RNA processing 5’ capping

1. RNA processing
5’ capping
Dr. Emasushan Minj
Assistant Professor
Department of Botany

2. RNA processing
• In eukaryotes, transcription and translation take place in different
cellular compartments like nucleus and cytoplasm respectively.
• In prokaryotes, transcription of mRNA and translation occur
simultaneously. Processing of mRNA undergo little or no
modification.
• In contrast, pre-tRNA and pre-rRNA undergo processing like
cleavage, addition of nucleotides and chemical modification after
synthesis.
• Processing of eukaryotic pre-mRNA involves 5’ capping, 3’
cleavage/polyadenylation, splicing and RNA editing before being
transported to the cytoplasm, where they are translated by
ribosomes.

3. 5’ capping
• Eukaryotic mRNA has peculiar enzymatically appended cap structure, which
consisting of 7-methylguanosine reside joined through 5’-5’ triphosphate
bridge.
• During transcription 7-methylguanosine is added to 5’ end of nascent mRNA.
Capping initiated by dimeric enzyme associated with phosphorylated
carboxyl-terminal tail domain (CTD) of RNA polymerase II.
• One subunit of capping enzyme removes γ-phosphate, another subunit
transfer the GMP of GTP which creates 5’-5’ triphosphate structure.

4. • And final separation of enzyme
transfers methyl groups from s-
adenosylmethionine to the N7
position of the guanine at 5’ end
of RNA.
• If methyl group is present at N7
position called as cap0. This is
the first methylation step and
occurs in all eukaryotes.

5. • In some higher eukaryotes methyl group addition also occurs at
second base, if adenine is present and reaction involves at the N6
position.
• mRNA with methyl groups on the N7 position of the guanine and
the 2’-OH position of the second nucleotide at the 5’ end is
known as cap1.
• Similarly, if methyl group is present at both second and third
nucleotide then it is known as cap2.

6. Functions of 5’ cap
 Protection of mRNA from degradation.
 Transport of the mRNA from nucleus to cytoplasm.
 Binding of ribosome with mRNA.

7. Thank you