eukaryotic transcription (an overview)

overview of transcription

• DNA double helix locally denatures and one strand acts as a template

• incoming ribonucleoside triphosphates base-pair with bases in the template DNA strand

• RNA polymerase sequentially joins the rNTPs from 5’ to 3’

• polymerization is energetically favoured because the high-energy bond between the α and β phosphates is replaced by a lower-energy phosphodiester bond

+1

start site of transcription

stages of transcription

1. initiation

2. elongation

3. termination

initiation stage of transcription

polymerase binds to the promoter sequence, locally denatures the DNA, and catalyzes the first phosphodiester linkage

elongation stage of transcription

polymerase advances 3’ → 5’ down the template strand, denaturing the DNA and polymerizing the RNA

termination stage of transcription

polymerase recognizes a stop site, releases the completed RNA, and dissociated from DNA

similarities between prokaryotic and eukaryotic transcription

• σ factors confer specificity RNA polymerase

• DNA binding proteins regulate the rate of RNA synthesis by enhancing or impeding RNA polymerase binding to promoter regions

• sequences in the DNA proximal to the transcribed gene are critical for efficient transcription

polycistronic

prokaryotic mRNA, contains multiple transcripts encoding multiple proteins

monocistronic

eukaryotic mRNA, has modified G cap and poly A tail, contains a single translation start site

RNA polymerase I

• transcribes pre-rRNA

• high resistance to α-amanitin

RNA polymerase II

• transcribes mRNA, snRNA, siRNA, miRNA

• intermediate resistance to α-amanitin

RNA polymerase III

• transcribes tRNA, 5S rRNA, snRNA U6, 7S RNA, and other small stable RNAs

• low resistance to α-amanitin

role of clamp domain of RNA polymerase II

closes down on DNA while transcribing, prevents RNA pol II from falling off DNA transcript

common features between eukaryotic RNA polymerases

• all exist in multimeric complexes

• some show significant similarity to bacterial subunits

• most are essential

carboxy-terminal domain (CTD) on large subunit of RNA polymerase II

• near exit site of RNA

• heptapeptide repeats

• unstructured

• phosphorylated during transcription (ex: polytene chromosomes)