Abstract: INTRODUCTION . MITOTIC RECOMBINATION: CROSSING-OVER AND GENE CONVERSION . MITOTIC RECOMBINATION IN YEAST . Characterization of Genetic Exchange .. Intrachromosomal Recombination .. Mitotic Recombination Contrasted with Meiotic Recombination .. Models of Recombination . Proteins Associated with Recombination . GENETIC CONTROL OF RECOMBINATION . Recombination and DNA Repair . DNA damtlge . Recombination in repair-deficient mutants . The question o[ recombinagenlc repair .. Interpretation of Dose-Resfi!.nse Relations for Induced Recombination .. Recombinagenic Effects oJ Thymidylate Stress . CONCLUDING REMARKS .

Abstract: The construction of plasmids which facilitate the study of interplasmidic and intraplasmidic recombination is described In this system, a single recombination event between two mutated Ter genes on separate plasmids or on one plasmid leads to a change in the host phenotype from sensitivity to resistance to tetracycline

Abstract: Transposition of the genetically related insertion elements gamma delta and Tn3 is thought to involve two steps. In the case of transposition from one replicon to another, the first step is fusion of the parent and target replicons with the element appearing in direct orientation at the two junctions. In a subsequent reaction, the cointegrate structure is resolved via a site-specific recombination event. I have constructed two plasmids, each carrying segments of gamma delta and Tn3, that contain the internal resolution site. The tnpR gene product encoded by either Tn3 or gamma delta mediates intramolecular recombination between these two sites. The product of this recombination is a hybrid region that contains gamma delta and Tn3 sequences fused at the point of crossover. DNA sequence analysis of such recombinants indicates that the recombination occurs within a 19-base-pair (bp) region of exact homology between gamma delta and Tn3. The site lies in the 160-bp center intercistronic region, 50 bp before the beginning of the tnpA gene. My results therefore suggest a model for the coupled regulation of the repressor (tnpR) and the transposase (tnpA) genes and site-specific recombination of transposition intermediates. The Tn3/gamma delta recombination system and bacteriophage lambda integration are compared.

Abstract: Immunoglobulin genes are shown to undergo dynamic rearrangements during differentiation as well as evolution. We have demonstrated that a complete immunoglobulin heavy chain gene is formed by at least two types of DNA rearrangement during B cell differentiation. The first type of rearrangement is V-D-J recombination to complete a variable region sequence and the second type is S-S recombination to switch a constant region sequence. Both types of recombination are accompanied by deletion of the intervening DNA segment. Structure and organization of CH genes are elucidated by molecular cloning and nucleotide sequence determination. Organization of H chain genes is summarized as VH-(unknown distance)-JH-(6.5 kb)-C mu-(4.5 kb)-C delta-(unknown distance)-C gamma 3-(34 kb)-C gamma 1-(21 kb)-C gamma 2b-(15 kb)-C gamma 2a-(14.5 kb)-C epsilon-(12.5 kb)-C alpha. The S-S recombination takes place at the S region which is located at the 5' side of each CH gene. Nucleotide sequence of the S region comprises tandem repetition of closely related sequences. The S-S recombination seems to be mediated by short common sequences shared among S regions. A sister chromatid exchange model was proposed as a mechanism for S-S recombination. Comparison of nucleotide sequences of CH genes indicates that immunoglobulin genes have scrambled by intervening sequence-mediated domain transfer during their evolution.

Abstract: We show here that recombination occurs at the normal rate in compound female mice containing two different complementing lethal haplotypes (th17/tw12 where there is a long stretch of homologous t-mutant chromatin. Thus the recombination suppression of a complete t-haplotype cannot be due to an intrinsic factor(s) which suppresses along the length of its own chromosome but is due to ‘mismatching’ of wild-type and mutant chromatin. Naturally occurring t-haplotypes of mouse chromosome 17 have several interesting genetic properties. First, they are always transmitted from males in much higher proportions than mendelian expectation; presumably this accounts for the maintenance of lethal and semilethal t-haplotypes at polymorphic levels in populations of wild mice, t-Haplotypes also show recombination suppression. The conventional map distance between genetic markers T and tf is 7–12 cM, whereas in (t/+) heterozygotes for naturally occurring t-haplotypes, recombination is suppressed and T and tf seem to be separated by only 0.1–0.5 cM (ref. 1). The region of recombination suppression extends to and includes the major histocompatibility complex (H–2)2. Thus t and H–2 effectively travel as a single unit— a ‘super gene’3. Although recombination suppression is known to be accompanied by failure of chiasmata formation4, the mechanism underlying the suppression has remained an enigma. Lyon suggested a disorder of t-heterochromatin5 and more recently a change in ‘intercalary’ middle repetitive DNA6. She proposed that either t-chromatin is intrinsically incapable of participating in crossing-over, or chiasma formation is prevented because of mismatching and mispairing of normal and abnormal chromatin. We have measured recombination between two chromosomes which carried extensive overlapping segments of t-chromatin. We report here that in this configuration, recombination occurs at a normal rate, and thus we conclude that cross-over suppression is due to mismatching.

Abstract: Serial section electron microscopic autoradiography was used to examine the relationship between recombination nodules and 3H-thymidine incorporation during pachytene in Drosophila melanogaster females. For both ellipsoidal and spherical recombination nodules, the number of nodules that are associated with an autoradiographic grain is higher than that expected by chance; this observation is consistent with the hypotheses that recombination involves DNA synthesis and that recombination nodules are the sites of meiotic recombination. Moreover, general DNA replication (S-phase) and synapsis (synaptonemal complex formation) were found to be temporally distinct events, contrary to previous reports; Drosophila females therefore are not exceptional in this regard.

Abstract: The spontaneous carrier lifetime τs was measured as a function of temperature for 1.6 µm wavelength InGaAsP/InP lasers. As a good approximation 1/τs∝Ith1/2, where Ith is the threshold current, in the temperature region from 140 K to 320 K. The measured lifetime, given by τs=1/(BeffN), where N is the carrier density, resulted in the effective recombination coefficient Beff=1 –2×10-10 cm3s-1 at room temperature. Gain spectra were measured for different temperatures. The gradient of room temperature peak gain with respect to carrier density was determined to be 1.2×10-16 cm2. At 140 K this gradient as well as the curvature of the gain spectra showed an ~10 times increase. The refractive index variation with carrier density was also obtained at different temperatures.

Abstract: In a preliminary report (Esposito 1978), evidence was presented which showed that heteroallelic recombination resulting in prototrophic colonies occurs at the 2-strand stage. A model utilizing replicative resolution of Holliday structures was proposed to explain how gene conversion at the 2-strand stage can result in exchange of outside markers. The object of the experiments reported herein was to present detailed genetic evidence for 2-strand recombination. In addition, we examined the features of mitotic recombination with respect to symmetry, length and polarity of heteroduplexes in wild type strains (REM1/REM1) and in strains bearing the hyper-recombination mutation rem1-1. To do this, we constructed strains so that prototrophs arising from heteroallelic recombination and recombinant for outside markers were detected by visual inspection. By analyzing these colonies genetically, we have inferred several features of mitotic recombination which distinguish it from its meiotic counterpart. Firstly, mitotic heteroduplexes are often symmetric while meiotic heteroduplexes are almost exclusively asymmetric. Secondly, heteroduplexes tend to be longer in mitosis that in meiosis. Thirdly, unlike meiotic conversion, mitotic conversion does not show strong polarity. Recombination in strains homozygous for the rem1-1 mutation also takes place at the 2-strand stage. The rem1-1 mutation, however, appears to alter the features of mismatch correction.

Abstract: The origin and maintenance of genetic recombination are unsettled evolutionary issues. Genetic variation affecting recombination frequency appears to be pervasive in nature, suggesting that natural selection must increase recombination frequency under some circumstances. However, theoretical arguments and experimental evidence indicate that the frequency of recombination should be reduced by natural selection.

Abstract: We have found that the RAD52 gene product, which is required for gene conversion and recombination in the yeast Saccharomyces cerevisiae, is not required for unequal mitotic sister-strand recombination.

Abstract: The rad alleles of 18 unlinked genes of S. pombe were tested for their level of spontaneous meiotic, spontaneous mitotic and UV-induced mitotic recombination in the ade7–50 × ade7–152 interval. The effects of these rad alleles on meiosis and cell morphology were also studied. None of these mutants showed a clear-cut reduction of spontaneous recombination rates, no matter whether they had lost or retained a caffeine-sensitive repair of UV-induced lesions, which has previously been interpreted as a recombinational pathway of DNA repair (Fabre, 1972a; Gentner, 1977; Gentner et al., 1978). rad1-1was the onlyly mutant with a reduced frequency of UV-induced recombination. Some mutants displayed an increased frequency of mitotic recombination, either spontaneosly (rad15-P, rad21–45), UV-induced (rad8–190) or both (rad2–44). Previous hypotheses on the contribution of recombination to DNA repair in S. pombe are reconsidered in the light of these data.

Abstract: Genes that suppress recombination when heterozygous have been found distributed as a polymorphism in wild and laboratory populations of Neurospora crassa. Three alleles, ssE, ssS and ssC, are associated, respectively, with the three wild types Emerson, St. Lawrence 74A and Costa Rica A. It is proposed that ss (synaptic sequence) genes modulate recombination by determining the pairing closeness of DNA duplexes in the vicinity of the nit-2 locus. When heterozygous, ss suppresses recombination 2- to 20-fold within the nit-2 locus, which it adjoins, but crossing over in intervals flanking nit-2 is not affected. The magnitude of suppression depends upon the ss alleles involved, and ss acts multiplicatively with rec-1; together, these genes modulate recombination within the nit-2 locus over a range exceeding 100-fold. The ss effect is not attributable to gross chromosomal rearrangement, but could be due to small inversions or insertions, such as transposable elements.

Abstract: Three-body electron-ion recombination in a molecular ambient gas is investigated theoretically. The limited success attained by a classical treatment is lost when necessary account is taken of the discreteness of the negative energy levels of the recombining electron. Thus the three-body recombination coefficients calculated in carbon dioxide and in water vapour differ from those measured by Warman et al. (1979) and Sennhauser et al. (1980) being much lower and falling off far more steeply with increase in temperature. It is noted that the ions involved in the laboratory study, (CO2+)n and H3O+(H2O)n, are open to dissociative recombination and it is pointed out that this process and three-body recombination supplement one another with striking consequences. The mechanism is in two stages: firstly, collisions between the electrons and the ambient gas molecules populate the Rydberg levels; secondly, the electrons in the Rydberg levels participate in radiationless transitions leading to dissociation in the same way as free electrons. A suitably descriptive name for the mechanism is collisional dissociative recombination. Detailed calculations give gas-density-dependent recombination coefficients in quite good agreement with those measured.

Abstract: The effects of the mutation pairs recB21 recF143 and recB21 uvrD152 on the frequency of genetic recombination were investigated in lambda phage-prophage crosses under homoimmune conditions. To prevent recombinants from being formed by the phage red system, these experiments were performed with phages and prophages carrying red and gam mutations. Both spontaneous and damage-induced recombination was measured, the phages being either undamaged or treated with trimethylpsoralen and 360-nm light to cross-link the phage DNA. Control and damaged phages were allowed to infect lysogenic host cells under conditions in which phage gene expression was repressed and phage DNA replication was blocked by lambda immunity. Although the double mutations recB21 recF143 and recB21 uvrD152 reduced recombination in Hfr by F- crosses to 0.3 to 0.02% of the wild-type controls, the presence of these pairs of mutations in the host lysogens had relatively little effect on the results of the phage-prophage crosses. In the latter system, recB21 recF143 reduced spontaneous and damaged-induced recombination by less than threefold whereas recB21 uvrD152 increased it to three times the wild-type level, the increase being attributable to the uvrD mutation. Evidently, the gene products of recB,C uvrD, and recF wee not needed for lambda phage-prophage recombination under repressed conditions.

Abstract: The main point of this article is to present a simple physical picture of the recombination process of a carrier hopping in a disordered solid in the presence of molecular recombination centers (rc). First, an exact (asymptotic) calculation of the entire time dependence of the transient photocurrent (TP) and the number of surviving carriers n(t) is carried out for a suitable model system. It is shown that the t'(l-~)-, t ( l + ~ ) transition does exist in the TP however the transition occurs in over two decades in time (for a=0.5). Further, the time tr corresponding to the intersection of the two slopes in the log(TP)-log(t) plot is the same as the median time for the n(t) decay. The t, is next shown to have an alternate simple interpretation. In the low rc concentration limit, tr is determined from a calculation of the number of distinct sites visited by the hopping carrier, as a function of time (t) and electric field (E). The results are compared to recent measurements in a-Si:H. Introduction.There has been a considerable experimental [l-61 and theoretical [3,7-91 interest recently in carrier recombination processes in disordered solids. Measurements on non-geminate recombination have been carried out in a-Si:H and a-As7Se3 using the delayed collection field technique [3]. Recombination is observed in the decay ofphitogenerated carriers using transient optical absorption in a-Si:H [4] and a-As7Se3 [l] and in transient photocurrent measurements in a-Si:H [5] and a-As2Se3 [6]. The results-forsurviving carriers n(t) typically show an asymptotic algebraic decay n(t) f a (o < a < 1) [3,4]. Theoretical studies have been carried out with a variety of models and techniques. For hopping transport Shlesinger [7] used a conditional probability for the occurrence of a reaction which was determined from a first passage time distribution to reach a specific set of sinks, while Movaghar [8] solved the Master eqcation, with site specific random loss terms, in a self consistent approximation. Basically they derived the same results, and for dispersive trmsport obtained n(t)ra. We shall show that the form of their results can be obtained in a straightforward way using an idea known in the random walk literature [l01 which formed the basis of a model [l11 that succesfully accounted for the unusual field dependent trapping [l21 in a quasi-onedimensional molecular crystal. In the limit of low recombination center rc (or deep trap) concentration, to an excellent approximation, the recombination time tr can be determined from S(t) the expected number of distinct sites visited in time t. The hopping and/or trapping process can be quite complex but S(t) is calculated from the rc-free transportFirst we calculate the complete time dependence of both the transient photocurrent (TP) and n(t) for a carrier undergoing dispersive transport. We discuss the significance of thz results and then interpret the recombination time tr using S(t). Transient vhotocurrent (TP) and survivors n(ti We first model recombination in a disordered solid by a carrier hopping on a lattice in the presence of a distribution of absorbing sites. The transport is characterized by +(s,t), the probability density that the time interval between successive arrivals is t and the displacement is S. The equation for the continuous time random walk (CTRW) generated by +(s,t) has been shown [l31 to be an exact one for the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814118 C4-548 JOURNAL DE PHYSIQUE configilration averaged , where P(s,t) is the probability of finding the carrier at s at time t if it slnrtcd at S, at t = O in a spccific configuration of randomly distributed sites. Equivalently, the process of configuration averaging defines a self-energy operator 2 for a coherent medium [14]. The descriptions are sinlply related by fs,sl = u$(s-sr,u) [l $(u)]-I and therefore a selfconsistent calculation of 5 [8,14] can be used to determine a $(s,t). [$(s,u), ~ ( L I ) are the Laplace transform (LT) of $(s,t) and $(t) r zs +(s,t), respectively.] The CTRW approach is particularly fruitful for trapping or recombination problems. Specifically, there have been a .lumber of cases in the RW literature [15,16] which demonstrate that under certain conditions various transport proverties (including diffusion) on lattices depend onlv on the densitv of "defects" and not on thew spatial arrangements. l'he conditions include low defect concentration and spatial dimension d22. We will further demonstrate this fact by the two approaches to the problem taken in this note. An exact solution for can be obtained for a general &(s.t) and a periodic distribution of traps [l71 of concentration c = n-3, where n is the distance between traps in units of the lattice constant. The traps are taken to be absorbing sites and the zeroth and the time derivative of the first spatial moment of determine n(t) and TP, respectively. The details of this derivation will be given elsewhere. We further specialize the result to the case where $(U) --l-bua (O 2 transit times. In the limit of low concentration the sum in Eq. (2) can be replaced by an integral and ( T ~ ) ~ = (0.77)bf1w, W = f" d 3 ~ [lA(K, o)]-I (3)

Abstract: Molecules of adenovirus 2 DNA were isolated as nucleoprotein complexes late in infection of HeLa cells and examined by electron microscopy. Some were interpreted as representing intermediates in recombination events. The structures observed were consistent with predicted intermediates according to the model for genetic recombination proposed by Meselson and Radding [Meselson, M. S. & Radding, C. M. (1975) Proc. Natl. Acad. Sci. USA 72, 358-361] and constitute physical evidence from animal cells in support of this model.

Abstract: F42lac recombination with λplac5is normally twentyfold to fiftyfold higher than recombination between λplac5 and a chromosomal lac gene. The presence of an fi+ R1 plasmid in the same cells as F42lac dramatically reduces this enhanced recombination level while the fi- R1drd19 plasmid has little effect. When F42lac traJ90 is tested in a sup+ strain, it shows a sharp reduction in recombination with λplac5 that can be largely reversed by the presence of a supF mutation that partially suppresses the traJ90 nonsense mutation. It is concluded that the enhanced recombination between F42lac and λplac5 is largely dependent on the constitutive expression of F42lac fertility functions.

Abstract: Recombination results in the release of variation upon which selection is practised in plant breeding. However, according to the objectives of the programme it may be necessary to reduce recombination to limit the disturbance of arrangements of genes already well suited to agricultural needs or to increase recombination to maximize the likelihood of recombination's giving rise to transgressive segregation. Appropriate breeding manipulations are discussed. In addition, descriptions are provided of the induction of recombination between chromosomes that are distantly related in evolution and between which meiotic pairing and recombination does not normally take place.

Abstract: In females homozygous for the meiotic mutant c(3)G, a reasonable amount of intergenic recombination was found to occur, while no intragenic recombination could be detected at the white locus. On the other hand, a significant increase in both inter-and intragenic recombination was observed in females heterozygous for the same mutation. These results are discussed in the context of the synaptonemal complex formation during chromosome pairing in the mutant.

Abstract: This method of calculating recombination frequency from F2 hybrid data is based upon delineation of the potential phenotypes by combinatorial analysis using a Punnett square, assessment of the true occurrence of each type of recombinant and calculation of the recombination frequency from these data. The example used will be the calculation of the recombination frequency between loci one of which has two codominant alleles, i.e. those of the major histocompatibility complex, designated L and U (RTI haplotype), and the other is the linked growth and reproduction complex (Grc), which is scored by the recessive gene for small body weight (dw-3), designated d, and its normal counterpart, + (Kunz et al. 1980). The parental haplotypes are Ld and U+, and the recombinant haplotypes are L + and Ud. The definitions of the symbols used are:

Abstract: Hybrids lambda H lambda T80 are formed due to recombination of the phage lambda att80 and phi 80 prophage partially deleted in the region of structural genes. Genetic structure of 22 independently isolated lambda H lambda T80 hybrids was determined by the restriction method and it was shown that recombination took place in the genes A, C, D and H. The frequencies of hybrid formation diminish from 1.10(-3) to 4.10(-5) for this gene order, which suggests that the polar divergence of nucleotide sequencies in the region of structural genes exists. It was found that formation of hybrids with recombination in the region of "weak" homology (gene H) was possible only when the region of "strong" homology was present in the deleted phi 80 prophage to initiate recombination.