ELONGATION-FACTOR SII CONTACTS THE 3'-END OF RNA IN THE RNA-POLYMERASE-II ELONGATION COMPLEX

Elongation factor SII (also known as TFIIS) is an RNA polymerase II bi nding protein that allows bypass of template arrest sites by activatin g a nascent RNA cleavage reaction. Here we show that SII contacts the 3'-end of nascent RNA within an RNA polymerase II elongation complex a s detected by photoaffinity labeling, Photocross-linking was dependent upon the presence of SII, incorporation of 4-thio-UMP into RNA, and i rradiation and was sensitive to treatment by RNase and proteinase. A t ranscriptionally active mutant of SII lacking the first 130 amino acid s was also cross-linked to the nascent RNA, but SII from Saccharomyces cerevisiae, which is inactive in concert with mammalian RNA polymeras e II, failed to become photoaffinity labeled, SII-RNA contact was not detected after a labeled oligoribonucleotide was released from the com plex by nascent RNA cleavage, demonstrating that this interaction take s place between elongation complex-associated but not free RNA. This s hows that the 3'-end of RNA is near the SII binding site on RNA polyme rase II and suggests that SII may activate the intrinsic RNA hydrolysi s activity by positioning the transcript in the enzyme's active site.