1

RNA processing enzymes are recruited by the tail of polymerase

2

Elongation polymerase is associated with a new set of

protein factors required for various types of RNA processing

RNA processing: – Capping of the 5’ end of the RNA– Splicing of the introns (most complicated)– Poly adenylation of the 3’ end

3

Function of poly(A) tail

• Increased mRNA stability• Increased translational efficiency• Splicing of last intron

4

Function of 5´cap

• Protection from degradation• Increased translational efficiency• Transport to cytoplasm• Splicing of first intron

5

RNA processing 15’ end cappingRNA processing 15’ end capping

• The “cap”: a methylated guanine joins to the RNA transcript by a 5’-5’ linkage

• The linkage contains 3 phosphates

• 3 sequential enzymatic reactions

• Occurs early

6

Splicing: joining the protein coding sequences

• Dephosphorylation of Ser5 within the CTD tail leads to dissociation of capping machinery

• Further phosphorylation of Ser2 recruits the splicing machinery

7

3’ end polyadenylation

• Linked with the termination of transcription• The CTD tail is involved in recruiting the

polyadenylation enzymes• The transcribed poly-A signal triggers the

reactions1. Cleavage of the message2. Addition of poly-A3. Termination of transcription

What is Pol II backsliding, pausing and arrest?• Backsliding is an event when RNA polymerase moves

backwards and newly made RNA gets inserted in the funnel. It can be caused by incorporation of wrong NTP or when 3’OH looses contact with active site Mg2+

• If the backsliding RNA piece is 2-4 nt long, this a reversible process and RNA polymerase can recover by itself. This is called pausing.

• If the backsliding RNA gets longer (7-10 nt), it gets trapped in funnel pore and RNA polymerase can not recover by itself. This is called arrest.

• Arrest can be overcome by specific elongation factors, which help RNA Pol II to cleave the arrested RNA

Mitochondrial RNA polymerase• Encoded in nuclear RNA and transported to

mitochondrial matrix• Contains only two subunits• One subunit similar to bacteriophage T7

RNA polymerase• Other subunit similar to bacterial s factor

s-like s

b

b’ aa

wT7-like

Chloroplast RNA polymerase• Encoded by chloroplast genome• Contains considerable homology to

bacterial a, b and b’ RNA pol subunits• No any s-like factors or general

transcription factors

s

b

b’ aab

b’ aa

w