Molecular Homology and Incompatibility in the IncFI Plasmid Group

Abstract

The usual grounds for the inclusion of a plasmid in a particular incompatibility group are its mutual incompatibility with a type plasmid of that group, and, in some cases, the demonstration of shared regions of specific homology, presumed to be related to DNA replication. Some plasmids classified as IncFI [incompatibility group FI] on genetical grounds share no homology with the previously described incompatibility regions of F on the basis of hybridization of specific radioactive probes to restriction enzyme digests of DNA from these plasmids. Others show homology with some or all of the regions of the F plasmid that can express incompatibility. The incompatibility behavior of these plasmids was examined to determine the relationship between the possession of regions of homology and the expression of incompatibility. Three plasmids, ColV3-K30, pHH507 and Entp307, show homology only with the secondary replicon of F and appear to use sequences homologous with the secondary F replicon in their replication. The results are consistent with the propositions that some contemporary IncFI plasmids arose by the integration of several replicons, and, in general, the replicon not being used for replication expresses its incompatibility, as does the replicon being used for replication. Incompatibility of 2 plasmids with F does not necessarily demonstrate relatedness of the plasmids to each other, and inclusion within the IncFI group can result from the possession of any of several combinations of inc sequences.