Come, today we will learn about Post Transcriptional Modification, so let’s start. Post-transcriptional modification or co-transcriptional modification is a set of biological processes common to most eukaryotic cells by which an RNA primary transcript is chemically altered after transcription from a gene to produce a mature, functional RNA molecule. which can then leave the nucleus and perform any of a variety of different functions in the cell. There are several types of post transcriptional modification achieved through a diverse class of molecular mechanisms.
An example is the conversion of precursor messenger RNA transcripts into mature messenger RNAs that are then capable of translation into proteins . This process involves three major steps that significantly modify the chemical structure of the RNA molecule: the addition of a 5′ cap , the addition of a 3′ polyadenylated tail, and RNA splicing . Such processing is important for the correct translation of eukaryotic genomes because the early precursor mRNA produced by transcription often contains both exons (coding sequences) and introns (non-coding sequences) ., Splicing removes introns and connects exons directly, while cap and tail facilitate the transport of mRNA into a ribosome and protect it from molecular degradation.
Post-transcriptional modifications can also occur during the processing of other transcripts that eventually become transfer RNA , ribosomal RNA , or any other type of RNA used by the cell.
mRNA processing
The pre-mRNA molecule undergoes three main modifications. These modifications are 5′ capping, 3′ polyadenylation and RNA splicing, which take place in the cell nucleus before the translation of RNA.
5′ processing
capping
Capping of the pre-mRNA involves the addition of 7 -methylguanosine (M7G ) to the 5′ end. To achieve this, removal of the terminal 5′ phosphatase is required, which is done with the aid of phosphatase enzymes. The enzyme guanosyl transferase then catalyzes the reaction, producing the diphosphate 5′ end. The 5′ end of the diphosphate then attacks the alpha phosphorus atom of the GTP molecule to add a guanine residue to the 5’5′ triphosphate link. The enzyme (guanine-N7-)-methyltransferase (“CAPMTS”) transfers a methyl group from S-adenosyl methionine to the guanine ring. [5] This type of cap, only ( M7g) is called a cap 0 structure. Nucleotides of adjacent ribose can also be methylated to give a cap 1. Methylation of nucleotides downstream of the RNA molecule producing cap 2, cap 3 structures and so on. In these cases 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 the specificity of a 3’5′ phosphodiester bond. [6]
3′ processing
cleavage and polyadenylation
Pre-mRNA processing at the 3′ end of the RNA molecule involves cleavage of its 3′ end and then the addition of approximately 250 adenine residues to form a poly(A) tail. Cleavage and adenylation reactions primarily occur when a polyadenylation signal sequence (5′- AAUAAA-3′) is located near the 3′ end of the pre-mRNA molecule, followed by another sequence, usually ( 5′-ca). -3′) and is the location of the crack. a GU-rich sequenceis also usually present downstream on the pre-mRNA molecule. Recently, it has been demonstrated that alternative signal sequences such as UGUA upstream of cleavage sites can also direct cleavage and polyadenylation in the absence of AAUAAA signal.
It is important to understand that these two signs are not mutually independent and often coexist. After the synthesis of sequence elements, many multi-subunit proteins are transcribed into the RNA molecule. These sequence specific binding proteins are cleaved and polyadenylation specificity factor (CPSF), cleavage factor I (CFI) and cleavage stimulation factor (CSTF) are transferred by RNA polymerase II. Three factors bind sequence elements. The AAUAAA signal is tied directly to the CPSF. For processing sites dependent on UGUA, The binding of the multi-protein complex is carried out by cleavage factor I (CF I). The resulting protein complex contains additional cleavage factor and the enzyme polyadenylate polymerase (PAP).
This complex cleaves RNA between the polyadenylation sequence and the GU-rich sequence at the cleavage site marked by the (5′-CA-3′) sequences. 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) tail is synthesized, it binds to multiple copies of the poly(A)-binding protein, which protects the 3′ end from ribonuclease digestion by enzymes including the CCR4-Notch complex. ) cleaves the RNA between the polyadenylation sequence and the GU-rich sequence at the cleavage site marked by the sequences. 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) tail is synthesized, it binds to multiple copies of the poly(A)-binding protein, which protects the 3′ end from ribonuclease digestion by enzymes including the CCR4-Notch complex. ) cleaves the RNA between the polyadenylation sequence and the GU-rich sequence at the cleavage site marked by the sequences. 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) tail is synthesized, it binds to multiple copies of the poly(A)-binding protein, which protects the 3′ end from ribonuclease digestion by enzymes including the CCR4-Notch complex.[6]
intron splicing
RNA splicing is the process by which introns, regions of RNA that do not code for proteins, are removed from the pre-mRNA and the remaining exons are fused to recreate a single continuous molecule. Exons are segments of mRNA that become “expressed” or translated into proteins. They are the coding parts of an mRNA molecule. [7] Although most RNA splicing occurs after complete synthesis and end-capping of the pre-mRNA, transcripts can be co-transcribed with multiple exons. [8]The splicing reaction is catalyzed by a large protein complex called the spliceosome that assembles from proteins and small nuclear RNA molecules that recognize splice sites in the pre-mRNA sequence. Many pre-mRNAs, including those encoding antibodies, can be spliced in a number of ways to generate different mature mRNAs that encode different protein sequences. This process is known as alternative splicing, and allows a large amount of protein to be produced from a limited amount of DNA.
Histone mRNA processing
Histones H2A, H2B, H3 and H4 form the core of a nucleosome and are thus called core histones. Core histones are processed differently because typical histone mRNAs lack many features of other eukaryotic mRNAs, such as poly(A) tails and introns. Thus, such mRNAs do not undergo splicing and their 3′ processing is independent of most cleavage and polyadenylation factors. Core histone mRNAs have a specialized stem-loop structure at the 3-prime end that is recognized by a stem-loop binding protein and a downstream sequence, called the histone downstream element (HDE) that recruits the U7 snRNA. Cleavage and polyadenylation specificity factor 73 cuts mRNA between stem-loop and HDE [9]
Histone variants, such as H2A.Z or H3.3, contain introns and are processed as normal mRNAs, which involve splicing and polyadenylation.
