This family consists of several bacterial MobC-like, mobilisation proteins. MobC proteins belong to the group of relaxases. Together with MobA and MobB they bind to a single cis-active site of a mobilising plasmid, the origin of transfer (oriT) region. The absence of MobC has several different effects on oriT DNA. Site- and strand-specific nicking by MobA protein is severely reduced, accounting for the lower frequency of mobilisation. The localized DNA strand separation required for this nicking is less affected, but becomes more sensitive to the level of active DNA gyrase in the cell. In addition, strand separation is not efficiently extended through the region containing the nick site. These effects suggest a model in which MobC acts as a molecular wedge for the relaxosome-induced melting of oriT DNA. The effect of MobC on strand separation may be partially complemented by the helical distortion induced by supercoiling. However, MobC extends the melted region through the nick site, thus providing the single-stranded substrate required for cleavage by MobA.