Mechanism of primer-template-dependent conversion of dNTP leads to dNMP by T5 DNA polymerase

T5 DNA polymerase catalyzes both 5' leads to 3' polymerization and 3' leads to 5' hydrolysis in a processive fashion. This knowledge has been utilized to obtain evidence indicating that the enzyme has a single primer-template binding site which can function as either polymerase or exonuclease, perhaps with the cooperation of additional or different side groups. Template-dependent conversion of dNTP leads to dNMP was observed with an excess of either primer-template or enzyme. With primer-template excess, practically all the enzymes were functional as polymerase; with enzyme excess, all primer-templates were extended during the first cycle of catalysis. These observations suggest that turnover takes place at the points of chain growth. Evidence is also provided which demonstrates that the enzyme is capable of switching its direction of catalysis from 3' leads to 5' to 5' leads to 3' without leaving the primer-template. A clear correspondence between the relative amount of hydrolysis of a terminally labeled residue on the primer and the relative amount of turnover suggests that (a) the probability of hydrolysis of a given type of residue in contact with the "active site" is constant, and (b) during each turnover episode enzyme usually takes only one step in the 3' leads to 5' direction. A simple probabilistic model of turnover is discussed.