Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

/pdf/microbial-cellulose-utilization-fundamentals-and-4yec12qx1c.pdf

Microbial cellulose utilization: fundamentals and biotechnology.

Physiological and genetic responses of bacteria to osmotic stress.

To succeed, many cells must alternate between life-styles that permit rapid growth in the presence of abundant nutrients and ones that enhance survival in the absence of those nutrients. One such change in life-style, the “acetate switch,” occurs as cells deplete their environment of acetate-producing carbon sources and begin to rely on their ability to scavenge for acetate. This review explains why, when, and how cells excrete or dissimilate acetate. The central components of the “switch” (phosphotransacetylase [PTA], acetate kinase [ACK], and AMP-forming acetyl coenzyme A synthetase [AMP-ACS]) and the behavior of cells that lack these components are introduced. Acetyl phosphate (acetyl∼P), the high-energy intermediate of acetate dissimilation, is discussed, and conditions that influence its intracellular concentration are described. Evidence is provided that acetyl∼P influences cellular processes from organelle biogenesis to cell cycle regulation and from biofilm development to pathogenesis. The merits of each mechanism proposed to explain the interaction of acetyl∼P with two-component signal transduction pathways are addressed. A short list of enzymes that generate acetyl∼P by PTA-ACKA-independent mechanisms is introduced and discussed briefly. Attention is then directed to the mechanisms used by cells to “flip the switch,” the induction and activation of the acetate-scavenging AMP-ACS. First, evidence is presented that nucleoid proteins orchestrate a progression of distinct nucleoprotein complexes to ensure proper transcription of its gene. Next, the way in which cells regulate AMP-ACS activity through reversible acetylation is described. Finally, the “acetate switch” as it exists in selected eubacteria, archaea, and eukaryotes, including humans, is described.

The acetate switch.

Proteins encoded by plasmid DNA are specifically labeled in uv-irradiated cells of Escherichia coli carrying recA and uvrA mutations because extensive degradation of the chromosome DNA occurs concurrently with amplification of plasmid DNA.

Simple method for identification of plasmid-coded proteins. [Escherichia coli, uv radiation]

With the goal of solving the whole-cell problem with Escherichia coli K-12 as a model cell, highly accurate genomes were determined for two closely related K-12 strains, MG1655 and W3110. Completion of the W3110 genome and comparison with the MG1655 genome revealed differences at 267 sites, including 251 sites with short, mostly single-nucleotide, insertions or deletions (indels) or base substitutions (totaling 358 nucleotides), in addition to 13 sites with an insertion sequence element or defective prophage in only one strain and two sites for the W3110 inversion. Direct DNA sequencing of PCR products for the 251 regions with short indel and base disparities revealed that only eight sites are true differences. The other 243 discrepancies were due to errors in the original MG1655 sequence, including 79 frameshifts, one amino-acid residue deletion, five amino-acid residue insertions, 73 missense, and 17 silent changes within coding regions. Errors in the original MG1655 sequence (<1 per 13 000 bases) were mostly within portions sequenced with out-dated technology based on radioactive chemistry.

/pdf/highly-accurate-genome-sequences-of-escherichia-coli-k-12-4m7om4wqr6.pdf

Highly accurate genome sequences of Escherichia coli K‐12 strains MG1655 and W3110

Mutants of Escherichia coli deficient in the fermentative NAD-linked lactate dehydrogenase (ldh) have been isolated. These mutants showed no growth defects under anaerobic conditions unless present together with a defect in pyruvate formate lyase (pfl). Double mutants (pfl ldh) were unable to grow anaerobically on glucose or other sugars even when supplemented with acetate, whereas pfl mutants can do so. The ldh mutation was found to map at 30.5 min on the E. coli chromosome. The ldh mutant FMJ39 showed no detectable lactate dehydrogenase activity and produced no lactic acid from glucose under anaerobic conditions as estimated by in vivo nuclear magnetic resonance measurements. We also found that in wild-type strains the fermentative lactate dehydrogenase was conjointly induced by anaerobic conditions and an acidic pH. Despite previous findings that phosphate concentrations affect the proportion of lactic acid produced during fermentation, we were unable to find any intrinsic effect of phosphate on lactate dehydrogenase activity, apart from the buffering effect of this ion.

Mutants of Escherichia coli deficient in the fermentative lactate dehydrogenase.

Fermenting anaerobic cultures of Escherichia coli were observed by the nonintrusive technique of in vivo, whole-culture nuclear magnetic resonance. Fermentation balances were calculated for hexoses, pentoses, sugar alcohols, and sugar acids. Substrates more reduced than glucose yielded more of the highly reduced fermentation product ethanol, whereas more-oxidized substrates produced more of the less-reduced fermentation product acetate. These relationships were made more obvious by the introduction of ldhA mutations, which abolished lactate production, and delta frd mutations, which eliminated succinate. When grown anaerobically on sugar alcohols such as sorbitol, E. coli produced ethanol in excess of the amount calculated by the standard fermentation pathways. Reducing equivalents must be recycled from formate to account for this excess of ethanol. In mutants deficient in hydrogenase (hydB), ethanol production from sorbitol was greatly decreased, implying that hydrogen gas released from formate by the formate-hydrogen lyase system may be partially recycled, in the wild type, to increase the yield of the highly reduced fermentation product ethanol.

Anaerobic fermentation balance of Escherichia coli as observed by in vivo nuclear magnetic resonance spectroscopy.

Our previous work resulted in the isolation of mutant strains of Escherichia coli K-12 which were able to oxidize furans and thiophenes as a result of mutations in several novel genes. Some of the genes involved in thiophene oxidation were cloned into the multicopy vector pUC19. The plasmid pKA10 carries a 3.8-kb chromosomal fragment which encodes a previously undiscovered gene involved in thiophene oxidation. Three proteins with approximate molecular sizes of 48, 30, and 26 kDa were overproduced by cells carrying pKA10. Maxicell experiments and DNA sequence analysis indicated that the 48- and 26-kDa proteins are encoded by pKA10, whereas the 30-kDa protein is apparently chromosomally derived. A cassette specifying kanamycin resistance was inserted into various sites on pKA10. An insertion which abolished the 48-kDa protein also abolished thiophene oxidation. Chromosomal integration of pKA10::Kan allowed us to locate the chromosomal insert of pKA10 at 84 min on the E. coli genetic map by transduction. Since no previously identified genes involved in thiophene metabolism are located in this region, we designated the gene for the 48-kDa protein as thdF. Sequencing of the 3.8-kb insert revealed an overlap of several hundred bases with the regulatory and structural regions of the tnaA gene, which is also located at 84 min. The 26-kDa protein is probably truncated tnaA protein. An open reading frame corresponding to the 48-kDa thdF protein was located next to the tnaA gene, which encodes tryptophanase, but was transcribed in the opposite sense.

Molecular cloning and sequence of the thdF gene, which is involved in thiophene and furan oxidation by Escherichia coli.

When grown at high osmotic pressure, some strains of Escherichia coli K-12 synthesized substantial levels of free sugar and accumulated proline if it was present in the growth medium. The sugar was identified as trehalose by chemical reactivity, gas-liquid chromatography, and nuclear magnetic resonance spectroscopy. Strains of E. coli K-12 could be divided into two major classes with respect to osmoregulation. Those of class A showed a large increase in trehalose levels with increasing medium osmolarity and also accumulated proline from the medium, whereas those in class B showed no accumulation of trehalose or proline. Most class A strains carried suppressor mutations which arose during their derivation from the wild type, whereas the osmodefective strains of class B were suppressor free. When amber suppressor mutations at the supD, supE, or supF loci were introduced into such sup0 osmodefective strains, they became osmotolerant and gained the ability to accumulate trehalose in response to elevated medium osmolarity. It appears that the original K-12 strain of E. coli carries an amber mutation in a gene affecting osmoregulation. Mutants lacking ADP-glucose synthetase (glgC) accumulated trehalose normally, whereas mutants lacking UDP-glucose synthetase (galU) did not make trehalose and grew poorly in medium of high osmolarity. Trehalose synthesis was repressed by exogenous glycine betaine but not by proline.

Accumulation of trehalose by Escherichia coli K-12 at high osmotic pressure depends on the presence of amber suppressors.

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Mutants of Escherichia coli deficient in the fermentative lactate dehydrogenase.

Anaerobic fermentation balance of Escherichia coli as observed by in vivo nuclear magnetic resonance spectroscopy.

Molecular cloning and sequence of the thdF gene, which is involved in thiophene and furan oxidation by Escherichia coli.

Accumulation of trehalose by Escherichia coli K-12 at high osmotic pressure depends on the presence of amber suppressors.