Role of two component signaling response regulators in acid tolerance of Streptococcus mutans

In bacteria, two-component systems (TCS) involving the products of a histidine kinase gene (hk) and a response regulator gene (rr) play important roles in adaptation to environmental changes. Fourteen hk-rr homologs and one orphan rr homolog were identified in the Streptococcus mutans UA159 genome database. There have been no comprehensive evaluations of the roles of rr homologs in the acid tolerance of S. mutans. The TCS genes (tcs) of S. mutans were designated smtcs01-15. Mutants of S. mutans UA159 with deletions of rr and hk-rr were constructed. Acid tolerance was evaluated by comparing the doubling times at pH 7.2 and pH 5.5 between the wild-type and mutant strains. Excluding smtcs10 and 12, for which viable mutants could not be obtained, a total of 13 rr deletion mutants were constructed. The rr deletions in smtcs03, 05, 08, and 13 resulted in diminished acid tolerance in comparison with UA159. The hk-rr double-mutants exhibited acid sensitivity levels similar to those of the corresponding rr mutants. The results of the present study reveal the involvement of the rr genes of smtcs03 and 05 in acid tolerance. Deletion of hk and/or rr in smtcs03 generated an acid-sensitive phenotype. In contrast, for smtcs05, while deletion of rr resulted in reduced acid tolerance, a single-deletion of hk had no effect on acid tolerance. We implicated two rr genes in the acid tolerance of S. mutans. In particular, smtcs05 is a novel tcs, the sole rr of which is involved in the acid tolerance of S. mutans.