RNA polymerase

Transcription machinery:

RNA polymerase

• Transcription is the synthesis of RNA under the direction of DNA

• Mediated by RNA polymerase (RNAPs)

• Polymerisation (extension of the RNA strand) proceeds in a 5’-3’ direction and uses the non-coding strand (3’-5’) as the template

Eukaryotic RNA polymerases

• Have three RNA polymerases which are responsible for transcribing different nuclear genes

• Multisubunit enzymes = 10 or more subunits

• RNA polymerase I = transcribes rRNA genes (in the nucleolus)

• RNA polymerase II = transcribes protein-encoding genes

• RNA polymerase III = transcribes tRNA genes

• Different RNAPs recognise different promoters

• Additional RNAPs for transcription of organelle genome

Archaeal RNA polymerases

• One RNAP consisting of 11-12 subunits

• Most similar to eukaryotic RNAP II

• Recognises promoters that are similar to those recognised by eukaryotic RNAP II

• Two transcription factors recognise conserved sequences in promoter and recruit RNAP

• Similar mechanism to transcription initiation in eukaryotes (except that this involves more transcription factors)

Bacterial RNA polymerase

• Five subunits: α2ββ’ω (the core enzyme)

• Catalytic components

• Beta (β) subunit = ribonucleoside triphosphate binding site

• Beta prime (β’) subunit = DNA binding abilities • Alpha (α) subunits = required for assembly of the core enzyme

• Omega (ω) subunits = function not well understood but involved in maintaining the core enzyme • Sigma (σ) factor = associates with the core enzyme to form the holoenzyme (α2ββ’ωσ) and guides the complex to specific promoters to initiate transcription • Recognise the -35 and -10 (Pribnow box) sequences

Bacterial promoters and sigma factors

• Specific sigma factors recognise specific promoters allowing regulation of gene expression under particular conditions

• E.g. σ32 guides the RNA polymerase specifically to promoters that control the expression of genes involved in the heat shock response