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 synthes is is blocked. To relieve arrest, the nascent transcript must be cleav ed from the 3' end. RNAs of 7-17 nt are liberated and transcription co ntinues 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 pyro phosphate. This generates the same set of cleavage products as fatter SII, but the fragments produced with pyrophosphate have 5'-triphosphat e termini. Thus, the active site of RNA polymerase II, in the presence of pyrophosphate, appears to be capable of cleaving phosphodiester li nkages as far as 17 nt upstream of the original site of polymerization , leaving the ternary complex intact and transcriptionally active.