THE ACTIVE-SITE OF RNA-POLYMERASE-II PARTICIPATES IN TRANSCRIPT CLEAVAGE WITHIN ARRESTED TERNARY COMPLEXES

RNA polymerase II may become arrested during transcript elongation, in which case the ternary complex remains intact but further RNA synthesis is blocked. To relieve arrest, the nascent transcript must be cleaved from the 3' end. RNAs of 7-17 nt are liberated and transcription continues from the newly exposed 3' end, Factor SII increases elongation efficiency by strongly stimulating the transcript cleavage reaction. We show here that arrest relief can also occur by the addition of pyrophosphate. This generates the same set of cleavage products as fatter SII, but the fragments produced with pyrophosphate have 5'-triphosphate termini. Thus, the active site of RNA polymerase II, in the presence of pyrophosphate, appears to be capable of cleaving phosphodiester linkages as far as 17 nt upstream of the original site of polymerization, leaving the ternary complex intact and transcriptionally active.