The speed of the Escherichia coli fork in vivo depends on the DnaB:DnaC ratio.

The DnaC protein is required for loading the DnaB helicase at oriC. Thus DnaC promotes the formation of the pre-replication complex, but must leave the complex in order for the DnaB protein to function as a helicase. In vitro, a slight excess of DnaC inhibits the movement of replication forks by inhibiting DnaB helicase activity (Allen and Kornberg, 1991). Here we show that inhibition of DNA replication by excess DnaC also occurs in vivo. The rate of replication-fork movement was measured by flow cytometry. Initiation of replication was inhibited with rifampicin and the rate of fork movement monitored during replication runout by measuring the increase in the fraction of the cell population with fully replicated chromosomes. The replication rate was inversely related to the amount of excess DnaC protein. Initiation of replication was also inhibited. Co-overexpression of DnaB protein alleviated the inhibition of replication caused by moderate excess of DnaC. The results show that DnaC interacts with replication forks during elongation in vivo, probably by binding to DnaB and inhibiting its helicase activity. Therefore, the ratio of DnaC to DnaB and the affinity of DnaC for a helicase hexamer at an established replication fork are of great importance for the rate of replication fork movement also in vivo.