Structure of a transcription elongation complex (TEC) and the TEC nucleotide addition cycle. (Left panel) Proposed TEC structure. Electron microscopy studies of bacterial and yeast RNAPs reveal a groove surrounded by jaw-like protrusions that may clamp around downstream duplex DNA and a tunnel that may function as a channel through which RNA exits from the active site. Chemical experiments show that RNA polymerase (RNAP) in a TEC protects about 17 bp of DNA on either side of the active site position, keeps the upstream half of the DNA melted, and maintains an about 8 bp RNA:DNA hybrid upstream from the active site. RNAP adds nucleotides to nascent RNA in a bipartite active site that coordinates the RNA 3' end and the nucleoside triphosphate alpha phosphate via two Mg2+ ions. (Right panel) States of the TEC active site. The vertical pathway shows a minimal RNA chain elongation cycle; the horizontal pathway depicts additional states of the TEC thought to arise when the RNAP active site (double circle) slides relative to the DNA:RNA heteroduplex. Arrested and hypertranslocated TECs are catalytically inactive because the 3' end is displaced from the active site. The four steps of the chain elongation cycle are all required; however, the order of the last two steps and the relative rates of all four steps are unknown and may vary at different template positions.