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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