Averell L. Gnatt studied the protein structure of RNA polymerase II, the enzyme that carries out much of eukaryotic transcription. In 2001, he and colleagues determined an X-ray crystallography structure of an actively transcribing complex at a spatial detail of . The complex contained both the DNA template and the growing RNA, allowing their paths through the enzyme to be followed.[ref]
Incoming paired DNA entered a cleft and separated before the active site. A nine-base-pair DNA-RNA hybrid held the newly copied region, while the growing RNA end lay at the site where the next nucleotide would be added. Thus only a short local stretch of the product remained paired to its template: the rest of the RNA could leave as copying continued. Comparison with the free enzyme also showed closure of a protein clamp over the bound DNA and RNA, connecting a change in protein structure with stable engagement during transcription.[ref]
Structural Basis of Transcription: An RNA Polymerase II Elongation Complex at 3.3 Å Resolution by Averell L. Gnatt and others. Abstract; nine-base-pair hybrid, RNA exit groove and clamp closure
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