New roles of DNA and SopB in polymerization of SopA of Escherichia coli F plasmid

Active equi-paritioning of the F plasmid is achieved by its sopABC gene. SopA binds to the sopAB promoter region and SopB binds to sopC. SopA also polymerizes in the presence of ATP and Mg(II), which is stimulated by SopB. Non-specific DNA is known to inhibit SopA polymerization and disassemble SopA polymer. This study followed kinetics of polymerization and de-poly-merization of SopA by turbidity measurement and found new effects by DNA and SopB. Plasmid DNA, at low concentrations, shortened the lag (nucleation) phase of SopA polymerization and also caused an ini-tial ‘burst ’ of turbidity. Results with two non-specific 20-bp DNAs indicated sequence/length dependence of these effects. sopAB operator DNA only showed inhib-ition of SopA polymerization. Results of turbidity de-crease of pre-formed SopA polymer in the presence of ethylenediaminetetraacetic acid showed that SopB also accelerates disassembly of the SopA polymer. The steady-state level of turbidity in the presence of SopB and plasmid DNA indicated synergy between SopB and DNA in the disassembly. SopB protein showed no effect on SopA polymerization, when SopB was specif-ically bound to DNA. This result and others with trun-cation mutants of SopB suggested that a proper configuration of the domains of SopB is important for SopA-SopB interactions