PROJECT SUMMARY/ABSTRACT Streptococcus mutans-induced caries formation continues to be a major problem in developed and developing nations, which according to the World Health Organization, impacts 60-90% of children worldwide (63). Despite widespread water fluoridation and the implementation of educational programs aimed at improving oral health in the United States and abroad, recent reports reveal no significant improvement in the prevalence or severity of caries in the primary dentition (64). In addition, nearly 100% of adults either have or have had caries in their permanent teeth, contributing to the world's estimated direct costs for dental treatment that totaled $298 billion in 2010 (31, 64, 65). The survival and cariogenic potential of S. mutans in the human oral cavity is directly related to the availability and transport of essential metal ions, particularly iron and manganese (11, 12, 48, 55). In the present grant application, we profess a novel approach to alleviating tooth decay that is centered on investigating virulence gene regulation by the S. mutans SloR metalloregulatory protein. During the previous grant term, we confirmed SloR-dependent repression of S. mutans metal ion transport genes and virulence factors when sustainable levels of manganese are achieved (such as during a mealtime), and loss of this repression when metal ions become limiting (such as between meals). These observations led us to propose that modifications to SloR that render it constitutively repressive regardless of exogenous manganese availability can significantly attenuate S. mutans virulence gene expression and impede the process of cariogenesis. This application sets out to address this hypothesis by elucidating the structural basis for SloR activation and DNA binding in S. mutans using molecular, biochemical, and next-generation sequencing approaches. The proposed research is highly significant because it can lead to an improved understanding of metal ion homeostasis in S. mutans and lend support to the growing body of evidence that implicates regulators of bacterial metal ion uptake/transport as key therapeutic targets (1, 4, 8, 35, 37, 40, 43). Moreover, these studies can reveal the mechanisms that underlie the inherent ability of S. mutans to regulate virulence gene expression in the human mouth where significant changes in metal ion availability, acid pH and oxidative stress are part of its obligate biofilm lifestyle.