D protein of miniF plasmid acts as a repressor of transcription and as a site-specific resolvase.

Two activities of the D protein of the miniF plasmid have been found. Divergent promoters in ori-1 ("primary" replicative origin) of miniF are both repressed in cells which produce D protein. The mobilization of plasmids containing the ori-1 region by the F conjugation system is also repressed by D protein. In the former case D appears to act as a transcriptional repressor, whereas in the latter case D protein acts by resolving cointegrates of F and the mobilized plasmid. D protein resolves dimers whose monomer units contain the rfsF sequence needed for recA-independent, site-specific recombination of F. The nucleotide sequence of the D gene was determined. The D gene region contains two oppositely-oriented open reading frames which have the same reading phase and substantially overlap. Transposon insertion mutants were used to show that the gene for D protein occupies the top-strand (left-to-right) open reading frame.