Gene activator protein. Molecular model of catabolite gene activator protein (CAP, yellow) complexed with deoxyribonucleic acid (DNA, red and blue) and RNA polymerase (green and pink). CAP activates genes that enable bacteria to use an alternative energy source when glucose, the preferred energy source, is unavailable. Falling levels of glucose cause an increase in the messenger molecule cAMP, which binds to CAP enabling CAP to bind to DNA. cAMP binds at 2 sites either side of the center of the CAP molecule. CAP binds to DNA at specific sites, causing it to bend. This enhances the ability of the enzyme RNA polymerase to make mRNA copies of the targeted gene.