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 1 5’ 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 reactions 1.Cleavage of the message 2.Addition of poly-A 3.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 Mg 2+ 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’b’ a a w T7-like

Chloroplast RNA polymerase Encoded by chloroplast genome Contains considerable homology to bacterial ,  and  ’ RNA pol subunits No any s -like factors or general transcription factors s b b’b’ a ab b’b’ a a w