Mutation in the plasmid pUB110 Rep protein affects termination of rolling circle replication

We isolated a mutant of plasmid pUB110 that has the following properties in Bacillus subtilis: (i) it is toxic for recA and add cells, particularly at elevated temperature; (ii) it has a copy number threefold higher than that of the parental plasmid, and the extra copies are present as multimers; and (iii) it can efficiently complement replication of a cmp- satellite plasmid, despite being cmp+. All these properties are due to a single change in the plasmid replication protein, i.e., Gly at position 148 to Glu. These properties of the mutant Rep protein reflect a diminished ability to terminate rolling circle replication. We propose that the Rep protein may have a diminished affinity for the plasmid origin; alternatively, it may be impaired for recognition of the plasmid conformations which distinguish initiation and termination.