Genetic control of DNA initiation in Escherichia coli

Abstract

We describe the isolation, and properties of a mutant (CT28) of Escherichia coli with a temperature-sensitive defect in DNA initiation that is reversible. The mutation ( dna-28) responsible for this defect is shown to be located in the same region of the map as the dnaC group of DNA initiation mutants. A terminalized culture of CT28 initiates DNA synthesis synchronously immediately upon lowering the temperature, and will do so in the presence of chloram-phenicol. During prolonged incubation at the non-permissive temperature, the cells accumulate a capacity to initiate multiple rounds of replication per chromosome. The variation in the susceptibility of the argH − and thyA − alleles to reversion by pulse mutagenesis with nitrosoguanidine during a synchronous round of DNA replication, suggests that this round of replication is bidirectional and commences from an origin in the vicinity of 60 to 65 minutes. CT28 contains two temperature-sensitive mutations. These have been mapped and separated into two derivative strains. One of these, CT28-3b, carries the dna-28 mutation of the C locus, and like the parental double mutant is reversibly temperature-sensitive for an initiation function; but it is more temperature-sensitive than either the double mutant or the other single mutant derivative, CT28-1. The other, CT28-1, is not defective in DNA replication or initiation of replication at the non-permissive temperature.