Structure-function analysis of RNA polymerases I and III.

Recent advances in elucidating the structure of yeast Pol I and III are based on a combination of X-ray crystal analysis, electron microscopy and homology modelling. They allow a better comparison of the three eukaryotic nuclear RNA polymerases, underscoring the most obvious difference existing between the three enzymes, which lies in the existence of additional Pol-I-specific and Pol-III-specific subunits. Their location on the cognate RNA polymerases is now fairly well known, suggesting precise hypotheses as to their function in transcription during initiation, elongation, termination and/or reinitiation. Unexpectedly, even though Pol I and III, but not Pol II, have an intrinsic RNA cleavage activity, it was found that TFIIS Pol II cleavage stimulation factor also played a general role in Pol III transcription.