 Transcription elongation complex

A transcription elongation complex is the working assembly of RNA polymerase, its DNA template and the growing RNA during transcript extension. The RNA $3^{\prime }$ end is positioned for addition of the next nucleotide; a short DNA-RNA hybrid helps organize the nucleic acids inside the enzyme. Additional proteins can associate with this core assembly.

A 2001 RNA polymerase II crystal structure at $3.3\text{A˚}$ resolution showed entering duplex DNA, unwinding before the active site, and a nine-base pair DNA-RNA hybrid. Compared with free RNA polymerase II, the protein clamp closed around the nucleic acids. These observations connected a structurally identifiable assembly with ongoing transcription.^[ref]

For a sequence example, a template segment read $3^{\prime }$-TAC-$5^{\prime }$ specifies the complementary RNA segment $5^{\prime }$-AUG-$3^{\prime }$. Each addition extends the RNA while the complex advances relative to the template. This illustrates information flow; the three-letter example does not specify the complete set of contacts that stabilizes a real transcription elongation complex.