Control of Initiation of DNA Synthesis in Escherichia coli B/r

The pattern of new growing forks in the Escherichia coli genome after a period of inhibition of DNA synthesis has been examined. Synchronous cultures of E. coli B/r were treated with nalidixic acid to stop DNA synthesis for various lengths of time. After removal of the nalidixic acid the location of active growing forks was determined by measuring the reversion rate of a histidine requirement induced by the "growing-point mutagen," nitrosoguanidine. It was found that the ability to initiate a new round of DNA synthesis upon release from the nalidixic acid block was attained at the cell age at which a new round of replication would have been initiated had DNA synthesis not been blocked. However, if the nalidixic acid was removed before the normal initiation age, the initiation of the next round of synthesis was delayed past the normal time, which indicates that initiation of new rounds is not completely independent of DNA synthesis.