Eukaryotic Transcription Overview

Overview

This section explains the steps, enzymes, and regulatory elements involved in transcription in eukaryotic cells, highlighting key differences from prokaryotic transcription.

Eukaryotic vs. Prokaryotic Transcription

• Eukaryotic transcription occurs in the nucleus and requires transporting and protecting mRNA before translation.
• Eukaryotes use three distinct RNA polymerases, while prokaryotes use one.
• Eukaryotic mRNAs are typically monogenic, encoding a single protein.

RNA Polymerases in Eukaryotes

• RNA polymerase I (in nucleolus) transcribes all rRNAs except 5S rRNA.
• RNA polymerase II (in nucleus) transcribes all protein-coding nuclear pre-mRNAs.
• RNA polymerase III (in nucleus) transcribes 5S rRNA, tRNAs, and small nuclear RNAs.
• Polymerase sensitivity to -amanitin toxin aids in identifying which polymerase transcribes a gene.

Initiation: Promoters and Transcription Factors

• Eukaryotic RNA polymerases need transcription factors to bind promoter regions before transcription can start.
• The TATA box (TATAAA) is a conserved promoter sequence at -25 to -35 bases from the transcription start site.
• Basal transcription factors (TFIIs) assemble at the promoter to recruit RNA polymerase II.
• TFIID, containing TATA-binding protein (TBP), is central in assembling the transcription initiation complex.
• Other promoter elements (CAAT box, GC-rich boxes, octamer boxes) enhance transcription initiation efficiency.

Regulation and Complexity

• Additional transcription factors bind enhancers/silencers to regulate how often a gene is transcribed.
• Promoters for RNA polymerases I and III differ in sequence and complexity from those for polymerase II.

Elongation and Termination

• Polymerase is released from the transcription factors after initiation, proceeding to elongate pre-mRNA in the 5' to 3' direction.
• DNA is compacted in nucleosomes; the FACT complex moves histones aside allowing transcription machinery access.
• RNA polymerase II synthesizes beyond the gene's end; this extra sequence is removed during processing.
• RNA polymerase I uses a specific sequence for termination, while III uses hairpin structures similar to prokaryotes.

Evolution of Promoter Sequences

• Promoters can evolve rapidly and contribute to gene expression changes independently of coding region mutations.
• Promoters may occur within, upstream, or even downstream of the genes they regulate.

Key Terms & Definitions

• Transcription Factor — protein helping RNA polymerase bind to DNA promoter regions.
• Promoter — DNA sequence where transcription machinery assembles to start RNA synthesis.
• TATA Box — conserved DNA sequence essential for eukaryotic transcription initiation.
• Basal Transcription Factors — general factors required for RNA polymerase II to initiate transcription.
• FACT Complex — protein complex that modifies chromatin structure for transcription access.
• Monogenic mRNA — mRNA that encodes only one protein product.
• Enhancer/Silencer — DNA regions that increase or decrease transcription frequency.
• -Amanitin — toxin used to distinguish RNA polymerase activity by their sensitivity.
• Nucleosome — DNA segment wrapped around histone proteins, forming chromatin structure.

Action Items / Next Steps

• Review the table summarizing locations, products, and sensitivities of the three RNA polymerases.
• Study the functions of transcription factors and promoter elements (TATA, CAAT, GC-rich, octamer boxes).
• Learn the differences in transcription initiation, elongation, and termination among the three eukaryotic polymerases.