Methylation-dependent transcription controls plasmid replication of the CloDF13 cop-1(Ts) mutant.

The CloDF13 cop-1(Ts) mutant expresses a temperature-dependent plasmid copy number. At 42 degrees C the mutant shows a "runaway" behavior, and cells harboring this plasmid are killed. The cop-1(Ts) mutation is a G-to-A transition that disturbs one of the two methylation sites which are located opposite in the stem-loop structure within a region involved in both the initiation of primer synthesis for DNA replication and the termination of the cloacin operon transcript. We demonstrate that the mutation results in an increased primer (RNA II) synthesis resulting from nonconditional enhanced RNA II promoter activity, which at 42 degrees C causes a decrease in the amount of active replication repressor molecules (RNA I) synthesized from the opposite strand. We found that the absence of Dam methylation abolishes the mutant phenotype and that under this condition the high mutant level of RNA II synthesis is reduced, which is accompanied by a restoration of the regulation by RNA I. The role of methylation in the regulation of plasmid replication is discussed.