Replication of ColE2 and ColE3 plasmids: Two ColE2 incompatibility functions

Summary We have identified and localized two incompatibility determinants (IncA and IncB) within a 1.3 kb segment of ColE2 sufficient for autonomous replication. The IncA determinant is localized in a region shorter than 250 bp and expresses incompatibility against both ColE2 and ColE3. The region which determines sensitivity to the IncA determinant seems to overlap with the region specifying the IncA determinant. The expression of the trans-acting factor(s) specifically required for replication of ColE2 interferes with expression of the IncA determinant against ColE2 but not against ColE3. The IncA determinant might be at least partly responsible for the copy number control of the plasmid. The IncB determinant is localized in a 50 bp region (origin) which is sufficient for initiation of replication in the presence of the trans-acting factor(s). The IncB determinant is specific for ColE2 and seems to be due to titration of the trans-acting essential replication factor(s) by binding.