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 b y RNA polymerase I (polI). Poll terminators from yeast through to mamm als 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 seque nces influence the position and efficiency of 3' end formation. Recent progress in understanding the mechanism of RNA chain elongation by ot her polymerases suggests a model for polI termination in which termina tion is seen as one of the several outcomes possible when a polymerase encounters a pause site.