Increased mutation frequency in redox-impaired Escherichia coli due to RelA- and RpoS-mediated repression of DNA repair.

Balancing of reducing equivalents is a fundamental issue in bacterial metabolism and metabolic engineering. Mutations in the key metabolic genes ldhA and pflB of Escherichia coli are known to stall anaerobic growth and fermentation due to a buildup of intracellular NADH. We observed that the rate of spontaneous mutation in E. coli BW25113 (DeltaldhA DeltapflB) was an order of magnitude higher than that in wild-type (WT) E. coli BW25113. We hypothesized that the increased mutation frequency was due to an increased NADH/NAD(+) ratio in this strain. Using several redox-impaired strains of E. coli and different redox conditions, we confirmed a significant correlation (P < 0.01) between intracellular-NADH/NAD(+) ratio and mutation frequency. To identify the genetic basis for this relationship, whole-genome transcriptional profiles were compared between BW25113 WT and BW25113 (DeltaldhA DeltapflB). This analysis revealed that the genes involved in DNA repair were expressed at significantly lower levels in BW25113 (DeltaldhA DeltapflB). Direct measurements of the extent of DNA repair in BW25113 (DeltaldhA DeltapflB) subjected to UV exposure confirmed that DNA repair was inhibited. To identify a direct link between DNA repair and intracellular-redox ratio, the stringent-response-regulatory gene relA and the global-stress-response-regulatory gene rpoS were deleted. In both cases, the mutation frequencies were restored to BW25113 WT levels.