PROTEIN-PROTEIN INTERACTIONS DURING TRANSCRIPTION ACTIVATION - THE CASE OF THE ESCHERICHIA-COLI CYCLIC-AMP RECEPTOR PROTEIN

The Escherichia coli cyclic AMP receptor protein (CRP) is a homodimeri c transcription activator triggered by cyclic AMP. Escherichia coli co ntains more than 100 differ ent promoters that can be activated by CRP : in most cases the CRP acts by making direct contact with RNA polymer ase. Remarkably, there is considerable variation in the location of th e DNA site for CRP from one CRP-dependent promoter to another. Genetic methods have been used to locate the activating regions of CRP that m ake contact with RNA polymerase at promoters of different architecture s. At promoters where the DNA site for CRP is centred near to position s -61, -71 or -81 (i.e. 61, 71 or 81 base pairs upstream of the transc ript start-point, respectively), a single surface-exposed loop (Activa ting Region 1) in the downstream subunit of the CRP dimer makes contac t with RNA polymerase. The contact site in RNA polymerase is located i n one of the C-terminal domains of two RNA polymerase alpha subunits. At promoters where the DNA site for CRP is centred near to position -4 1, both subunits of the CRP dimer make contact with RNA polymerase via three separate surface exposed regions (Activating Regions 1, 2 and 3 ). At these promoters, where bound CRP overlaps with RNA polymerase-bi nding elements, the C-terminal domains of the polymerase alpha subunit s are displaced and bind upstream of CRP. Activation at a number of E. coli promoters is dependent on binding of two CRP dimers, with one di mer bound near to position -41 and the other dimer bound further upstr eam. In these cases, both bound CRP dimers contact RNA polymerase. The CRP dimer bound around position -41 contacts RNA polymerase via Activ ating Regions 1, 2 and 3, whereas the upstream hound CRP dimer contact s one of the displaced alpha C-terminal domains via Activating Region 1 in the downstream CRP subunit. Thus in these cases, codependence on two activators is due to simultaneous contacts between separate activa tors and RNA polymerase. This mechanism allows great flexibility, as a ny activator that can contact the C-terminal domain of the RNA polymer ase alpha subunits can act cooperatively with CRP.