Location of the C-terminal domain of the RNA polymerase alpha subunit in different open complexes at the Escherichia coli galactose operon regulatory region.

Hydroxyl radical footprinting has been used to study different open complexes between Escherichia coli RNA polymerase and the galactose operon regulatory region, which contains two overlapping promoters, P1 and P2. Complexes at P1 were studied by exploiting a P2- mutant and complexes at P2 were studied with a P1-mutant. We have identified the precise location of alpha binding in both binary RNA polymerase-galP1 and RNA polymerase-P2 complexes from the effects of deletion of the C-terminal domain of the RNA polymerase alpha subunit: alpha binds to different sites at the upstream end of each complex. Transcription initiation at galP1 can be activated by the cyclic AMP receptor protein (CRP). Addition of CRP to the RNA polymerase-galP1 complex displaces the C-terminal domain of alpha, which then binds to a different site upstream of CRP in the ternary CRP-RNA polymerase-galP1 complex. Thus, the C-terminal domain of alpha can occupy three different sites at the gal operon regulatory region. We have also examined the effect of disrupting the Activating Region of CRP on interactions between CRP and the C-terminal domain of alpha in ternary CRP-RNA polymerase-galP1 complexes. Footprinting experiments show that these substitutions interfere with the contact between CRP and alpha but do not affect the position of alpha binding to its site upstream of bound CRP.