Regulation of DNA replication by iterons: an interaction between the ori2 and incC regions mediated by RepE-bound iterons inhibits DNA replication of mini-F plasmid in Escherichia coli.

In bacteria, plasmids and some DNA viruses, DNA replication is initiated and regulated by binding of initiator proteins to repetitive sequences. To understand the control mechanism we used the plasmid mini-F, whose copy number is stringently maintained in Escherichia coli, mainly by its initiator protein RepE and the incC region. The monomers of RepE protein bound to incC iterons, which exert incompatibility in trans and control the copy number of mini-F plasmid in cis. Many incompatibility defective mutants carrying mutations in their incC iterons had lost the affinity to bind to RepE, while one mutant retained high level binding affinity. The mutated incC mini-F plasmids lost the function to control the copy number. The copy number of the wild-type mini-F plasmid did not increase in the presence of excess RepE. These results suggested that the control of replication by incC iterons does not rely on their capacity to titrate RepE protein. Using a ligation assay, we found that RepE proteins mediated a cross-link structure between ori2 and incC, for which the dimerization domain of RepE and the structure of incC seem to be important. The structure probably causes inhibition of extra rounds of DNA replication initiation on mini-F plasmids, thereby keeping mini-F plasmid at a low copy number.