Role of Escherichia coli DNA polymerase I in conferring viability upon the dnaN159 mutant strain

The Escherichia coli dnaN159 allele encodes a mutant form of the P-sliding clamp (P159) that is impaired for interaction with the replicative DNA polymerase (Pol), Pol III. In addition, strains bearing the dnaN159 allele require functional Pol I for viability. We have utilized a combination of genetic and biochemical approaches to characterize the role(s) played by Poll I in the dnaN159 strain. Our findings indicate that elevated levels of Pol I partially suppress the temperature-sensitive growth phenotype of the dnaN159 strain. In addition, we demonstrate that the P clamp stimulates the processivity of Poll I in vitro and that 0159 is impaired for this activity. The reduced ability of 0159 to stimulate Pol I in vitro correlates with our finding that single-stranded DNA (ssDNA) gap repair is impaired in the dnaN159 strain. Taken together, these results suggest that (i) the beta clamp-Pol I interaction may be important for proper Pol I function in vivo and (ii) in the absence of Pol 1, ssDNA gaps may persist in the dnaN159 strain, leading to lethality of the dnaN159 Delta pol4 strain.