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Structure-Function Analysis of the Human TFIIB-Related Factor II Protein Reveals an Essential Role for the C-Terminal Domain in RNA Polymerase III Transcription

Ashish Saxena,^1,2 Beicong Ma,³ Laura Schramm,^2, and Nouria Hernandez^2,3*

Genetics Program, Stony Brook University, Stony Brook, New York 11794,¹ Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724,² Howard Hughes Medical Institute, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724³

Received 3 June 2005/ Returned for modification 26 July 2005/ Accepted 4 August 2005

The transcription factors TFIIB, Brf1, and Brf2 share related N-terminal zinc ribbon and core domains. TFIIB bridges RNA polymerase II (Pol II) with the promoter-bound preinitiation complex, whereas Brf1 and Brf2 are involved, as part of activities also containing TBP and Bdp1 and referred to here as Brf1-TFIIIB and Brf2-TFIIIB, in the recruitment of Pol III. Brf1-TFIIIB recruits Pol III to type 1 and 2 promoters and Brf2-TFIIIB to type 3 promoters such as the human U6 promoter. Brf1 and Brf2 both have a C-terminal extension absent in TFIIB, but their C-terminal extensions are unrelated. In yeast Brf1, the C-terminal extension interacts with the TBP/TATA box complex and contributes to the recruitment of Bdp1. Here we have tested truncated Brf2, as well as Brf2/TFIIB chimeric proteins for U6 transcription and for assembly of U6 preinitiation complexes. Our results characterize functions of various human Brf2 domains and reveal that the C-terminal domain is required for efficient association of the protein with U6 promoter-bound TBP and SNAP_c, a type 3 promoter-specific transcription factor, and for efficient recruitment of Bdp1. This in turn suggests that the C-terminal extensions in Brf1 and Brf2 are crucial to specific recruitment of Pol III over Pol II.

Present address: Department of Biological Sciences, St. John's University, Jamaica, NY 11439.

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