MUTATION IN THE PLASMID PUB110 REP PROTEIN AFFECTS TERMINATION OF ROLLING CIRCLE REPLICATION

We isolated a mutant of plasmid pUB110 that has the following properti es in Bacillus subtilis: (i) it is toxic for recA and add cells, parti cularly at elevated temperature; (ii) it has a copy number threefold h igher than that of the parental plasmid, and the extra copies are pres ent 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., G ly 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 .