The divergent promoters mediating transcription of the par locus of plasmid RP4 are subject to autoregulation.

The partitioning region of broad-host-range plasmid RP4 contains four genes (parA, parB, parC, and parD) that encode products essential for partition activity. Two divergently arranged promoters located in the intercistronic region between parC and parD mediate transcription of these genes. The transcriptional initiation sites for both promoters were determined by primer extension. Transcriptional fusions were used to show that parA, parB, and parC are combined in an operon, while parD constitutes a separate transcription unit. Both parCBA (genes in order of transcription) and parD are negatively autoregulated at the level of transcription by the gene products of parA and parD, respectively. parD promoter mutants which have become insensitive to repression by parD were isolated. Comparison of wild type and the mutant parD promoter sequences indicated that three short repeats are likely involved in the negative regulation of this promoter. Potentially these sequence elements comprise target sites for the ParD protein.