1. Lecture 19 RNA Processing

2. General features of RNA processing (AAUAAA) Transcription (heterogeneous nuclear RNA) Capping, methylation, poly A addition Splicing, transport Processing

3. Pre-mRNA is capped shortly after the initiation of transcription.

4. Methylation of the first two bases may also occur

5. Polyadenylation of 3 ends Transcription and RNA capping RNA Polymerase II Poly (A) signal 3’ cleavage at specific site ~10-30 nt downstream of poly (A) addition site Cleavage and polyadenylation Poly (A) polymerase adds 100-200 adenylyl residues to 3’ end cleaved RNA

6. RNA Editing LDL receptor binding Lipoprotein assembly CAA ApoB-100 (found in LDL) RNA editing by deamination of C UAA Stop ApoB-48 (found in chylomicrons)

7. Introns: Must be removed before translation

8. No high energy cofactor is required for splicing of Group I and II introns Types of introns • Group I • A few rRNAs Self-Splicing introns • Group II • Mitochondrial and chloroplast mRNAs • tRNA • Some tRNAs • Requires ATP • Others mRNA • Largest class • Nuclear mRNAs • Requires ‘spliceosome’ and ATP

9. Splicing occurs via a transesterification reaction

10. Group I Intron Self-Splicing Group II Introns

11. Ribozymes have enzymatic properties • Saturable • Binding of guanosine cofactor • Specificity • Catalyzes reactions at only certain bases • Internal guide sequence • Can catalyze multiple reactions • Usually does not happen because it is a self splicing mechanism

12. Splicesomes mRNA processing is mediated by snRNPs(small nuclear ribonucleoproteins) or snurps

13. Splicesome assembly:

14. Splicesome action • The catalytic center of the spliceosome is formed by U2 and U6 snRNAs which are base paired. • Base pairing of U2 snRNA with the branch point is thought to displace and activate the adenosine whose 2’ OH attacks the 5’ splice site

15. Alternative processing can result in different polypeptides