5’ capping

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 N 7 position of the guanine at 5’ end of RNA. If methyl group is present at N 7 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 N 6 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