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Eukaryotic Transcription

Eukaryotic Transcription. Eukaryotic Transcriptional. Transcription control is the most important mode of control in eukaryotes. Three RNA polymerases: RNA Polymerase I: synthesis of pre-rRNA, which is processed into 28S, 5.8S, and 18S rRNAs

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Eukaryotic Transcription

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  1. Eukaryotic Transcription

  2. Eukaryotic Transcriptional • Transcription control is the most important mode of control in eukaryotes. • Three RNA polymerases: • RNA Polymerase I: synthesis of pre-rRNA, which is processed into 28S, 5.8S, and 18S rRNAs • RNA polymerase III: synthesis of tRNA, 18 S rRNA, and small, stable RNAs • RNA polymerase II: synthesis of mRNAs and four small nuclear RNAs that take part in RNA splicing

  3. Eukaryotic Transcriptional • The main purpose is the execution of precise developmental decisions (irreversible). • Cis-acting control elements are located many kb away from the start site. • Promoter region is poorly characterized.

  4. TATA box About 25-35 bp upstream of the start site +1 5’ TATAAA mRNA start -34 to -26 Well-defined transcription start

  5. Initiator • Instead of a TATA box, some eukaryotic gene contain an alternative promoter element, called an initiator. • Initiator is highly degenerative. +1 5’ Y Y A N T/A Y Y Y Y = pyrimidine (C or T) N = any

  6. CpG island • Genes coding for intermediary metabolism are transcribed at low rates, and do not contain a TATA box or initiator. • Most genes of this type contain a CG-rich stretch of 20-50 nt within ~100 bp upstream of the start site region. • A transcription factor called SP1 recognizes these CG-rich region. • Gives multiple alternative mRNA start sites. mRNA ~100 bp CpG island Multiple 5’-start sites

  7. Enhancers • Located several Kb away from the start site. • Usually ~100-200 bp long, containing multiple 8- to 20-bp control elements. • Cell-type specific • Direction-less (invertible) +1 Enhancer Enhancer

  8. Promoter Proximal Elements • Occur within ~200 bp of the start site. • Contain up to ~20 bp. • Cell-type specific • Invertible +1 Promoter Proximal element

  9. A hypothetic mammalian promoter region Promoter Proximal Element +1 Enhancer Intron Enhancer TATA Enhancer -200 -30 -10~-50 Kb +10~50 Kb Exon

  10. A hypothetic yeast promoter region +1 TATA Enhancer ~-90

  11. mRNA processing Poly (A) site Exon Intron Termination transcription Cap 5’ The 5’-Cap is added to Nascent RNAs (pre-RNAs) shortly after initiation by RNA polymerase II. Pre-mRNAs are associated with hnRNP (heterogeneous ribonucleoprotein particles) proteins containing conserved RNA-binding domains

  12. Poly (A) site Exon Intron Termination Cap 5’ Cap endonuclease 5’ Poly(A) polymerase 5’ A100-250 Pre-mRNAs are cleaved at specific 3’ sites and rapidly polyadenylated

  13. Cleavage and polyadenylation Poly(A) signal Poly(A) site 5’ G/U rich AAUAAA Pre-mRNA 5’ AAAAAA100-250 AAUAAA

  14. Poly (A) site Exon Intron Termination Cap 5’ endonuclease 5’ Poly(A) polymerase 5’ A100-250 RNA splicing 5’ A100-250

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