Regulation of cid and lrg expression by CcpA in Streptococcus mutans

Abstract

<p class="first" id="d10274705e206">The <i>Streptococcus mutans</i> Cid/Lrg system represents an ideal model for studying this organism’s ability to withstand various stressors encountered in the oral cavity. The <i>lrg</i> and <i>cid</i> operons display distinct and opposite patterns of expression in response to growth phase and glucose levels, suggesting that the activity and regulation of these proteins must be tightly coordinated in the cell and closely associated with metabolic pathways of the organism. Here, we demonstrate that expression of the <i>cid</i> and <i>lrg</i> operons is directly mediated by a global transcriptional regulator CcpA in response to glucose levels. Comparison of the <i>cid</i> and <i>lrg</i> promoter regions with the conserved CcpA binding motif revealed the presence of two potential <i>cre</i> sites (for CcpA binding) in the <i>cid</i> promoter (designated <i>cid-cre1</i> and <i>cid-cre2</i>), which were arranged in a similar manner to those previously identified in the <i>lrg</i> promoter region (designated <i>lrg-cre1</i> and <i>lrg-cre2</i>). We demonstrated that CcpA binds to both the <i>cid</i> and <i>lrg</i> promoters with a high affinity, but has an opposing glucose-dependent effect on the regulation of <i>cid</i> (positive) and <i>lrg</i> (negative) expression. DNase I footprinting analyses revealed potential binding sequences for CcpA in both <i>cid</i> and <i>lrg</i> promoter regions. Collectively, these data suggest that CcpA is a direct regulator of <i>cid</i> and <i>lrg</i> expression, and are suggestive of a potential mechanism by which Cid/Lrg-mediated virulence and cellular homeostasis is integrated with signals associated with both the environment and cellular metabolic status. </p>