Binding of CRP to DNA-dependent RNA polymerase from E. coli: modulation by cAMP of the interactions with free and DNA-bound holo and core enzyme.

The regulatory protein CRP (or CAP) from E. coli is shown to display two distinct patterns of binding interactions with DNA-dependent RNA polymerase. The free core enzyme, and both the core and the holo polymerase when bound to single-stranded DNA, can bind CRP in a cAMP-independent association reaction. Instead, the binding of CRP to free holoenzyme and to holo or core polymerase bound to native DNA was undetectable in the absence of cAMP. The specific ligand of CRP (cAMP) strengthens distinctively this class of interactions. In no case could any release of sigma-factor be demonstrated. Estimates of the dissociation constants were obtained for the various binding reactions which were investigated under quasi-physiological ionic conditions. These, together with the known values of the in vivo concentrations of CRP and RNA polymerase, suggest that the interactions described may have a functional significance.