Studies on the regulation of initiation of chromosome replication in Escherichia coli.

The total initiation frequency of chromosome replication in Escherichia coli is dependent on two factors; the timing or time interval between successive initiations on an individual chromosome (initiation pace) and the number of individual chromosomes which are being replicated per cell. We have examined these parameters in a dnaA^ts, conditionally-lethal, “initiation mutant” of an E. coli K12 strain growing at different permissive temperatures. Our results indicate that at temperatures between 30 and 35 °C the gene product of the dnaA167 allele becomes limiting with respect to the number of replicating chromosomes per cell, which decreases from two at 30 °C to one at 35 °C. However, over this same temperature range it is clear that cell growth is balanced and the initiation pace, as determined from the growth rate, increases with temperature and is indistinguishable from that of the dnaA⁺ parent. These results demonstrate that one can alter the total initiation frequency independently of the initiation pace, indicating the involvement of at least two cellular components in the regulation of initiation. They also suggest that while the dnaA product may be involved in determining the total number of initiation events which can occur per cell per doubling time it does not control the timing or pace at which successive initiation events are triggered on each chromosome, i.e. it is not the “pace-maker” for initiation.