Abstract
A new Escherichia coli mutant allele, named dnaR, that causes thermosensitive initiation of chromosome replication has been identified to be an allele of the prs gene, the gene for phosphoribosylpyrophosphate synthetase (Y. Sakakibara, J. Mol. Biol. 226:979-987, 1992; Y. Sakakibara, J. Mol. Biol. 226:989-996, 1992). The dnaR mutant became temperature resistant by acquisition of a mutation in the dnaA gene that did not affect the intrinsic activity for the initiation of replication. The suppressor mutant was capable of initiating replication from oriC at a high temperature restrictive for the dnaR single mutant. The thermoresistant DNA synthesis was inhibited by the presence of the wild-type dnaA allele at a high but not a low copy number. The synthesis was also inhibited by an elevated dose of a mutant dnaR allele retaining dnaR activity. Therefore, thermoresistant DNA synthesis in the suppressor mutant was dependent on both the dnaA and the dnaR functions. On the basis of these results, I conclude that the initiation of chromosome replication requires cooperation of the prs and dnaA products.