TFIIB

Transcription factor IIB (TFIIB) is a general transcription factor required for RNA polymerase II transcription initiation in eukaryotes and archaea. In human cells, the protein is encoded by the GTF2B gene. TFIIB participates in the assembly of the transcription preinitiation complex by bridging promoter DNA, via TBP, to RNA polymerase II and other general factors, thereby facilitating promoter recognition and start-site selection.

Structure: The protein consists of an N-terminal zinc ribbon motif that coordinates a zinc ion, followed by a flexible linker and a C-terminal core composed of two cyclin-like domains. The N-terminal region interacts with promoter-bound TBP and DNA and helps position Pol II. The C-terminal core engages RNA Pol II and TBP; a structural element called the B-finger protrudes into the Pol II active site and contributes to start-site selection and promoter clearance.

Function: TFIIB binds to the TBP-DNA complex at the promoter and helps recruit Pol II to form the PIC. It stabilizes promoter DNA around the transcription start region, influences the choice of start site, and modulates promoter escape after nucleotide incorporation. TFIIB also coordinates with other general transcription factors such as TFIIA, TFIIE, TFIIH, and TFIIF during PIC assembly and transition to elongation.

Evolution and homologs: In archaea, a homologous factor called TFB performs a similar role in the archaeal transcription system, reflecting a shared ancestry with eukaryotic TFIIB. In yeast, SUA7 encodes functional TFIIB.

Regulation and significance: TFIIB is essential for viability in many organisms. It can be regulated by post-translational modifications and interactions with other transcriptional regulators. Mutations or dysregulation affecting TFIIB function can alter transcriptional initiation, with broad effects on gene expression.