also known as CRP, catabolite regulator protein (or cyclic AMP receptor protein). The protein mediates the action of cyclic AMP (cAMP) on transcription in that cAMP and CAP must first combine. The cAMP-CAP complex then binds to the promoter regions of Escherichia coli and stimulates transcription of its operon. Since a cell component increases rather than inhibits transcription, this type of regulation of gene expression is called positive transcriptional control.