A replication region of the IncHI plasmid, R27, is highly homologous with the RepFIA replicon of F

A region of the IncHl plasmid R27 has been found to share very close nucleotide sequence homology with the RepFIA replicon of F. This region has been located on a 1.6 kb segment of R27 plasmid DNA, and corresponds to ori‐2 and the E gene of F. The IncC repeat sequence region shows reduced homology, with the F repeats being an imperfect subset of a larger repeated sequence found in R27. The E gene homologue of R27 is able to initiate replication from the F ori‐2 sequence and to repress the E gene promoter of F. The results are consistent with the observed incompatibility behaviour of R27, and have a bearing on the specificity of interaction of E protein with its DNA‐binding sites.