Streptococcus mutans is considered as the major etiological agent in dental caries. S. mutans is also an important agent of infective endocarditis. The organism colonizes the oral cavity by forming diverse, multispecies biofilms on the tooth surface, known as dental plaque. This pathogen has developed multiple mechanisms to adapt and to flourish in the hostile environment of the oral cavity. S. mutans has the ability to respond rapidly and efficiently to various environmental fluxes, including severe nutrient limitation, fluctuations in pH and temperature, and changes in oxidative and osmotic tensions. Exposure of bacteria to these adverse environments can induce a stress tolerance response through expression of a wide variety of genes that provides cross-protection against diverse environmental challenges. Stress tolerance genes are regulated by unique groups of transcriptional regulators, including alternate sigma factors. However, unlike other pathogens, S. mutans and other streptococci do not encode any alternate sigma factors. On the other hand, some genes that putatively function in stress responses in other bacteria are present in the S. mutans genome, but their role in stress and virulence has not been investigated in S. mutans in great detail. One such group is the Clp (caseinolytic protease) family of proteins that are involved in thermal and oxidative stress responses. The Clp family is composed of a small cytoplasmic serine protease, called ClpP, and various ATPases. S. mutans encodes five Clp ATPases and ClpP associates with a partner Clp ATPase to form a functional complex that specifically targets damaged or misfolded proteins for degradation or translocation. While the ATPase component determines the substrate specificity, ClpP degrades the damaged protein. In most pathogens, ClpP plays a very important role in virulence. For example, in S. mutans, ClpP is required for optimal biofilm formation and acid-tolerance response. Although ClpP play important roles for the pathogenesis, very little is known about the regulation of the clp genes in this pathogen. Regulation of the clp genes is very complex, and can vary significantly depending on the organism. The main goal of this proposal is to gain insight into the expression of the clpP gene during S. mutans growth under normal and stressful conditions. Furthermore, we also propose to study an important and unique transcriptional repressor, CtsR, which is important for clp genes, including clpP, expression during adaptive response. We anticipate that completion of this project will lead to the identification of novel regulatory pathway for expression of stress response genes in S. mutans. Moreover, knowledge acquired from this project can also be extended to analysis of other important gram-positive pathogens such as S. pnemoniae and S. pyogenes, and may lead to the identification of novel drug targets.