Post-transcriptional changes in mRN A and tRNA
RNA transcripts in eukaryotes are modified, or processed, before leaving the nucleus to yield functional mRNA.
Eukaryotic RNA transcripts can be processed in two ways:
1. Covalent alteration of both the 3′ and 5′ ends.
2. Removal of intervening sequences.
The pre-mRNA (primary transcript that will be processed to functional mRNA) molecule undergoes three main modifications. These modifications are 5′ capping, 3′ polyadenylation, and RNA splicing, which occur in the cell nucleus before the RNA is translated.
Capping of the pre-mRNA involves the addition of 7-methylguanosine (m7G) to the 5′ end. In order to achieve this, the terminal 5′ phosphate requires removal, which is done by the aid of a phosphatase enzyme. The enzyme guanosyl transferase then catalyses the reaction which produces the diphosphate 5′ end. The diphosphate 5′ prime end then attacks the a phosphorus atom of a GTP molecule in order to add the guanine residue in a 5’5′ triphosphate link. The enzyme S-adenosyl methionine then methylates the guanine ring at the N-7 position.
This type of cap, with just the (m7G) in position is called a cap 0 structure. The ribose of the adjacent nucleotide may also be methylated to give a cap 1. Methylation of nucleotides downstream of the RNA molecule produce cap 2, cap 3 structures and so on. In these cases the methyl groups are added to the 2′ OH groups of the ribose sugar. The cap protects the 5′ end of the primary RNA transcript from attack by ribonucleases that have specificity to the 3’5′ phosphodiester bonds.
Cleavage and Polyadenylation :
The pre-mRNA processing at the 3′ end of the RNA molecule involves cleavage of its 3′ end and then the addition of about 200 adenine residues to form a poly(A) tail. The cleavage and adenylation reactions occur if a polyadenylation signal sequence (5′- AAU AAA-3′) is located near the 3′ end of the pre-mRNA molecule, which is followed by another sequence, which is usually (5′-CA-3′). Poly(A) polymerase then adds about 200 adenine units to the new 3′ end of the RNA molecule using ATP as a precursor. As the poly(A) tails is synthesised, it binds multiple copies of poly(A) binding protein, which protects the 3’end from ribonuclease digestion.
RNA splicing is the process by which introns, regions of RNA that do not code for protein, are removed from the pre-mRNA and the remaining exons connected to re-form a single continuous molecule. Although most RNA splicing occurs after the complete synthesis and end-capping of the pre-mRNA, transcripts with many exons can be spliced co-transcriptionally. The splicing reaction is catalyzed by a large protein complex called the spliceosome assembled from proteins and small nuclear RNA molecules that recognize splice sites in the pre-mRNA sequence.RNA
tRNA splicing is another rare form of splicing that usually occurs in tRNA. The splicing reaction involves a different biochemistry than the spliceomsomal and self-splicing pathways. Ribonucleases cleave the RNA and ligases join the exons together. This form of splicing does also not require any RNA components for catalysis. In biochemistry, a ligase is an enzyme that can catalyse the joining of two molecules (ligation or glue together) by forming a new chemical bond, with concomitant hydrolysis of ATP or other similar molecules.
Most of higher eukaryotic genes coding for mRNA, tRNA and some coding for rRNA are interrupted by unrelated regions called introns. The other parts of the genes are called exons. Exons contain information that appears in the functional mRNA. Genes for mRNA have 0 to 60 introns. Genes for tRNA have 0 to 1 intron.