Genetic and physiologic analysis of the groE operon and role of the HrcA repressor in stress gene regulation and acid tolerance in Streptococcus mutans

Our working hypothesis is that the major molecular chaperones DnaK and GroE play central roles in the ability of oral bacteria to cope with the rapid and frequent stresses encountered in oral biofilms, such as acidification a nd nutrient limitation. Previously, our laboratory partially characterized the dnaK operon of Streptococcus mutans (hrcA-grpE-dnaK) and demonstrated t hat dnaK is up-regulated in response to acid shock and sustained acidificat ion (G. C. Jayaraman, J. E. Penders, and R. A. Burne, Mol. Microbiol. 25:32 9-341, 1997). Here, we show that the groESL genes of S. mutans constitute a n operon that is expressed from a stress-inducible sigma (A)-type promoter located immediately upstream of a CIRCE element. GroEL protein and mRNA lev els were elevated in cells exposed to a variety of stresses, including acid shock. A nonpolar insertion into hrcA was created and used to demonstrate that HrcA negatively regulates the expression of the groEL and dnaK operons . The SM11 mutant, which had constitutively high levels of GroESL and rough ly 50% of the DnaK protein found in the wild-type strain, was more sensitiv e to acid killing and could not lower the pH as effectively as the parent. The acid-sensitive phenotype of SM11 was, at least in part, attributable to lower F1F0-ATPase activity. A minimum of 10 proteins, in addition to GroES -EL, were found to be up-regulated in SM11 The data clearly indicate that H rcA plays a key role in the regulation of chaperone expression in S. mutans and that changes in the levels of the chaperones profoundly influence acid tolerance.