RNA Polymerase and Promoters

Transcription of Eukaryotic Genes

• Eukaryotic transcription is more complicated than prokaryotic transcription
• Three separate polymerases are involved in eukaryotic transcription: RNA polymerase I, II, and III
• Transcription factors (TFs) bind to specific DNA sequences to initiate transcription
• Chromatin structure affects transcription; actively transcribed genes are in euchromatin, while inactive genes are in heterochromatin

RNA Polymerases

• RNA polymerase I synthesizes precursor rRNA in the nucleolus
• RNA polymerase II synthesizes nuclear precursors of mRNA, snoRNA, snRNA, and miRNA
• RNA polymerase III synthesizes tRNA, 5S rRNA, and some snRNA and snoRNA

Promoters for RNA Polymerase II

• The TATA or Hogness box is a consensus sequence found in some genes transcribed by RNA pol II
• The CAAT box is another consensus sequence found in some genes
• Enhancers are cis-acting DNA sequences that increase the rate of transcription initiation
• RNA polymerase II is inhibited by α-amanitin

Posttranscriptional Modification of RNA

• Primary transcripts of eukaryotic RNA and rRNA are modified by cleavage
• Eukaryotic mRNA is extensively modified co- and post-transcriptionally

Posttranscriptional Modification of mRNA

• The primary transcripts undergo extensive co- and posttranscriptional modification in the nucleus
• Modifications include:
  • Addition of 5' cap
  • Addition of poly-A tail
  • Removal of introns

Mechanism of Splicing

• Uracil-rich small nuclear RNAs (snRNA) form small nuclear ribonucleoprotein particles (snRNPs) that mediate splicing
• snRNPs facilitate removal of introns by forming base pairs with consensus sequences at each end of the introns
• The binding of snRNPs brings the sequences of neighboring exons into correct alignment for splicing
• The 2'-OH group of an adenosine residue attacks the phosphate at the 5'-end of the intron, forming an unusual 2'→5' phosphodiester bond and creating a "lariat" structure