Distinctive genetic features exhibited by the Y-family DNA polymerases in Bacillus subtilis

Translesional DNA polymerases form a large family of structurally related proteins, known as the Y-polymerases. Bacillus subtilis encodes two Y-polymerases, referred herewith as Pol Y1 and Pol Y2. Pol Y1 was expressed constitutively and did not mediate UV mutagenesis. Pol Y1 overexpression increased spontaneous mutagenesis. This effect depended on Pol Y1 polymerase activity, Pol Y1 interaction with the beta-clamp, and did not require the presence of the RecA protein. In addition, Pol Y1 overexpression delayed cell growth at low temperature. The growth delay was mediated by Pol Y1 interaction with the beta-clamp but not by its polymerase activity, suggesting that an excess of Pol Y1 in the cell could sequester the beta-clamp. In contrast, Pol Y2 was expressed during the SOS response, and, in its absence, UV-induced mutagenesis was abolished. Upon Pol Y2 overproduction, both UV-induced and spontaneous mutagenesis were stimulated, and both depended on the Pol Y2 polymerase activity. However, UV mutagenesis did not appear to require the interaction of Pol Y2 with the beta-clamp whereas spontaneous mutagenesis did. In addition, Pol Y2-mediated spontaneous mutagenesis required the presence of RecA. Together, these results show that the regulation and the genetic requirements of the two B. subtilis Y-polymerases are different, indicating that they fulfil distinct biological roles. Remarkably, Pol Y1 appears to exhibit a mutator activity similar to that of Escherichia coli Pol IV, as well as an E. coli UmuD-related function in growth delay. Pol Y2 exhibits an E. coli Pol V-like mutator activity, but probably acts as a single polypeptide to bypass UV lesions. Thus, B. subtilis Pol Y1 and Pol Y2 exhibit distinctive features from the E. coli Y-polymerases, indicating that different bacteria have adapted different solutions to deal with the lesions in their genetic material.