General Transcription Factors

Eukaryotic transcription initiation requires six general transcription factors: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH. These proteins bind sequentially to gene promoter regions alongside RNA polymerase II to regulate gene expression. Each factor plays a distinct role in assembling the transcription pre-initiation complex at the promoter.

TFIID is a multiprotein complex containing the TATA-binding protein (TBP) subunit, which recognizes and binds to the TATA box region found in most promoters. This binding initiates the recruitment of other general transcription factors. TFIID serves as the foundational anchor for assembling the transcription machinery at the promoter.

TFIIB positions the preformed polymerase-TFIIF complex at the transcription start site after attaching to the TFIID-DNA complex. This positioning is critical for proper initiation of RNA synthesis. TFIIB essentially acts as a bridge between the general transcription factors and RNA polymerase II.

TFIIH is recruited to the promoter after TFIIE tethers to the DNA. The DNA helicase subunit of TFIIH unwinds the double-stranded DNA, exposing the template strand for RNA synthesis. This unwinding is essential for forming the transcription pre-initiation complex and enabling polymerase to access the template.

Tissue-specific transcription factors are only expressed in particular cell types, restricting which genes can be activated in those cells. For example, beta globin is present in all cells but only expressed in red blood cells because the transcription factors binding its promoter are exclusive to those cells. This ensures genes are expressed only where needed.

Combinatorial control occurs when multiple transcription factors simultaneously regulate a single gene's expression. The combination of several transcriptional activators and repressors enables differential regulation and adaptation to environmental changes without requiring additional genes. This system provides precise, flexible control over gene expression.

TFIIA stabilizes the TFIID-DNA complex after binding, preventing premature dissociation. TFIIF facilitates and stabilizes the interaction of RNA polymerase II with other transcription factors. Together, these factors ensure the transcription pre-initiation complex remains stable and properly assembled at the promoter.