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CAP is the Catabolite Activator Protein which leads to positive control of lac operon. CAP binds to cAMP and forms active CAP-cAMP complex. This complex binds to a site known as CAP binding site which is upstream to a promoter region and causes a bend in DNA. This bending favors the interaction between CAP and RNA polymerase leading to transcription.

This positive control helps in keeping the lac operon β€œON” only in presence of lactose and not in presence of any other sugar molecules. CAP is also known as CRP (cyclic AMP Repressor Protein) and cga (Catabolite gene activator).