The copR gene product of plasmid pIP501 acts as a transcriptional repressor at the essential repR promoter.

The amount of the rate-limiting replication initiator protein RepR of plasmid pIP501 is negatively controlled by an antisense RNA (RNAIII) and a dispensable protein (CopR). Deletions or mutations in either component cause a 10-20-fold copy number increase. RNAIII induces transcription attenuation of the repR mRNA; the mode of CopR action remained unclear. To test the function of CopR, transcriptional fusions of promoters pI, pII and pIII with lacZ were integrated into the Bacillus subtilis chromosome. CopR and/or RepR were supplied in trans, and LacZ synthesis measured. The results show that CopR represses the repR promoter pII. Neither CopR nor RepR autoregulate their promoters. Gel mobility shift assays indicate that CopR binds to a 44 bp DNA fragment comprising the inverted repeat upstream of pII.