Replication and incompatibility properties of plasmid pUB110 in Bacillus subtilis

Within plasmid pUB110 we have identified a 1.2 kb segment necessary and sufficient for driving autonomous replication in Rec+ cells at a wild-type copy number. This region can be divided into three functionally discrete segments: a 24 base pair (bp) region that acts as an origin, a 949 bp determinant of an essential replication protein, repU, and a 358 bp incompatibility region, incA, overlapping with the repU gene. The synthesis of the IncA determinant/s proceeds in the direction opposite to that of RepU. The positively (RepU) and negatively (IncA) trans-acting products seem to be involved in the control of plasmid replication. The RepU product has an Mr of 39 kDa, could be overproduced in Escherichia coli, and binds to the pUB110 origin region. Outside the minimal replicon a cis-acting, orientation dependent, 516 bp determinant is required (i) to compete with a coexisting incompatible plasmid and (ii) for segregational stability.