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J. Bacteriol. doi:10.1128/JB.01961-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Modulation of covR expression in Streptococcus mutans UA159

Department of Microbiology, University of Kansas Medical Center, Kansas City, KS 66160

^* To whom correspondence should be addressed. Email: ibiswas{at}kumc.edu.

	Abstract

The biofilm-forming Streptococcus mutans is a Gram-positive bacterium that resides in the human oral cavity, and is considered to be the primary etiological agent in the formation of dental caries. The global response regulator CovR, which lacks a cognate sensor kinase, is essential for pathogenesis and biofilm-formation of this bacterium, but it is not clear how covR expression is regulated in S. mutans. In this communication, we present the results of our studies examining various factors that regulate the expression of covR in S. mutans UA159. Southern hybridization and PCR analysis indicated that CovR is an orphan response regulator in various isolates of S. mutans. The transcriptional start site for covR was found to be 221 base pairs upstream of the ATG start codon, and sited-directed mutagenesis of the upstream TATAAT box confirmed our findings. Expression of covR is growth phase dependent, with maximal expression observed during exponential growth phase. While changes to the growth temperature did not significantly affect expression of covR, increasing the pH or the concentration of Mg²⁺ in the growth medium leads to an increase in covR expression. The results of semiquantitative RT-PCR analysis and in vivo transcriptional fusion reporter assays indicated that CovR autoregulates its own expression; this was verified by EMSA and DNase I protection assays, which demonstrated direct binding of CovR to the promoter region. Apparently, regulation by Mg²⁺ and autoregulation of covR are not linked. A detailed analysis of the regulation of CovR may lead to a better understanding of pathogenesis of S. mutans as well as further insight in the prevention of dental caries.