Transcription of the repA gene of the Pseudomonas plasmid pPS10 is initiated from a sigma 70 type promoter located 81 bp upstream from the repA gene, extends through the repA gene and the adjacent open reading frame, and ends 1114 nucleotides downstream. The repA promoter is repressed by interactions of the RepA protein with a region of 44 bp that extends from the -10 box of the promoter to the dnaA box of the origin of replication. The core of the repA operator region is formed by two in-phase invertedly repeated sequences of 8 bp, S1 and S2, that flank the -35 box of the promoter, and that share homology with the internal sequences of the iterons present in the origin of replication. RepA enters at the operator region first by protein-DNA interactions and subsequently by protein-protein interactions. These sequential interactions lead to the formation of high, medium and low-mobility electrophoretic complexes. Formation of the high-order complexes seems to be important for an efficient repression of the promoter. Interactions of RepA with the repA promoter region (repPO) occur more efficiently than with the origin of replication.