The cAMP receptor protein (CRP) in the presence of cAMP can modulate the transcription initiation of many operons of E. coli. The protein is inactive in the absence of cAMP; when cAMP is present, the cAMP binds to CRP, causing a conformational change to an active form. We are studying how cAMP causes the conformational change in an effort to determine how a transcriptional regulatory factor can itself be regulated and to determine how an allosteric change in a protein can be accomplished. We have isolated, using mutagen-induced as well as site-specific mutagenesis, several classes of mutations within the crp gene that encodes CRP: crp* mutations that allow CRP to function in the absence of cAMP, crp*-intragenic suppressor mutations that force a crp* mutant to require cAMP, and crp** mutations that have even more cAMP independence than crp* mutants. Our current model for how cAMP induces the allosteric change in CRP is that cAMP binding alters the relative orientation of specific amino acids that are involved in subunit- subunit alignment, domain-domain alignment, and positioning of the DNA-binding F alpha-helices. Based on the locations within the CRP molecule of the substituted amino acids that cause the change, we have identified regions of CRP that are involved in the cAMP-induced conformational change.
