Binding of the Escherichia coli MelR protein to the melAB promoter:: orientation of MelR subunits and investigation of MelR-DNA contacts

The Escherichia coli MelR protein is a melibiose-triggered transcription factor, belonging to the AraC family, that activates transcription initiation at the melAB promoter. Activation is dependent on the binding of MelR to four 18 bp sites, centred at position -42.5 (site 2'), position -62.5 (site 2), position -100.5 (site 1) and position -120.5 (site 1') relative to the melAB transcription start point. Activation also depends on the binding of CRP to a single site located between MelR binding site 1 and site 2. All members of the AraC family contain two helix-turn-helix (HTH) motifs that contact two segments of the DNA major groove at target sites on the same DNA face. In this work, we have studied the binding of MelR to different sites at the melAB promoter, focusing on the orientation of binding of the two MelR HTH motifs, and the juxtaposition of the different bound MelR subunits with respect to each other. To do this, MelR was engineered to contain a single cysteine residue adjacent to either one or the other HTH motif. The MelR derivatives were purified, and the cysteine residues were tagged with p-bromoacetamidobenzyl-EDTA-Fe, an inorganic DNA cleavage reagent. Patterns of DNA cleavage after MelR binding were then used to determine the positions of the two HTH motifs at target sites. In order to simplify our analysis, we exploited an engineered derivative of the melAB promoter in which MelR binding to site 2 and site 2', in the absence of CRP, is sufficient for transcription activation. To assist in the interpretation of our results, we also used a shortened derivative of MelR, MelR173, that is able to bind to site 2 but not to site 2'. Our results show that MelR binds as a direct repeat to site 2 and site 2' with the C-terminal HTH located towards the promoter-proximal end of each site. The orientation in which MelR binds to site 2' appears to be determined by MelR-MelR interactions rather than by MelR-DNA interactions. In complementary experiments, we used genetic analysis to investigate the importance of different residues in the two HTH motifs of MelR. Epistasis experiments provided evidence that supports the proposed orientation of binding of MelR at its target site.