The mechanism of transcription termination by RNA polymerase I

Eukaryotic ribosomal gene transcription units are bordered at their 3' ends by short DNA sequences which specify site-specific termination by RNA polymerase I (polI). PolI terminators from yeast through to mammals appear to follow similar rules: they contain a site for a sequence-specific DNA-binding protein; they function only in one orientation; 3' ends are formed upstream of the binding site; and 5' flanking sequences influence the position and efficiency of 3' end formation. Recent progress in understanding the mechanism of RNA chain elongation by other polymerases suggests a model for polI termination in which termination is seen as one of the several outcomes possible when a polymerase encounters a pause site.