Staphylococcus aureus is a major community and hospital-acquired pathogen causing superficial skin and life-threatening infections worldwide. The continued emergence of multiple-antibiotic resistant strains of S. aureus, especially methicillin and vancomycin-intermediate resistant strains, is of significant public health concern. This organism has evolved into a series of two-component signal transduction systems (TCS) in order to sense its immediate surroundings and to modulate cellular responses and the expression of virulence genes. Our long-term goals are to elucidate the regulatory mechanisms controlling the expression of genes involved in critical pathways in order to identify novel targets for delivering efficacious preventive and/or therapeutic agents against S. aureus. The specific hypothesis is that a novel S. aureus TCS (yhcSR} controls the genes/operons required for bacterial growth. Our hypothesis has been formulated on the basis of our preliminary data that 1) the down-regulation of YhcS (histdine kinase) expression inhibits bacterial growth in a dose-dependent manner; 2) the over-expression of either YhcS or YhcR also impedes bacterial growth; and 3) yhcR encodes a response regulator (DNA-Binding protein) and has many different homologs in various Gram-positive pathogens. Therefore, the objectives of this proposal are to characterize the yhcSR system, identify genes/operons that are controlled by yhcSR, and determine the genes/operons that yhcSR directly regulates, which are required for bacterial survival. We will pursue the following specific aims to test our central hypothesis. First, we will characterize the yhcSR system to address why it is important for bacterial growth. Second, we will identify the target genes directly regulated by yhcSR and determine the importance of the identified gene products for bacterial growth. Third, we will determine what controls yhcSR expression. These results will not only provide new insights into the regulatory mechanisms of bacterial physiology, but may also lead to the identification of novel targets for developing preventive and therapeutic interventions.