Escherichia coli encodes three SOS-induced DNA polymerases: pol II, pol IV, and pol V. We show here that each of these polymerases confers a competitive fitness advantage during the stationary phase of the bacterial life cycle, in the absence of external DNAdamaging agents known to induce the SOS response. When grown individually, wild-type and SOS pol mutants exhibit indistinguishable temporal growth and death patterns. In contrast, when grown in competition with wild-type E. coli, mutants lacking one or more SOS polymerase suffer a severe reduction in fitness. These mutants also fail to express the ''growth advantage in stationary phase'' phenotype as do wild-type strains, instead expressing two additional new types of ''growth advantage in stationary phase'' phenotype. These polymerases contribute to survival by providing essential functions to ensure replication of the chromosome and by generating genetic diversity.GASP ͉ genetic diversity ͉ competitive fitness ͉ stationary phase survival ͉ SOS DNA polymerases O f the five DNA polymerases present in Escherichia coli, three are induced as part of the SOS regulon in response to DNA damage (1). Pol V, encoded by umuDC (2-4), and pol IV, encoded by dinB (5), are members of the recently named Y-family of error-prone polymerases (6), whereas B-family pol II, encoded by polB (7-11), copies DNA with high fidelity (12). Although we generally think of these enzymes as being either error-free or error-prone polymerases, under certain conditions, an error-prone polymerase can catalyze an error-free reaction and vice versa (1). SOS pols take part in diverse biochemical pathways in dividing and stationary-phase cells. In dividing cells, pol V is responsible for generating base substitution mutations targeted at DNA damage sites (13,14). Pol II is instrumental in rescuing stalled replication forks on damaged DNA (15, 16), but in contrast to error-prone pol V, the pol II-catalyzed replicationrestart process occurs with no measurable increase in mutational load (15). Indeed, pols II and V behave as ''flip sides of a coin''-pol II acting early to help catalyze ''error-free'' replication-restart and pol V coming into play only later to perform translesion synthesis at persisting DNA damage sites (13,15,17,18). Pol IV may be primarily involved in generating simple ''nontargeted'' frameshift mutations (19,20), perhaps while helping to rescue replication forks that become stalled on undamaged DNA (21). Pol IV is also involved in copying bulky template adducts (22,23). In nondividing cells, pols IV and II play a central balancing role during adaptive mutation, a process in which nonproliferating microbial populations accumulate mutations when placed under nonlethal selective pressure (24). Pol IV is responsible for making most of the lacZ adaptive frameshift mutations occurring on a plasmid (25, 26), with pol II acting to moderate the level of mutation (27).A stationary-phase-specific process, the expression of the ''growth advantage in stationary phase'' (GASP) phenotype, depends...
Glycation, or nonenzymatic glycosylation, is a chemical reaction between reactive carbonyl-containing compounds and biomolecules containing free amino groups. Carbonyl-containing compounds include reducing sugars such as glucose or fructose, carbohydrate-derived compounds such as methylglyoxal and glyoxal, and nonsugars such as polyunsaturated fatty acids. The latter group includes molecules such as proteins, DNA, and amino lipids. Glycation-induced damage to these biomolecules has been shown to be a contributing factor in human disorders such as Alzheimer's disease, atherosclerosis, and cataracts and in diabetic complications. Glycation also affects Escherichia coli under standard laboratory conditions, leading to a decline in bacterial population density and long-term survival. Here we have shown that as E. coli aged in batch culture, the amount of carboxymethyl lysine, an advanced glycation end product, accumulated over time and that this accumulation was affected by the addition of glucose to the culture medium. The addition of excess glucose or methylglyoxal to the culture medium resulted in a dose-dependent loss of cell viability. We have also demonstrated that glyoxylase enzyme GloA plays a role in cell survival during glycation stress. In addition, we have provided evidence that carnosine, folic acid, and aminoguanidine inhibit glycation in prokaryotes. These agents may also prove to be beneficial to eukaryotes since the chemical processes of glycation are similar in these two domains of life.
The penicillin-binding proteins (PBPs) catalyze the synthesis and modification of bacterial cell wall peptidoglycan. Although the biochemical activities of these proteins have been determined in Escherichia coli, the physiological roles of many PBPs remain enigmatic. Previous studies have cast doubt on the individual importance of the majority of PBPs during log phase growth. We show here that PBP1b is vital for competitive survival of E. coli during extended stationary phase, but the other nine PBPs studied are dispensable. Loss of PBP1b leads to the stationary phase-specific competition defective phenotype and causes cells to become more sensitive to osmotic stress. Additionally, we present evidence that this protein, as well as AmpC, may assist in cellular resistance to beta-lactam antibiotics.
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