A simple mechanism for co-dependence on two activators at an Escherichia coli promoter.

The Escherichia coli melAB promoter is co-dependent upon two transcription activators, MelR and the cyclic AMP receptor protein, CRP. In this study we demonstrate positive co-operativity between the binding of MelR and CRP at the melAB promoter, which provides a simple mechanism for its co-dependence. MelR binds to four sites, centred at positions -42.5, -62.5, -100.5 and -120.5 relative to the melAB transcription start point. When MelR is pre-bound, CRP is able to bind to a target located between MelR at positions -62.5 and -100.5. This increases the occupation of the two downstream sites for MelR, which is essential for transcription activation. We have identified residues within activating region 1 (AR1) of CRP that are important in transcription activation of the melAB promoter. At simple CRP-dependent promoters, the surface of CRP containing these residues is involved in contacting the RNA polymerase alpha subunit. Our results show that, at the melAB promoter, the surface of CRP containing AR1 contacts MelR rather than RNA polymerase. Thus, MelR and CRP activate transcription by a novel mechanism in which they bind co-operatively to adjacent sites and form a bacterial enhanceosome.