Positive regulation of the Escherichia coli L-rhamnose operon is mediated by the products of tandemly repeated regulatory genes.

The rhaC gene, whose product is the positive activator of the genes required for L-rhamnose utilization, has been cloned along with the rhamnose structural genes. The rhaC sequence shows two partially overlapping reading frames, encoding two proteins of molecular weight 32,000 and 35,000 RhaS and RhaR. Both proteins show significant homology to AraC, the positive activator of the arabinose operon. S1 mapping located transcriptional start points and showed that RhaR, and possibly RhaS, positively regulate transcription from the structural gene promoters as well as transcription from their own promoter. In-vivo dimethyl sulfate footprinting and DNase I footprinting indicate that the RhaR protein may bind to DNA elements upstream from its RNA polymerase binding site.