The capacity of S. mutans to generate acid and to survive in an acidIc, highly cariogenic environment are among key elements of the organism's virulence. Results from studies by others have shown that S. mutans has the ability to rapidly adapt to the acidification of its cytoplasm. Results from our laboratory have shown that a genetically well-defined mutation in the gene (recA) of S. mutans renders the cells acid-sensitive. This observation indicated the need for DNA damage repair during normal cellular metabolism. Further, our studies have shown that cells grown at pH values of 5.0 induced the synthesis of a novel DNA repair system. The inducible system is RecA-independent and is not present in cells grown at neutral pH values. The repair system provides S. mutans with resistance to acid treatment, treatment with hydrogen peroxide, and irradiation by ultraviolet-light (UV). Thus, it appears that acid-adaptation by S. mutans includes the up-regulation of a system designed to cope with the DNA- damaging effects of life under acidic conditions. The maintenance of chromosomal integrity is central to the ability of a micro-organism to survive in its milieu and to replicate. A DNA repair system, designed to function at low pH growth conditions, would be of clear benefit to S. mutans and is likely part of the organism's repertoire of acid adaptive mechanisms. Therefore, we propose to elucidate the nature of this novel repair system; the mechanism(s) by which S. mutans effects its induction; and its role in virulence.
Our specific aims are as follows: we will extend our physiological characterization of the inducible system in the wild-type and recA strains for the purpose of understanding its ability to repair DNA, its regulation, and its relationship to a known repair pathways; we will use biochemical techniques to identify the damaged-DNA-binding component of the inducible system and to clone the gene encoding the binding component; and, we will evaluate the effect of the recA mutation on caries formation in the rat. Finally, we will use a library of insertionally-inactivated genes in the S. mutans chromosome to isolate genes involved with acid adaptation and the low pH-inducible repair system. The results of our studies will provide the following: new information regarding the genetic regulation of acid-adaptation and survival of the oral streptococci growing at low pH values; the definition of a novel, undefined DNA repair system and its role in the cariogenicity of S. mutans; and, insights into the role of DNA repair in cellular homeostasis. The successful completion of our project goals will provide a broad new understanding of the molecular mechanisms of streptococcal persistence in the oral cavity and its associated virulence. Lastly, we will obtain insight into the primary metabolic goal of a bacterium, that is, the maintenance and replication of its genetic stock under varying environmental conditions.
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