Initiation of DNA replication in Escherichia coli

Summary

When a culture of E. coli strain carrying a temperature-sensitive DNA initiation mutation, dna-167 or dnaC2, is exposed to a nonpermissive temperature for a certain period of time, and then transferred back to a permissive temperature, DNA synthesis is resumed even in the presence of chloramphenicol. This shows that thermolabile components coded by either of these mutated genes can be reactivated after return to permissive temperatures, and consequently initiation of a new replication cycle can occur in the absence of concomitant protein synthesis in both strains. The reinitiation of replication occurring after lowering the temperature is sensitive to rifampicin in the dna-167 cells, but not in the dnaC2 mutant. The capacity for initiating a new round of replication is very labile in the dna-167 mutant, but not in the dnaC2 mutant, when a culture of the mutant is maintained at a nonpermissive temperature in the presence of rifampicin. Mechanisms of blocking of the initiation process with these mutants are discussed.

After a prolonged exposure of an early-exponential phase culture to high temperatures, reinitiation of DNA replication never exceeds a doubling in both strains, when the temperature is lowered in the presence of chloramphenicol. However, after an exposure of a late-exponential phase culture to a nonpermissive temperature, more than one round of replication occurs in both strains even in the presence of chloramphenicol.