mRNA processing

5’capping
Polyadenylation
Splicing

1. mRNA Processing Darshan S

2. CONTENTS 1 Introduction 5’capping 2 3 Polyadenylation Splicing 4 Conclusion 5 Reference 6

3. 1 Introduction Many of the RNA molecules in bacteria and virtually all RNA molecules in eukaryotes are processed to some degree after synthesis

4. 1 Introduction mRNAs are synthesized by either cellular or viral enzymes RNA processing- a series of covalent modifications Facilitate recognition of mRNAs by the protein synthesizing machinery Most RNA-processing reactions were discovered in viral systems

5. Principles of Virology, ASM Press 1 Introduction

6. 2 5’Capping m 7 GpppNp

7. 2 5’Capping Principles of Virology, ASM Press

8. 2 5’Capping Protects mRNAs from 5’ exonucleolytic attack Essential for the efficient translation of most mRNAs de novo synthesis by cellular enzymes, synthesis by viral enzymes and acquisition of preformed 5’ cap

9. 2 5’Capping Synthesis by cellular enzymes Pre mRNA are substrates for cellular capping enzymes RNA pol II Cotranscriptional reaction that takes place when nascent RNA is only 20 to 30 nucleotides in length Phosphorylation of paused RNA pol II at specific serines in the C-terminal domain of the largest subunit is the signal for binding of the capping enzyme and capping of nascent RNA

10. 2 5’Capping Principles of Virology, ASM Press

11. 2 5’Capping Acquisition of viral 5’Cap from cellular mRNA 5’ caps of orthomyxoviral and bunyaviral mRNAs are produced by cellular capping enzymes Viral cap dependent endonucleases cleave cellular transcripts to produce the primers needed for viral mRNA synthesis- cap snatching The 5’ terminal segments and caps of influenza virus mRNAs are obtained from cellular pre-mRNA in nucleus Bunyaviral mRNA synthesis is primed with 5’ terminal fragments cleaved from mature cellular mRNAs in cytoplasm

12. 3 Polyadenylation Addition of poly(A) tail to 3’ end

13. 3 Polyadenylation History Principles of Virology, ASM Press

14. 3 Polyadenylation Stability and increases the efficiency of translation 3’ terminal stemloop structures instead poly(A) tail in mRNAs that encode histones and also in case of reoviral and arenaviral mRNAs Carried out by either cellular or viral enzymes

15. 3 Polyadenylation By cellular Enzymes Transcription of a gene proceeds beyond the site at which poly(A) tail is added The 3’ end of the mRNA is determined by endonucleolytic cleavage of its pre-mRNA Poly(A) is then added to the new 3’ terminus Termination of transcription Degradation of RNA downstream to the cleavage site

16. 3 Polyadenylation Principles of Virology, ASM Press Lehninger principles of Biochemistry , WH Freeman

17. 3 Polyadenylation By Viral Enzymes Either posttranscriptional or during viral mRNA synthesis Vaccinia virus- 2 processes with one enzyme Other viruses- during mRNA synthesis Poly(U) sequence present at the 5’ end of the (-) strand RNA template Principles of Virology, ASM Press

18. 4 Splicing Non-coding regions are removed and coding regions are joined together

19. 4 Splicing Discovery Nuclear precursors of mRNAs are larger than mRNAs transported in cytoplasm and heterogenous in size Both ends of hnRNA retained 1993- Phillip Sharp and Richard Roberts Adenoviral late mRNAs

20. 4 Splicing Principles of Virology, ASM Press

21. 4 Splicing 4 classes of introns Group I: Found in some nuclear, mitochondrial and chloroplast genes that code for rRNAs , mRNAs and tRNAs Group II: Found in the primary transcripts of mitochondrial or chloroplast mRNAs in fungi,algae and plants Rare examples of introns found in bacteria

22. 4 Splicing Lehninger principles of Biochemistry , WH Freeman

23. 4 Splicing Group III Introns Largest class Found in nuclear mRNA primary transcripts Spliceosomal introns Splicing by lariat mechanism Spliceosome is made up of specialized RNA-protein complexes: snRNPs

24. 4 Splicing Lehninger principles of Biochemistry , WH Freeman

25. 4 Splicing Lehninger principles of Biochemistry , WH Freeman

26. 4 Splicing Differential RNA processing Lehninger principles of Biochemistry , WH Freeman

27. 4 Splicing Principles of Virology, ASM Press

28. 5 Conclusion

29. 5 Conclusion Eukaryotic mRNAs are modified by addition of a 7-methylguanosine residue at the 5’ end and by cleavage and polyadenylation at 3’ end Introns are removed by splicing Group I introns require guanosine cofactor Some group I and group II introns are capable of self splicing Group III introns are spliced with the aid of RNA-protein complexes called snRNPs Complex transcripts can have either more than one site for cleavage and polyadenylation or both

30. 6 Reference Flint, Racaniello, 2015, Principles of virology, 4 th edition David L Nelson, M Cox, 2013, Lehninger Principles of Biochemistry, sixth edition, W H Freeman and Company

31. THANK YOU!