The FBP Interacting Repressor Targets TFIIH to Inhibit Activated Transcription

Abstract

FUSE-binding protein (FBP) binds the single-stranded far upstream element of active c-myc genes, possesses potent transcription activation and repression domains, and is necessary for c-myc expression. A novel 60 kDa protein, the FBP interacting repressor (FIR), blocked activator-dependent, but not basal, transcription through TFIIH. Recruited through FBP's nucleic acid-binding domain, FIR formed a ternary complex with FBP and FUSE. FIR repressed a c-myc reporter via the FUSE. The amino terminus of FIR contained an activator-selective repression domain capable of acting in cis or even in trans in vivo and in vitro. The repression domain of FIR targeted only TFIIH's p89/XPB helicase, required at several stages in transcription, but not factors required for promoter selection. Thus, FIR locks TFIIH in an activation-resistant configuration that still supports basal transcription.

Far Upstream Element Binding Protein Plays a Crucial Role in Embryonic Development, Hematopoiesis, and Stabilizing Myc Expression Levels

“…22,23,54,55 FIR then restores TFIIH helicase activity to basal levels, depressing transcription. 7 Falling levels of supercoiling combined with Figure 10 Scheme for FBP operating as a molecular cruise control to help set MYC levels and to constrain fluctuations around the set point. A and B, 1: MYC transcription is initiated by a conventional transcription factor(s) represented by symbols.…”

“…6 Similarly, the FBP/FIR/FUSE system acts through this subunit and is proposed to act as a molecular cruise control to dampen fluctuations in MYC expression. 4,5,7,10 If this proposal has merit, then Myc levels in Fubp1 À/À cells should vary more than in WT cells. The relatively uniform expression of Myc in MEFs made it a good substrate to assess the variation elicited on loss of FBP.…”

“…4e6 Positive regulation of MYC is at least in part counteracted by FBP interacting repressor (FIR) that binds FBP and FUSE and opposes TFIIH helicase activity, retarding the transcription cycle. 5,7 Besides MYC, FBP regulates the expression of the cyclin-dependent kinase inhibitor p21, stathmin, and USP29, a powerful stabilizer of p53 induced by oxidative stress. 8e10 Because the FBP/FIR system reads single-stranded DNA sequences exposed by transcriptionally generated dynamic supercoiling, it is proposed that the FBP/ FIR/FUSE system serves as a molecular cruise control that monitors the mechanical output of promoters in real time while providing both positive and negative feedback.…”

“…7,24 FBP, FBP2, and FBP3 have three, four, and two tyrosine-rich motifs, respectively. 14,24 The amino termini of FBP and FBP2 include an a-helix that interacts with RNA recognition motif 2 domain of FIR, 2,4,7,16,23 whereas an A/V substitution in FBP3 precludes this interaction. 16 A variety of in vitro and tissue culture experiments have associated FBP binding with a complex set of DNA and RNA targets.…”

Myc in model organisms: A view from the flyroom

“…Wg may inhibit dmyc expression by inducing Halfpint (Hfp), a homolog of human FBP interacting repressor (FIR), a pre-mRNA splicing factor also called PUF60 [40]. In human cells, FIR inhibits c-myc transcription and interacts with FBP, a protein that binds an upstream element at the c-myc locus [41]. In the fly, hfp mutant cells have elevated dmyc transcript levels and the ZNC fails to form [40].…”