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Evolution of compatible replicons of the related IncQ-like plasmids, pTC-F14 and pTF-FC2

Department of Microbiology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

Correspondence
Douglas E. Rawlings
der{at}sun.ac.za

Two closely related but compatible plasmids of the IncQ-2 and IncQ-2 groups, pTF-FC2 and pTC-F14, were discovered in two acidiphilic chemolithotrophic bacteria. Cross-complementation and cross-regulation experiments by the replication proteins were carried out to discover what changes were necessary when the plasmids evolved to produce two incompatibility groups. The requirement of a pTC-F14 oriV for a RepC DNA-binding protein was plasmid specific, whereas the requirement for the RepA helicase and RepB primase was less specific and could be complemented by the IncQ-2 plasmid pTC-FC2, and the IncQ-1 plasmid pIE1108. None of the IncQ-1 plasmid replication proteins could complement the pTC-F14 oriV, and pTC-F14 and RSF1010 were incompatible. This incompatibility was associated with the RepC replication protein and was not due to iteron incompatibility. Replication of pTC-F14 took place from a 5·7 kb transcript that originated upstream of the mobB gene located within the region required for mobilization. A pTC-F14 mobB–lacZ fusion was regulated by the pTC-F14 repB gene product and was plasmid specific, as it was not regulated by the RepB proteins of pTF-FC2 or the IncQ-1 and IncQ-1 plasmids. Plasmid pTC-F14 appears to have evolved independently functioning iterons and a plasmid-specific RepC-binding protein; it also has a major replication transcript that is independently regulated from that of pTF-FC2. However, the RepA and RepB proteins have the ability to function with either replicon.