This project is concerned with the structural and functional aspects of a dGTPase of the bacterium E. coli, which has the unusual specificity: dGTP-> dG + PPPi. Although this activity was reported a number of years ago, its function in the cell has been unknown. Recently, we discovered a novel mutator activity in E. coli resulting from a defect in the dgt gene, encoding the above activity. Thus, insight into the function of the Dgt dGTPase may be gleaned from studies of the dgt mutator effect. As one hypothesis, we are considering the possibility that Dgt may be an activity aimed at sanitizing the cellular dNTP pool by removing aberrant dGTP derivatives. These derivatives, if not removed, will be incorporated into DNA and cause mutations. Alternatively, the protein functions in regulating the canonical dGTP pool in the cell. Our studies of the Dgt function include several approaches: (i) a genetic analysis of the dgt mutator effect in the bacterium E. coli, (ii) a biochemical analysis of the Dgt protein, including study of its regulation and substrate specificity, and (iii) structural analysis of Dgt protein by Nuclear Magnetic Resonance and X-ray crystallography methods. NMR studies will be aimed at possibly identifying the physiologically relevant substrate. It is noted that other mutation avoidance pathways working at the dNTP level, such as hydrolysis of the oxidative stress-related contaminant 8-oxo-dGTP, are characterized by a more conventional biochemical transformation in which the pyrophosphate moiety is cleaved rather than the entire triphosphate chain. Thus, it is likely that further study of the Dgt enzyme will provide insight into a novel, and likely important, cellular mechanism of mutation avoidance, whose significance may likely extend beyond the E. coli model system. The crystal structure of Dgt has revealed a hexametric structure containing two allosterically active ssDNA molecules, and kinetic experiments have shown that DNA binding leads to Dgt activation, primarily by lowering the km for the dGTP substrate.
