uvrA is an acid-inducible gene involved in the adaptive response to low pHin Streptococcus mutans

The pH-inducible acid tolerance response (ATR) is believed to play a major role in acid adaptation and virulence of Streptococcus mutans. To study thi s phenomenon in S. mutans JH1005, differential display PCR was used to iden tify and clone 13 cDNA products that had increased expression in response t o pH 5.0 compared to that of pH 7.5-grown cells. One of these products, con firmed to be pH inducible by RNA dot blot and reverse transcription-PCR ana lyses, had 67% identity to a uvrA-UV repair excinuclease gene in Bacillus s ubtilis. Further sequence analysis of the uvrA homologue using the S. mutan s genome database revealed that the complete gene was encoded in an open re ading frame (ORE) of 2,829 bp (944 amino acids; 104.67 kDa). Immediately 3' of uvrA was an ORF encoding a putative aminopeptidase gene (pepP). uvrA kn ockouts were constructed in S. mutans strains JH1005, NGS, and UA159 using allelic-exchange mutagenesis, replacing the entire gene with an erythromyci n resistance cassette. As with uvrA mutants in other bacteria, the S. mutan s uvrA mutants were extremely sensitive to UV irradiation. The uvrA mutant of S. mutans JH1005 was also more sensitive than the wild type to growth at pH 5.0, showing a 15% reduction in growth rate and a 14% reduction in fina l resting culture density. Acid-adapted S. mutans JH1005 uvrA mutants were shown to be more resistant to UV irradiation than was the parent but were u nable to survive exposure to a killing pH of 3.0. Moreover, agarose gel ele ctrophoretic analysis of chromosomal DNA isolated from uvrA-deficient cells exposed to low pH demonstrated more DNA damage than that for the wild-type strain. Here we suggest that uvrA and the nucleotide excision repair pathw ay are involved in the repair of acid-induced DNA damage and are associated with successful adaptation of S. mutans to low pH.