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 “ON” 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).