Lucinda Notley-McRobb

rpoS mutations and loss of general stress resistance in Escherichia coli populations as a consequence of conflict between competing stress responses.

The general stress resistance of Escherichia coli is controlled by the RpoS sigma factor (phi(S)), but mutations in rpoS are surprisingly common in natural and laboratory populations. Evidence for the selective advantage of losing rpoS was obtained from experiments with nutrient-limited bacteria at different growth rates. Wild-type bacteria were rapidly displaced by rpoS mutants in both glucose- and nitrogen-limited chemostat populations. Nutrient limitation led to selection and sweeps of rpoS null mutations and loss of general stress resistance. The rate of takeover by rpoS mutants was most rapid (within 10 generations of culture) in slower-growing populations that initially express higher phi(S) levels. Competition for core RNA polymerase is the likeliest explanation for reduced expression from distinct promoters dependent on phi(70) and involved in the hunger response to nutrient limitation. Indeed, the mutation of rpoS led to significantly higher expression of genes contributing to the high-affinity glucose scavenging system required for the hunger response. Hence, rpoS polymorphism in E. coli populations may be viewed as the result of competition between the hunger response, which requires sigma factors other than phi(S) for expression, and the maintenance of the ability to withstand external stresses. The extent of external stress significantly influences the spread of rpoS mutations. When acid stress was simultaneously applied to glucose-limited cultures, both the phenotype and frequency of rpoS mutations were attenuated in line with the level of stress. The conflict between the hunger response and maintenance of stress resistance is a potential weakness in bacterial regulation.

The relationship between external glucose concentration and cAMP levels inside Escherichia coli: implications for models of phosphotransferase-mediated regulation of adenylate cyclase.

The concentration of glucose in the medium influences the regulation of cAMP levels in Escherichia coli. Growth in minimal medium with micromolar glucose results in 8- to 10-fold higher intracellular cAMP concentrations than observed during growth with excess glucose. Current models would suggest that the difference in cAMP levels between glucose-rich and glucose-limited states is due to altered transport flux through the phosphoenolpyruvate: glucose phosphotransferase system (PTS), which in turn controls adenylate cyclase. A consequence of this model is that cAMP levels should be inversely related to the saturation of the PTS transporter. To test this hypothesis, the relationship between external glucose concentration and cAMP levels inside E. coli were investigated in detail, both through direct cAMP assay and indirectly through measurement of expression of cAMP-regulated genes. Responses were followed in batch, dialysis and glucose-limited continuous culture. A sharp rise in intracellular cAMP occurred when the nutrient concentration in minimal medium dropped to approximately 0.3 mM glucose. Likewise, addition of > 0.3 mM glucose, but not < 0.3 mM glucose, sharply reduced the intracellular cAMP level of starving bacteria. There was no striking shift in growth rate or [14C] glucose assimilation in bacteria passing through the 0.5 to 0.3 mM concentration threshold influencing cAMP levels, suggesting that neither metabolic flux nor transporter saturation influenced the sensing of nutrient levels. The (IIA/IIBC)Glc PTS is 96-97% saturated at 0.3 mM glucose so these results are not easily reconcilable with current models of cAMP regulation. Aside from the transition in cAMP levels initiated above 0.3 mM, a second shift occurred below 1 muM glucose. Approaching starvation, well below saturation of the PTS, cAMP levels either increased or decreased depending on unknown factors that differ between common E. coli K-12 strains.

Assessing the effect of reactive oxygen species on Escherichia coli using a metabolome approach

A two-dimensional thin-layer chromatographic analysis of [14C]-labelled metabolites in Escherichia coli was employed to follow metabolic shifts in response to superoxide stress. Steady-state challenge with paraquat at concentrations inducing SoxRS-controlled genes resulted in several alterations in metabolite pools, including a striking increase in valine concentration. Elevated valine levels, together with increased glutathione and alkylperoxidase, are proposed to constitute an intracellular protection mechanism against reactive oxygen species. As shown by this example of metabolome analysis, novel cellular responses to environmental challenge can be revealed by following the total complement of metabolites in a cell.

The influence of cellular physiology on the initiation of mutational pathways in Escherichia coli populations

The factors affecting the direction of evolutionary pathways and the reproducibility of adaptive responses were investigated under closely related but non–identical conditions. Replicate chemostat cultures of Escherichia coli were compared when adapting to partial or severe glucose limitation. Four independent populations used a reproducible sequence of early mutational changes under both conditions, with rpoS mutations always occurring first before mgl. However, there were interesting differences in the timing of mutational sweeps: rpoS mutations appeared in a clock–like fashion under both partial and severe glucose limitation, while mgl sweeps arose under both conditions but at different times. Interestingly, malT and mlc mutations appeared only under severe limitation. Even though the ancestors were genotypically identical, the semi–differentiated properties of bacteria growing with mild or severe glucose limitation sent the populations in characteristic directions. Mutation supply and the fitness contribution of mutations were estimated and demonstrated to be potential influences in the choice of particular adaptation pathways under severe and mild glucose limitation. Predicting all the mutations fixed in adapting populations is beyond our current understanding of evolutionary processes, but the interplay between ancestor physiology and the initiation of adaptation pathways is demonstrated and definable in bacterial populations.

Substrate Specificity and Signal Transduction Pathways in the Glucose-Specific Enzyme II (EII Glc ) Component of the Escherichia coli Phosphotransferase System

Escherichia coli adapted to glucose-limited chemostats contained mutations in ptsG resulting in V12G, V12F, and G13C substitutions in glucose-specific enzyme II (EII(Glc)) and resulting in increased transport of glucose and methyl-alpha-glucoside. The mutations also resulted in faster growth on mannose and glucosamine in a PtsG-dependent manner. By use of enhanced growth on glucosamine for selection, four further sites were identified where substitutions caused broadened substrate specificity (G176D, A288V, G320S, and P384R). The altered amino acids include residues previously identified as changing the uptake of ribose, fructose, and mannitol. The mutations belonged to two classes. First, at two sites, changes affected transmembrane residues (A288V and G320S), probably altering sugar selectivity directly. More remarkably, the five other specificity mutations affected residues unlikely to be in transmembrane segments and were additionally associated with increased ptsG transcription in the absence of glucose. Increased expression of wild-type EII(Glc) was not by itself sufficient for growth with other sugars. A model is proposed in which the protein conformation determining sugar accessibility is linked to transcriptional signal transduction in EII(Glc). The conformation of EII(Glc) elicited by either glucose transport in the wild-type protein or permanently altered conformation in the second category of mutants results in altered signal transduction and interaction with a regulator, probably Mlc, controlling the transcription of pts genes.

Regulation of mutY and Nature of Mutator Mutations in Escherichia coli Populations under Nutrient Limitation

Previous analysis of aerobic, glucose-limited continuous cultures of Escherichia coli revealed that G:C-to-T:A (G:C-->T:A) transversions were the most commonly occurring type of spontaneous mutation. One possible explanation for the preponderance of these mutations was that nutrient limitation repressed MutY-dependent DNA repair, resulting in increased proportions of G:C-->T:A transversions. The regulation of the mutY-dependent DNA repair system was therefore studied with a transcriptional mutY-lacZ fusion recombined into the chromosome. Expression from the mutY promoter was fourfold higher under aerobic conditions than under anaerobic conditions. But mutY expression was higher in glucose- or ammonia-limited chemostats than in nutrient-excess batch culture, so mutY was not downregulated by nutrient limitation. An alternative explanation for the frequency of G:C-->T:A transversions was the common appearance of mutY mutator mutations in the chemostat populations. Of 11 chemostat populations screened in detail, six contained mutators, and the mutator mutation in four cultures was located in the region of mutY at 66 min on the chromosome. The spectrum of mutations and rate of mutation in these isolates were fully consistent with a mutY-deficiency in each strain. Based on PCR analysis of the region within and around mutY, isolates from three individual populations contained deletions extending at least 2 kb upstream of mutY and more than 5 kb downstream. In the fourth population, the deletion was even longer, extending at least 5 kb upstream and 5 kb downstream of mutY. The isolation of mutY mutator strains from four independent populations with extensive chromosomal rearrangements suggests that mutY inactivation by deletion is a means of increasing mutation rates under nutrient limitation and explains the observed frequency of G:C-->T:A mutations in glucose-limited chemostats.