Processing of precursor mRNA Processing of precursor rRNA Processing of precursor tRNA RNA editing

1. Post transcriptional
modifications
Dr. Roshan Kumar Mahat
(PhD Medical Biochemistry)
Assistant Professor
Department of Biochemistry

2. Objectives:
1. Processing of precursor mRNA
2. Processing of precursor rRNA
3. Processing of precursor tRNA
4. RNA editing

3. Processing of precursor mRNA
Processing of mRNA from hnRNA
includes:
1. Capping (5’cap)
2. Tailing (Polyadenylation)
3. Removal of extra RNA at 3’end
4. Splicing

4. Figure: Formation of the primary transcript and its processing during maturation of
mRNA in a eukaryotic cell.

5. The 5’ cap of mRNA

6. Polyadenylation at 3’ end of mRNA

7. Splicing
Splicing is the process of removal of
introns and joining of exons to form
functional mRNA.
Classes of introns (4 classes):
Group I
Group II
mRNA introns –spliceosome mediated
splicing
tRNA introns- requires endonuclease
Self Splicing

8. Splicing of group I introns
1 2
1 2
21
Step-1
Step-2
Step-3

9. SplicingofgroupIIintrons
1 2
1
1 2
2
Step-1
Step-2
Step-3

10. RNA pairing interactions in the formation of spliceosome complexes. The U1
snRNA has a sequence near its 5’ end that is complementary to the splice site
at the 5’ end of the intron. Base pairing of U1 to this region of the primary
transcript helps define the 5’ splice site during spliceosome assembly. U2 is
paired to the intron at a position encompassing the A residue (shaded light red)
that becomes the nucleophile during the splicing reaction.
Spliceosome mediated splicing
5’ splice site 3’ splice siteBranch site

11. Figure:Assemblyofspliceosomes.TheU1andU2snRNPsbind,
thentheremainingsnRNPs(theU4-U6complexandU5)bindto
formaninactivespliceosome.Internalrearrangementsconvertthis
speciestoanactivespliceosomeinwhichU1andU4havebeen
expelledandU6ispairedwithboththe59splicesiteandU2.
Thisisfollowedbythecatalyticsteps,whichparallelthoseofthe
splicingofgroupIIintrons
Step-1
Step-2
Step-3
Step-4
Step-5
1
1
1
1
1
1
2
2
2
2
2
2

12. FIGURE: Overview of
the processing of a
eukaryotic mRNA.

13. Alternative splicing:
• Alternative splicing allows synthesis
of range of functionally distinct
proteins from the primary transcript of
a single gene.
• Alternative splicing occurs by altering
the patterns of exons from a single
primary transcript.

14. Mechanisms of alternative splicing:

18. Disorders of splicing:
1. Beta thalassemia:
The G-T sequence is mutated to A-T
sequence resulting in defective RNA
splicing. The defective β-globin mRNA
can not be utilized for translation.
2. Systemic lupus erythematosus:
Autoantibodies raised against U1 RNA
of spliceosome.

19. Processing of precursor rRNA
Posttranscriptional processing is not
limited to mRNA.
Ribosomal RNAs of bacterial,
archaeal, and eukaryotic cells are
made from longer precursors called
preribosomal RNAs, or pre-rRNAs.

20. Figure: Processing of pre-rRNA transcripts in bacteria.

21. Figure:Processingofpre-rRNA
transcriptsineukaryotes.

22. Figure: rRNA and
ribosome synthesis

23. Processing of precursor tRNA
The four different events of post-
transcriptional processing of
pre-tRNA molecules are as
follows:
1. Removal of extra nucleotides
from 5’ and 3’ ends
2. removal of intron from anticodon
site
3. Modification of bases
4. Addition of CCA sequence

24. Figure: post transcriptional processing of tRNA
RNAse P
RNAse D
Endonucleases
RNA ligase
1
2
3
4
5
Nucleotidyl transferase

25. Fig: Overview of tRNA
synthesis and its processing

26. RNA editing:
Figure: RNA editing of apo-B pre mRNA