EXPLORING MECHANISMS BY WHICH CODY REGULATES THE ACTIVITY OF THE SAE-TWO COMPONENT SYSTEM TO CONTROL VIRULENCE IN STAPHYLOCOCCUS AUREUS

Abstract

Staphylococcus aureus is a ubiquitous Gram-positive bacterium that colonizes up to 30% of the human population. As an opportunistic human pathogen, the bacterium is a leading cause of skin and soft tissue infections, endocarditis, and bacteremia. Drug resistance compounds the problem, and the lack of an efficacious vaccine emphasizes the need for new therapies to combat staphylococcal infections. One exciting approach is to target the regulation of virulence genes. Virulence genes are often regulated by multiple factors that respond to different environmental cues. Some of the factors act directly on virulence genes; others control virulence indirectly by controlling the expression or activity of other regulators. My dissertation research focuses on CodY, an amino acid- and GTP-responsive global transcriptional regulator of many metabolic pathways, and the mechanism by which it funnels nutrient depletion signals through a major regulator of the SaeR/S two-component system. Previous work by others and I in the Brinsmade lab and elsewhere showed that overexpressing the sensor kinase gene saeS and the response regulator gene saeR does not increase the expression of the Sae regulon genes. To explain this surprising result, my thesis provides experimental support for the hypothesis that CodY controls the cellular fraction of activated SaeR (SaeR~P) to adjust the expression of virulence factor genes. In Chapter II, I show that transcriptional regulation of the sae locus is dispensable for CodY-dependent upregulation of the Sae regulon. Moreover, CodY-deficient S. aureus strains have higher SaeS kinase activity, correlating with increased membrane branch-chain fatty acids derived from isoleucine. In Chapter III, I show that CodY regulates SaeS kinase activity during periods of nutrient sufficiency by repressing the genes that code for enzymes and proteins that catalyze branched-chain fatty acid synthesis. Additional work herein focuses on the molecular mechanism by which the branched-chain fatty acids alter the SaeS signaling complex. My work contributes to the elucidation of a novel method of post-transcriptional virulence regulation by branched-chain fatty acids. Understanding the molecular basis of this regulation is an important first step in characterizing potentially new anti-virulence targets with therapeutic potential.