Copy-number control of the Escherichia coli chromosome: a plasmidologist's view

The homeostatic system that sets the copy number, and corrects over-replication and under-replication, seems to be different for chromosomes and plasmids in bacteria. Whereas plasmid replication is random in time, chromosome replication is tightly coordinated with the cell cycle such that all origins are initiated synchronously at the same cell mass per origin once per cell cycle. In this review, we propose that despite their apparent differences, the copy-number control of the Escherichia coli chromosome is similar to that of plasmids. The basic mechanism that is shared by both systems is negative-feedback control of the availability of a protein or RNA positive initiator. Superimposed on this basic mechanism are at least three systems that secure the synchronous initiation of multiple origins; however, these mechanisms are not essential for maintaining the copy number.