Properties and functions of Escherichia coli: Pol IV and Pol V

Escherichia coli possesses two members of the newly discovered class of Y DNA polymerases (Ohmori et al., 2001): Pol IV (dinB) and Pol V (umuD'C). Polymerases that belong to this family are often referred to as specialized or error-prone DNA polymerases to distinguish them from the previously discovered DNA polymerases (Pol I, II, and III) that are essentially involved in DNA replication or error-free DNA repair. Y-family DNA polymerases are characterized by their capacity to replicate DNA, through chemically damaged template bases, or to elongate mismatched primer termini. These properties stem from their capacity to accommodate and use distorted primer templates within their active site and from the lack of an associated exonuclease activity. Even though both belong to the Y-family, Pol IV and Pol V appear to perform distinct physiological functions. Although Pol V is clearly the major lesion bypass polymerase involved in damage-induced mutagenesis, the role of Pol IV remains enigmatic. Indeed, compared to a wild-type strain, a dinB mutant exhibits no clear phenotype with respect to survival or mutagenesis following treatment with DNA-damaging agents. Subtler dinB phenotypes will be discussed below. Moreover, despite the fact that both dinB and umuDC loci are controlled by the SOS response, their constitutive and induced levels of expression are dramatically different. In noninduced cells, Pol V is undetectable by Western analysis. In contrast, it is estimated that there are about 250 copies of Pol IV per cell. On SOS induction, it is believed that only about 15 molecules of Pol V are assembled per cell (S. Sommer, personal communication), whereas Pol IV levels reach approximately 2500 molecules. In fact, despite extensive knowledge of the individual enzymatic properties of all five E. coli DNA polymerases, much more work is needed to understand how their activities are orchestrated within a living cell.