Staphylococcus aureus is a leading cause of nosocomial and community associated infections. The organism owes its ability to cause disease to the production of a repertoire of virulence- and antibiotic resistance- determinants, as well as its adaptability to environmental challenges. Our long term goal is to define the regulatory mechanisms that control S. aureus pathogenicity. Classically, S. aureus virulence factor expression has been considered to be regulated at the level of transcript synthesis. The specific hypothesis of this proposal is that S. aureus regulates these factors by modulating their mRNA turnover. This hypothesis is based on the following observations: 1) we have shown that S. aureus regulates the mRNA turnover of mRNA species, including most recognized virulence factors, in response to endogenous and exogenous cues, 2) our preliminary data establish that alterations in mRNA decay correlate with changes in protein production, 3) we have found that the organism produces several molecular components that, by virtue of analogous systems, are likely to transiently affect mRNA turnover, and 4) modulation of mRNA turnover is a common mechanism of regulating protein production in other bacterial pathogens. The specific aims of this proposal are: 1. Characterize factors that govern log-phase (native) S. aureus mRNA turnover. We believe that the native RNA turnover functions can be altered in response to conditions that the organism faces during pathogenesis. As a prerequisite to determining how these functions are altered, it is crucial to understand the principle components of native mRNA turnover;they will be identified. 2. Identify factors that transiently modulate mRNA turnover. Small non-coding RNAs (sRNAs), RNA binding proteins, and RNA degradation machinery auxiliary factors influence mRNA turnover and translation within other bacterial pathogens. Our preliminary data indicate that several, if not all, of these RNA turnover modulatory factors exist within S. aureus. We will define the effects of S. aureus sRNA-like molecules on mRNA expression, mRNA turnover and protein production. Moreover, we will identify other trans-acting factors that mediate alterations in S. aureus virulence factor mRNA stability. 3. Characterize the effects of modulating mRNA stability on virulence factor protein production.