Transcription from the rha operon psr promoter.

S1 nuclease mapping experiments performed with RNA extracted from cell lines that were unable to metabolize L-rhamnose demonstrated that L-rhamnose and not a metabolite was the inducer of the L-rhamnose operons of Escherichia coli. In vitro transcription studies showed that purified RhaR activates transcription from the psr promoter in the presence of L-rhamnose. In the absence of L-rhamnose, RhaR binds to the psr promoter but does not activate transcription until L-rhamnose is added.