The Bacteriophage P1 hot Gene Product Can Substitute for the Escherichia coli DNA Polymerase III θ Subunit

Abstract

ABSTRACT The θ subunit ( holE gene product) of Escherichia coli DNA polymerase (Pol) III holoenzyme is a tightly bound component of the polymerase core. Within the core (α-ε-θ), the α and ε subunits carry the DNA polymerase and 3′ proofreading functions, respectively, while the precise function of θ is unclear. holE homologs are present in genomes of other enterobacteriae, suggestive of a conserved function. Putative homologs have also been found in the genomes of bacteriophage P1 and of certain conjugative plasmids. The presence of these homologs is of interest, because these genomes are fully dependent on the host replication machinery and contribute few, if any, replication factors themselves. To study the role of these θ homologs, we have constructed an E. coli strain in which holE is replaced by the P1 homolog, hot . We show that hot is capable of substituting for holE when it is assayed for its antimutagenic action on the proofreading-impaired dnaQ49 mutator, which carries a temperature-sensitive ε subunit. The ability of hot to substitute for holE was also observed with other, although not all, dnaQ mutator alleles tested. The data suggest that the P1 hot gene product can substitute for the θ subunit and is likely incorporated in the Pol III complex. We also show that overexpression of either θ or Hot further suppresses the dnaQ49 mutator phenotype. This suggests that the complexing of dnaQ49 -ε with θ is rate limiting for its ability to proofread DNA replication errors. The possible role of hot for bacteriophage P1 is discussed.