phototactic microorganisms collect in a beam of actinic light. The instrument allows the reproducible measurement of rate and degree of phototaxis. Using this device, we have carried out a systematic study of the influence of physical parameters on the phototactic response in Euglena gracilis, z-strain. When Euglena is grown under conditions of artificial day/night cycles, circadian rhythms of phototactic ability are observed which parallel similar rhythms previously observed in the photosynthetic capacity of other algae. Maintenance of a light-grown culture in complete darkness results in a loss of the phototactic response after about 72 hr. A dark-grown culture develops chlorophyll and phototactic capacity simultaneously. These results suggest a relation between phototaxis and photosynthesis. From the results of light intensity dependence studies of phototaxis, it is concluded that the actinic light triggers rather than powers the response. The upper limit for the quantum threshold of phototaxis is estimated to be 100 quanta per photoreceptor at 460 mμ. Measurements of the dependence of phototaxis on the wavelength of the actinic light yield an action spectrum which agrees very well with the absorption spectrum of the eyespot pigments. The rate of phototaxis vs. temperature closely resembles the temperature dependence of enzymatic activity.

Abstract: — The absorption and emission spectra of pyrimidine derivatives show that sub-stituting an amino or hydroxyl group at the 2nd and 4th positions of the pyrimidine ring (to give the derivatives found in biological systems) creates excitation patterns quite different from those for pyrimidine itself: the lowest singlet and triplet states are of (?,π*) and not (n,π*). The relative intensities of fluorescence and phosphorescence depend upon at least two factors: (a) the extent of intersystem crossing which depends mostly on the relative energies of the lowest (π,π*) singlet and lowest (n,π*) triplet states, where the energy of the latter is influenced strongly by the environment; and (b) the extent of mixing of (π,π*) and (n,π*) triplet states, which depends initially upon the closeness of the energies of the two states. The measured and inferred energies for these various states are summarized. In most cases the intensity of luminescence is lower than that found in purines, presumably much of the excitation energy is dissipated in photochemical changes. Also the lifetime of phosphorescence is shorter than in purines. Substitution at the 5th carbon increases the luminescence yield. When a bromine atom is substituted on the 5th carbon of uracil no effect on intersystem crossing is found—presumably the electron density of the π* orbital is minimal in the vicinity of the bromine. Polarization measurements show that the two lowest π-π* transition moments in 2-aminopyrimidine are oriented almost perpendicular to each other in the molecular plane whereas phosphorescence is polarized out of plane.

Abstract: — By combining independent, dominant, niodifier genes of the albino cl1 mutant it is possible to produce a spectrum of phenotypes ranging from normal green to albino. Analysis of plastid pigments reveals that the albino possesses the ability to produce as much or more chlorophyll than normal siblings and that this ability is not impaired by the presence of the modifier genes. The modifiers do influence, however, the amount of carotene and xanthophyll the plants produced. The level of the three plastid pigments (chlorophyll, carotene and xanthophyll) vary simultaneously and, in most modified phenotypes, occur in approximately the same concentrations relative to their normal siblings. Since chlorophyll production appears to be normal in these mutants, it is suggested that the modifier genes do not directly influence the concentration of this pigment. Rather, the ultimate amount of chlorophyll will not rise above that which can be protected from photodestruction by the carotenoid levels determined by the various modifier genotypes.

Abstract: — The photodegradation of riboflavin by 436 mμ monochromatic light and of lumichrome by white nonfiltered light was studied in a set of organic solvents including ethanol, acetone, dioxane, pyridine and acetic acid. For comparison, water was used as a solvent. Photolysis was carried out in the presence of atmospheric oxygen. Riboflavin and lumichrome were found to be effectively stabilized towards the action of light by hydrogen bonds with solvent molecules, and consequently are most light-stable in water solutions. The overall scheme of riboflavin photolysis in organic solvents seems to be the same as in aqueous solutions. Lumichrome has been found as the main product of riboflavin photolysis in the organic solvents tested.

Abstract: — Light-induced electron spin resonance (ESR) absorption is not observed in degassed benzene solutions of chlorophyll a with hydroquinone. In the presence of methanol, however, the characteristic hyperfine pattern of p-benzosemiquinone ion radical is formed. This photo-reduction of chlorophyll a by hydroquinone is enhanced apparently by a small amount of triethylamine and inhibited by β-carotene. Chlorophyll excited in the presence of oxygen produces a single broad ESR signal in benzene solutions which is enhanced by hydroquinone and quenched by β-carotene, alcohol, and a mine. Their quenching effect is in parallel with their inhibition of the photoauto-oxidation of chlorophyll. The high quenching capacity of triethylamine for this ESR signal is reduced by the addition of β-carotene. In methanol solutions, the initial production of the single broad signal is followed by gradual development of the hypertine structure of p-benzo-semiquinone ion radical. This development can be accelerated by triethylamine. The mechanism of these changes is discussed on the primary interaction of excited chlorophyll with oxygen.

Abstract: — The action spectrum and dose-rate dependence for photoreactivation of mutation to prototrophy in late-lag-phase cultures of Escherichia coli H3r30 (which lacks active photo-reactivating enzyme) are roughly similar to those for photoprotection from killing in other strains. It is suggested that photoreactivation of this mutation in H/r30 is an indirect effect, similar in mechanism to photoprotection. The action spectrum and dose-rate dependence for photoreactivation of mutation to prototrophy in late-lag-phase cultures of E. coli H3r30-R (which possesses active photoreactivating enzyme) are roughly similar to those for photoreactivation of killing in most other strains. It is suggested that photoreactivation of this mutation in H/r30-R is a direct effect at long wavelengths, but that there is an indirect component at short wavelengths. A quite different interpretation of these data is noted. Finally, it is found that, under the conditions of these experiments, indirect photoreactivation of killing in H/r30 and H/r30-R is weak or nonexistent.

Abstract: — The electron paramagnetic resonance (EPR) of the triplet excited state of solutions of a series of porphyrins and aromatic amino-acids has been studied at 77°K. It has been shown that for some of the compounds it is possible to observe EPR for transitions with δM=±1, ±2. It has been shown that photoreactions in solid solutions of porphyrins and aromatic amino-acids proceed through the triplet excited state. Reactions of photosensitized deamination of aliphatic amino-acids in solid aqueous solutions at 77°K by aromatic compounds have been studied.

Abstract: — Seven chromatographically separable products were shown to be formed when an aqueous solution of tryptophan was exposed to the light of a 100-W bulb at pH 9 in the presence of methylene blue and oxygen. Some of these products were detected, though in much smaller quantities, even when tryptophan was irradiated in the absence of methylene blue and/or oxygen. Contrary to reports in the literature, none of the common derivatives of tryptophan, such as tryptamine, indole acetic acid, indole aldehyde, anthranilic acid or kynurenine, were detected on irradiation of the amino acid by visible light. Such irradiation of tryptamine and indole acetic acid gave 1–2 components which were chromatographically identical with those obtained from tryptophan; irradiation of indole aldehyde gave no detectable breakdown products. Exposure of tryptophan to ultraviolet light or when treated with hydrogen peroxide did not result in the formation of any of the products obtained with visible light. The results presented here suggest that during exposure of tryptophan to visible light, the indole ring is first oxygenated resulting in the formation of dioxindole derivatives. One of the products of irradiation of tryptophan with visible light was tentatively identified as dioxin-dolylalanine.