RepA negatively autoregulates the transcription of the repABC operon of the Rhizobium etli symbiotic plasmid basic replicon

The basic replicon of Rhizobium etli CE3, like other members of the repABC plasmid family, is constituted by the repABC operon. RepC is essential for replication, and RepA and RepB play a role in plasmid segregation. It has b een shown that deletion derivatives lacking the repAB genes have an increas ed copy number, indicating that these genes participate in the control of p lasmid copy number. RepA is also a trans-incompatibility factor. To underst and the regulation of the repABC operon, in this paper: (i) the transcripti on start site of the repABC operon was determined; (I!) the promoter region was identified by site-directed mutagenesis of the putative -35 and -10 he xameric elements; and (iii) RepA was recognized as a negative regulator of the transcription of the repABC operon.