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) a nd a dispensable protein (CopR). Deletions or mutations in either comp onent cause a 10-20-fold copy number increase. RNAIII induces transcri ption attenuation of the repR mRNA; the mode of CopR action remained u nclear. To test the function of CopR, transcriptional fusions of promo ters pI, pII and pIII with lacZ were integrated into the Bacillus subt ilis chromosome. CopR and/or RepR were supplied in trans, and LacZ syn thesis measured. The results show that CopR represses the repR promote r pit. Neither CopR nor RepR autoregulate their promoters. Gel mobilit y shift assays indicate that CopR binds to a 44 bp DNA fragment compri sing the inverted repeat upstream of pII.