Eukaryotic Transcription Overview

Overview

This lecture covers the steps of eukaryotic transcription, emphasizing the roles of three RNA polymerases, transcription factors, and differences in promoter structures and regulation.

Eukaryotic vs. Prokaryotic Transcription

• Eukaryotic transcription occurs in a nucleus and uses three RNA polymerases, unlike prokaryotes.
• Eukaryotic mRNA is monogenic and must be transported to the cytoplasm and protected from degradation.

Steps in Eukaryotic Transcription

• Initiation starts with transcription factors binding to the promoter, then recruiting RNA polymerase.
• Elongation proceeds as RNA polymerase synthesizes pre-mRNA in the 5' to 3' direction.
• Termination mechanisms differ: RNA polymerase II transcribes past gene end; I and III require specific termination signals.

The Three RNA Polymerases

• RNA polymerase I (nucleolus): transcribes most rRNAs except 5S, insensitive to α-amanitin.
• RNA polymerase II (nucleus): transcribes all protein-coding pre-mRNAs, extremely sensitive to α-amanitin.
• RNA polymerase III (nucleus): transcribes 5S rRNA, tRNAs, and small nuclear RNAs, moderately sensitive to α-amanitin.

Promoters and Transcription Factors

• Eukaryotic promoters include the TATA box (TATAAA) at -25 to -35 bases, similar to prokaryotic -10 box.
• Additional elements like the CAAT box and GC/octamer boxes can enhance transcription.
• Basal transcription factors (TFII proteins) form a preinitiation complex and recruit RNA polymerase II.
• Enhancers and silencers regulate transcription frequency but are not required for transcription to occur.

Chromatin and Transcription Elongation

• Eukaryotic DNA is wrapped around histones in nucleosomes, forming chromatin.
• FACT complex (facilitates chromatin transcription) moves histones to allow polymerase passage and restores them after.

Evolution of Promoters

• Promoter sequences can evolve faster than protein-coding genes, affecting gene expression efficiency.

Key Terms & Definitions

• Transcription factor — protein that binds DNA and helps recruit RNA polymerase.
• RNA polymerase I/II/III — enzymes that transcribe rRNA, mRNA, or tRNA/small RNAs respectively.
• TATA box — common DNA promoter sequence (TATAAA) recognized by transcription factors.
• Basal transcription factors — proteins required to assemble the preinitiation complex for RNA polymerase II.
• FACT complex — protein complex that facilitates transcription through chromatin by moving histones.
• Enhancer/Silencer — DNA elements that increase or decrease transcription efficiency.

Action Items / Next Steps

• Review the roles of each RNA polymerase and the types of genes they transcribe.
• Study the structure and function of promoter elements (TATA, CAAT, GC boxes).
• Prepare to compare prokaryotic and eukaryotic transcription mechanisms for exams.