RNA polymerase active site. A model of the transition state for phosphodiesterbond formation in the active site of RNA polymerase. The 3′-hydroxyl group of the growing RNA chain attacks the α-phosphoryl group of the incoming nucleoside triphosphate, resulting in the release of pyrophosphate. This transition state is structurally similar to that in the active site of DNA polymerase (Figure 28.5).