Regulation of the gal operon. (a) Structure of the gal operon. (b) The Gal repressor does not prevent RNA polymerase from binding the promoter; rather, it prevents formation of the open promoter-polymerase complex that is required for transcription initiation. (c) Similar to regulation of the lac and ara operons, transcription of the gal operon is increased only when glucose is absent, and thus CRP-cAMP binding is required.