RESUMO
Gdown1, the substoichiometric 13th subunit of RNA polymerase II (pol II), has an important role in pausing during the initial stage of transcript elongation. However, Gdown1 quantitatively displaces the essential initiation factor TFIIF from free pol II and elongating pol II. Thus, it is not clear how or even if pol II can initiate in the presence of Gdown1. Using an in vitro transcription system with purified factors and pol II lacking Gdown1, we found that although Gdown1 is strongly inhibitory to transcription when prebound to pol II, a fraction of complexes do remain active. Surprisingly, when Gdown1 is added to complete preinitiation complexes (PICs), it does not inhibit initiation or functionally associate with the PICs. Gdown1 does associate with pol II during the early stage of transcript elongation but this association is competitive with TFIIF. By phosphorylating TFIIF, PICs can be assembled that do not retain TFIIF. Gdown1 also fails to functionally associate with these TFIIF-less PICs, but once polymerase enters transcript elongation, complexes lacking TFIIF quantitatively bind Gdown1. Our results provide a partial resolution of the paradox of the competition between Gdown1 and TFIIF for association with pol II. Although Gdown1 completely displaces TFIIF from free pol II and elongation complexes, Gdown1 does not functionally associate with the PIC. Gdown1 can enter the transcription complex immediately after initiation. Modification of TFIIF provides one pathway through which efficient Gdown1 loading can occur early in elongation, allowing downstream pausing to be regulated.