Transcription from the Escherichia coli melR promoter is dependent on the cyclic AMP receptor protein.

Expression of the melR gene is required for melibiose-dependent stimulation of transcription initiation at the promoter of the melAB operon. Using the S1 nuclease method we have located the melR transcription start point. Transcription from the melR promoter is dependent on cAMP-CRP: specific nucleotide sequences downstream of bp -59 with respect to the melR transcription start are sufficient for full promoter activity. Nucleotide sequence homologies suggest that the cAMP-CRP binding site is located from bp -52 to -31, in exactly the same position as at the galP1 promoter. Using DNase I footprinting we show that cAMP-CRP and RNA polymerase together bind tightly to the melR promoter sequence, creating a strong footprint from bp -70 to +20. Alone, cAMP-CRP binding is hardly detectable, whereas RNA polymerase alone creates a weak footprint centred around the -10 hexamer sequence. When the melR gene is expressed from a cAMP-CRP-independent promoter, melibiose-dependent transcription from the melAB promoter becomes independent of cAMP-CRP, showing that the melR promoter is the primary site of control by cAMP-CRP in the mel regulon.