Partition Functions of Mini-F Affect Plasmid DNA Topology in Escherichia coli

Abstract

Efficient segregation of the low copy number plasmid mini-F is dependent on partition functions encoded by the plasmid sopABCgenes. The sopregion encodes proteins SopA and SopB and a cis-acting element, sopC, which may function as a centromere analog. The SopC segment contains 12 imperfect 43 bp repeats to which the SopB protein binds. We have found that mutations in the sopgenes affect superhelicity of isolated plasmid DNA. Plasmids with mutations in sopBor a deletion of the sopCsegment were more highly negatively supercoiled than normal. In contrast, a mutation in the autoregulatory SopA protein resulted in plasmid DNA that was more relaxed. The SopAB proteins provided in transto a pBR322 plasmid carrying sopCresulted in the relaxation of negative supercoils. We suggest that binding of SopB protein to the cis-acting sopCsegment in vivo, alone or in conjunction with other proteins, produced a change in DNA topology in which positive superhelical turns were3 introduced locally. This higher-order nucleoprotein structure may allo interaction of plasmid mini-F with the partition apparatus.