Control of replication of the broad host range plasmid RSF1010: the incompatibility determinant consists of directly...

A 500 bp DNA fragment located in the vicinity of the origin of replication of plasmid RSF1010 was cloned into the plasmid vector pBR322 and shown to exhibit incompatibility against parental RSF1010. The rightmost region of this fragment contains three perfect 20 bp direct repeats and a fourth half-repeat of 11 bp, as shown by DNA sequencing. Deletion of the four repeats from the cloned fragment resulted in complete loss of incompatibility whereas partial deletion of the repeated sequence resulted in a concurrent decrease in the expression of incompatibility. We conclude that the incompatibility determinant of RSF1010 is defined by the four repeats and also that the incompatibility expressed is not very strong, since the presence of about 1.5 times as many copies of the repeated sequence as are normally in a cell does not cause a total switch off of RSF1010 replication, but only a 40% reduction in the rate of replication.