Eukaryotic Transcription Eukaryotic Transcriptional Transcription control is the

Eukaryotic Transcription

Eukaryotic Transcriptional • Transcription control is the most important mode of control in eukaryotes. • Three RNA polymerases: – RNA Polymerase I: synthesis of pre-r. RNA, which is processed into 28 S, 5. 8 S, and 18 S r. RNAs – RNA polymerase III: synthesis of t. RNA, 18 S r. RNA, and small, stable RNAs – RNA polymerase II: synthesis of m. RNAs and four small nuclear RNAs that take part in RNA splicing

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.

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

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 = pyrimidine (C or T) N = any

Cp. G 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 2050 nt within ~100 bp upstream of the start site region. • A transcription factor called SP 1 recognizes these CGrich region. • Gives multiple alternative m. RNA start sites. ~100 bp Cp. G island m. RNA Multiple 5’-start sites

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

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

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

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

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

Exon Poly (A) site Intron Termination Cap 5’ 5’ endonuclease Poly(A) polymerase A 100 -250 Pre-m. RNAs are cleaved at specific 3’ sites and rapidly polyadenylated

Cleavage and polyadenylation Poly(A) signal Poly(A) site 5’ AAUAAA G/U rich Pre-m. RNA 5’ AAUAAA AAAAAA 100 -250

Exon Poly (A) site Intron Termination Cap 5’ endonuclease 5’ Poly(A) polymerase A 100 -250 5’ RNA splicing 5’ A 100 -250