The level of supercoiling affects the regulation of DNA replication in Escherichia coli.

The chromosome of Escherichia coli is negatively supercoiled. This favours processes that unwind the two DNA strands, such as DNA replication. In this paper, we have investigated the effect of changed levels of overall chromosomal supercoiling on the initiation of DNA replication. Specifically, we have used flow cytometry to reveal effects on the synchrony of initiations of DNA replication in single cells. An increase in the level of supercoiling moderately reduced initiation synchrony. In contrast, decreased supercoiling led to pronounced asynchrony. We have excluded the possibility that this asynchrony is caused by changes in the level of the Dam methyltransferase or the DnaA protein. We suggest that the global level of supercoiling influences the topology of oriC and thereby the sequence of events leading to initiation of DNA replication in E. coli.