Dissection of a surface-exposed portion of the cAMP-CRP complex that mediates transcription activation and repression

The Escherichia coli cAMP receptor protein (CRP) is essential for the activ ation and repression of transcription initiation at promoters in the CytR r egulon. CRP performs these activities by making direct protein-protein inte ractions to the alpha-subunits of RNA polymerase and to the CytR regulator. Strikingly, it has been shown that amino acids of CRP that are critical fo r communication with the two partner proteins are located in close proximit y on the surface of CRP. Here, we have dissected this surface in order to p inpoint the 'repression region' of CRP and to assess whether it overlaps wi th the characterized 'activating region'. Our results established that resi dues 12, 13, 17, 105, 108 and 110 are essential for the interaction with Cy tR and confirmed that 'activating region' 2 of CRP is made up of residues 1 9, 21 and 101, In the crystallographic structure of the CRP-DNA complex, th e two sets of determinants are located immediately adjacent to each other f orming a consecutive surface-exposed patch. The 'repression region' is chem ically complementary to the characterized region on CytR that is essential for protein-protein communication to CRP. Moreover, the results provide ins ight into the mechanism by which CytR might prevent CRP-mediated transcript ion.