Interactions between the Escherichia coli MelR transcription activator protein and operator sequences at the melAB promoter.

The Escherichia coli MelR protein binds to two sites at the melAB promoter and activates transcription in response to melibiose. The binding of purified MelR to the melAB promoter and to a number of derivatives carrying point mutations has been studied. A melAB promoter fragment that is very weakly activated by the melR gene product has been made by creating multiple symmetrical changes in both MelR-binding sites. A library of random substitutions in MelR was screened to find mutants that were able to activate this mutant promoter. One of these mutants contained a substitution in a proposed helix-turn-helix region that most likely relaxes the sequence-specificity of MelR binding. The other mutants contain substitutions throughout the central part of MelR and appear to alter the conformation of MelR, such that activity is triggered at lower melibiose concentrations and by arabinose.