The Redox-Sensing Regulator Rex Modulates Central Carbon Metabolism, Stress Tolerance Response and Biofilm Formation by <em>Streptococcus mutans</em>

<div><p>The Rex repressor has been implicated in regulation of central carbon and energy metabolism in Gram-positive bacteria. We have previously shown that <em>Streptococcus mutans</em>, the primary causative agent of dental caries, alters its transcriptome upon Rex-deficiency and renders <em>S. mutans</em> to have increased susceptibility to oxidative stress, aberrations in glucan production, and poor biofilm formation. In this study, we showed that <em>rex</em> in <em>S. mutans</em> is co-transcribed as an operon with downstream <em>guaA</em>, encoding a putative glutamine amidotransferase. Electrophoretic mobility shift assays showed that recombinant Rex bound promoters of target genes avidly and specifically, including those down-regulated in response to Rex-deficiency, and that the ability of recombinant Rex to bind to selected promoters was modulated by NADH and NAD⁺. Results suggest that Rex in <em>S. mutans</em> can function as an activator in response to intracellular NADH/NAD⁺ level, although the exact binding site for activator Rex remains unclear. Consistent with a role in oxidative stress tolerance, hydrogen peroxide challenge assays showed that the Rex-deficient mutant, TW239, and the Rex/GuaA double mutant, JB314, were more susceptible to hydrogen peroxide killing than the wildtype, UA159. Relative to UA159, JB314 displayed major defects in biofilm formation, with a decrease of more than 50-fold in biomass after 48-hours. Collectively, these results further suggest that Rex in <em>S. mutans</em> regulates fermentation pathways, oxidative stress tolerance, and biofilm formation in response to intracellular NADH/NAD⁺ level. Current effort is being directed to further investigation of the role of GuaA in <em>S. mutans</em> cellular physiology.</p>