The PriA protein, a component of the X174-type primosome, was previously shown to be essential for damage-inducible DNA replication in Escherichia coli, termed inducible stable DNA replication. Here, we show that priA::kan null mutants are defective in transductional and conjugational homologous recombination and are hypersensitive to mitomycin C and gamma rays, which cause double-strand breaks. The introduction of a plasmid carrying the priA300 allele, which encodes a mutant PriA protein capable of catalyzing the assembly of an active primosome but which is missing the n-pas-dependent ATPase, helicase, and translocase activities associated with PriA, alleviates the defects of priA::kan mutants in homologous recombination, double-strand break repair, and inducible stable DNA replication. Furthermore, spa-47, which was isolated as a suppressor of the broth sensitivity of priA::kan mutants, suppresses the Rec ؊ and mitomycin C sensitivity phenotypes of priA::kan mutants. The spa-47 suppressor mutation maps within or very near dnaC. These results suggest that PriA-dependent primosome assembly is crucial for both homologous recombination and double-strand break repair and support the proposal that these processes in E. coli involve extensive DNA replication.Homologous recombination, a ubiquitous activity in both prokaryotes and eukaryotes, not only is a means by which to generate genetic diversity but also plays a crucial role in the repair of DNA damage, including double-strand breaks (DSBs) (8,12). Despite recent advances in our understanding of the mechanism of homologous recombination, the extent to which DNA synthesis might be involved in the process is largely unknown. PriA, a component of the priming system which primes DNA synthesis in the initiation of X174 phage and ColE1-type plasmid DNA replication, has recently been shown to play an essential role in the initiation of DNA damage-inducible chromosome replication in Escherichia coli, termed inducible stable DNA replication (iSDR) (25). Here, we show that priA null mutants are defective in homologous recombination and are hypersensitive to chemical and physical agents that cause DSBs. These results strongly support the notion that extensive DNA replication is involved in homologous recombination and DSB repair.The X174-type primosome, originally discovered in the study of the initiation of phage X174 DNA replication, consists of several E. coli proteins (see reference 23 for a review

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Constitutive stable DNA replication (cSDR), which uniquely occurs in Escherichia coli rnhA mutants deficient in ribonuclease HI activity, requires RecA function. The recA428 mutation, which inactivates the recombinase activity but imparts a constitutive coprotease activity, blocks cSDR in rnhA mutants. The result indicates that the recombinase activity of RecA, which promotes homologous pairing and strand exchange, is essential for cSDR. Despite the requirement for RecA recombinase activity, mutations in recB, recD, recJ, ruvA and ruvC neither inhibit nor stimulate cSDR. It was proposed that the property of RecA essential for homologous pairing and strand exchange is uniquely required for initiation of cSDR in rnhA mutants without involving the homologous recombination process. The possibility that RecA protein is necessary to counteract the action of Tus protein, a contra-helicase which stalls replication forks in the ter region of the chromosome, was ruled out because introduction of the tus::kan mutation, which inactivates Tus protein, did not alleviate the RecA requirement for cSDR.