Cooperative DNA binding of heterologous proteins: evidence for contact between the cyclic AMP receptor protein and RNA polymerase.

Four cAMP-independent receptor protein mutants (designated CRP* mutants) isolated previously are able to activate in vivo gene transcription in the absence of cAMP and their activity can be enhanced by cAMP or cGMP. One of the four mutant proteins, CRP598 (Arg-142 to His, Ala-144 to Thr), has been characterized with regard to its conformational properties and ability to bind to and support abortive initiation from the lac promoter. In the absence of cGMP, CRP598 shows a more open conformation than CRP, as indicated by its sensitivity to proteolytic attack and 5,5'-dithiobis(2-nitrobenzoic acid)-mediated subunit crosslinking. Binding of wild-type CRP to its site on the lac promoter and activation of abortive initiation by RNA polymerase on this promoter are effected by cAMP but not by cGMP. CRP598 can activate lacP+-directed abortive initiation in the presence of cAMP and less efficiently in the presence of cGMP or in the absence of cyclic nucleotide. DNase I protection ("foot-printing") indicates that cAMP-CRP binds to its site on the lac promoter whereas unliganded CRP* and cGMP-CRP* form a stable complex with the [32P]lacP+ fragment only in the presence of RNA polymerase, showing cooperative binding of two heterologous proteins. This cooperative binding provides strong evidence for a contact between CRP and RNA polymerase for activation of transcription. Although cGMP binds to CRP, it cannot replace cAMP in effecting the requisite conformational transition necessary for site-specific promoter binding. In contrast, the weakly active unliganded CRP*598 can be shifted to a functional state not only by cAMP but also by cGMP and RNA polymerase.