THE RELAXOSOME PROTEIN MOBC PROMOTES CONJUGAL PLASMID MOBILIZATION BYEXTENDING DNA STRAND SEPARATION TO THE NICK SITE AT THE ORIGIN OF TRANSFER

The frequency of conjugal mobilization of plasmid R1162 is decreased a pproximately 50-fold if donor cells lack MobC, one of the plasmid enco ded proteins making up the relaxosome at the origin of transfer (oriT) . The absence of MobC has several different effects on oriT DNA. Site- and strand-specific nicking by MobA protein is severely reduced, accou nting for the lower frequency of mobilization. The localized DNA stran d separation required for this nicking is less affected, but becomes m ore sensitive to the level of active DNA gyrase in the cell. In additi on, strand separation is not efficiently extended through the region c ontaining the nick site. These effects suggest a model in which MobC a cts as a molecular wedge for the relaxosome-induced melting of oriT DN A. The effect of MobC on strand separation may be partially complement ed by the helical distortion induced by supercoiling. However, MobC ex tends the melted region through the nick site, thus providing the sing le-stranded substrate required for cleavage by MobA.