Reinitiation of chromosome replication in a thermosensitive DNA mutant of Escherichia coli: II. Synchronization of chromosome replication after temperature shifts

Abstract

A short incubation at the non-permissive temperature, 10 to 15 minutes at 40 °C, suffices to induce chromosome reinitiation in CRT 266, a thermosensitive DNA mutant of Escherichia coli. In order to acquire the potentiality to reinitiate chromosome replication, protein synthesis is necessary, both during the 40 °C incubation and also during the first 15 minutes after returning to 30 °C. After a temperature shift in CRT 266, chromosome replication is initiated in only one of the two partially synthesized daughter chromosomes. The induced reinitiation results in a synchronous round of chromosome replication, as shown by the abrupt and time-dependent increase in (a) enzyme levels of inducible operons, and (b) efficiency of nitrosoguanidine-induced reversion of auxotrophic mutations after reinitiation of chromosome replication. The results suggest that the starting point of chromosome replication lies in the rec A region at seven o'clock on the circular E. coli chromosome and that replication proceeds clockwise from this origin. Under our conditions, i.e. glucose synthetic media and at 30 °C, replication of the E. coli CRT 266 chromosome is completed in 70 to 80 minutes.