MECHANISM OF ACTION OF RNA-POLYMERASE-II ELONGATION-FACTOR ELONGIN - MAXIMAL STIMULATION OF ELONGATION REQUIRES CONVERSION OF THE EARLY ELONGATION COMPLEX TO AN ELONGIN-ACTIVABLE FORM

We previously identified and purified Elongin by its ability to stimul ate the rate of elongation by RNA polymerase II in vitro (Bradsher, J. N., Jackson, K. W., Conaway, R. C., and Conaway, J. W. (1993) J. Biol . Chem. 268, 25587-25593), in this report, we present evidence that st imulation of elongation by Elongin requires that the early RNA polymer ase II elongation complex undergoes conversion to an Elongin-activable form. We observe (i) that Elongin does not detectably stimulate the r ate of promoter-specific transcription initiation by the fully assembl ed preinitiation complex and (ii) that early RNA polymerase II elongat ion intermediates first become susceptible to stimulation by Elongin a fter synthesizing 8-9-nucleotide-long transcripts. Furthermore, we sho w that the relative inability of Elongin to stimulate elongation by ea rly elongation intermediates correlates not with the lengths of their associated transcripts but, instead, with the presence of transcriptio n factor IIF (TFIIF) in transcription reactions. By exploiting adenovi rus 2 major late promoter derivatives that contain premelted transcrip tional start sites and do not require TFIIF, TFIIE, or TFIIH for trans cription initiation, rye observe (i) that Elongin is capable of strong ly stimulating the rate of synthesis of trinucleotide transcripts by a subcomplex of RNA polymerase II, TBP, and TFIIB and (ii) that the abi lity of Elongin to stimulate synthesis of these short transcripts is s ubstantially reduced by addition of TFIIF to transcription reactions. Here we present these findings, which are consistent with the model th at maximal stimulation of elongation by Elongin requires that early el ongation intermediates undergo a structural transition that includes l oss of TFIIF.