Gene activator protein. Computer artwork of catabolite gene activator protein (CAP, blue and red) bound to a molecule of deoxyribonucleic acid (DNA). CAP is a dimer, a protein made up of 2 identical subunits (red and blue). It 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 (black) either side of the centre 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.