Structure and function of the F plasmid genes essential for partitioning.

The F plasmid in Escherichia coli has its own partition mechanism controlled by the sopA and sopB genes, and by the cis-acting sopC region. The DNA sequence of the entire partition region and its flanking regions is described here. Two large open reading frames coding for 43,700 Mr and 35,400 Mr proteins correspond to sopA and sopB, respectively. The sopB reading frame is located immediately downstream from the sopA reading frame. Twelve 43 base-pair direct repeats exist in the sopC region without any spacer regions, and one pair of seven base-pair inverted repeats exists in each of the direct repeats. Analysis of deletions in the sopC region showed that the direct repeats play an important role in plasmid partition and IncD incompatibility. IncG incompatibility is exhibited by pBR322 derivatives carrying the sopB gene alone. When compared with the partition genes parA and parB of plasmid P1, homology in amino acid sequence was found between the SopA protein of F and the ParA protein of P1, and also between SopB protein of F and ParB protein of P1. In addition, homology was found between Rep proteins of F and P1.