Жалғанұридин: РНК құрамындағы маңызды өзгеріс

Chemical compound

Pseudouridine (5 ribosyluracil, abbreviated by the Greek letter psi Ψ) is an isomer of the nucleoside uridine in which the uracil is attached via a carbon carbon instead of a nitrogen carbon glycosidic bond. Pseudouridine is the most abundant RNA modification in cellular RNA. After transcription and following synthesis, RNA can be modified with over 100 chemically distinct modifications. These can potentially regulate RNA expression post transcriptionally, in addition to the four standard nucleotides and play a variety of roles in the cell including translation, localization and stabilization of RNA. Pseudouridine, being one of them, is the C5 glycoside isomer of uridine that contains a C C bond between C1 of the ribose sugar and C5 of uracil, rather than usual C1 N1 bond found in uridine. Uridine is converted to pseudouridine by rotating the uridine molecule 180𝄪 across its N3 C6 axis. The C C bond gives it more rotational freedom and conformational flexibility. In addition, pseudouridine has an extra hydrogen bond donor at the N1 position. Pseudouridine is a ubiquitous constituent of structural RNA (transfer, ribosomal, small nuclear (snRNA) and small nucleolar), and present in coding RNA, across the three phylogenetic domains of life and was the first discovered. It accounts for 4% of the nucleotides in the yeast tRNA This base modification is able to stabilize RNA and improve base stacking by forming additional hydrogen bonds with water through its extra imino group. There are 11 pseudouridines in the Escherichia coli rRNA, 30 in yeast cytoplasmic rRNA and a single modification in mitochondrial 21S rRNA and about 100 pseudouridines in human rRNA indicating that the extent of pseudouridylation increases with the complexity of an organism. Pseudouridine was also detected in Leishmania donovani genome. 18 pseudouridine modification sites were detected in the peptidyl transferase entry site and in the mRNA entry tunnel in protein translation. These modifications in the parasite lead to increased protein synthesis and growth rate. Pseudouridine in rRNA and tRNA has been shown to fine tune and stabilize the regional structure and help maintain their functions in mRNA decoding, ribosome assembly, processing and translation. Pseudouridine in snRNA has been shown to enhance spliceosomal RNA pre mRNA interaction to facilitate splicing regulation.

Ψ is ubiquitous in this class of RNAs and facilitates common tRNA structural motifs. One such structural motif is the TΨC stem loop which incorporates Ψ55. Ψ is commonly found in the D stem and anticodon stem and loop of tRNAs from each domain. In each structural motif the unique physicochemical properties of Ψ stabilize structures that would not be possible with the standard U. In the SARS CoV2 vaccine from BioNTech/Pfizer, also known as BNT162b2, tozinameran or Comirnaty, all U's have been substituted with N1 methylpseudouridine, a nucleoside related to Ψ that contains a methyl group added to N1 atom.

Ψ is found in the large and small ribosomal subunits of all domains of life and their organelles. In the ribosome Ψ residues cluster in domains II, IV, and V and stabilize RNA RNA and/or RNA protein interactions. The stability afforded by Ψ may assist rRNA folding and ribosome assembly. Ψ may also influence the stability of local structures which impact the speed and accuracy of decoding and proofreading during translation. PUS 1 is located in the nucleus and modifies tRNA at different locations, U44 of U2 snRNA, and U28 of U6 snRNA. Studies found that PUS 1 expression increased during environmental stress and is important for regulating the splicing of RNA. Also, that PUS 1 is necessary for taking the tRNA made in the nucleus and sending them to the cytoplasm.

TruD is able to modify a variety of RNA, and it is unclear how these different RNA substrates are recognized. PUS 7 modifies U2 snRNA at the position 35 and this modification will increase when the cells are in heat shock. Another modification is cytoplasmic tRNA in position 13, and position 35 in pre tRNATyr. PUS 7 modifies almost specificity does not depend on the type of RNA as mRNA show pseudouridylated by PUS 7. Recognize this the sequence of the RNA, UGUAR with the second U being the nucleotide that will be modified. The pseudouridylation of mRNA by PUS 7 increases during heat shock, because the protein moves from the nucleus to the cytoplasm. The modification is thought to increase the stability of mRNA during heat shock before the RNA goes to the nucleus or mitochondria, but more studies are needed. A chemical way to identify pseudouridine uses a compound called CMC or N cyclohexyl N′ β (4 methylmorpholinium) ethylcarbodiimide to specifically label and distinguish uridine from pseudouridine. CMC will bond both with pseudouridine and uridine, but holds tighter to the former, because of the third nitrogen able to form hydrogen bond. CMC bound to pseudouridine can then be imaged by tagging a signaling molecule. This method is still being worked on to become high throughput.

Pseudouridine exerts a subtle but significant influence on the nearby sugar phosphate backbone and also enhances base stacking. These effects may underlie the biological role of most, but perhaps not all of the pseudouridine residues in RNA. Certain genetic mutants lacking specific pseudouridine residues in tRNA or rRNA exhibit difficulties in translation, display slow growth rates, and fail to compete effectively with wild type strains in mixed culture. Pseudouridine modifications are also implicated in human diseases such as mitochondrial myopathy and sideroblastic anemia (MLASA) and Dyskeratosis congenita. Pseudouridylation has also been associated with the pathogenesis of maternally inherited diabetes and deafness (MIDD). In particular, a point mutation in a mitochondrial tRNA seems to prevent the pseudouridylation of one nucleotide, thus altering the tRNA tertiary structure. This may lead to higher tRNA instability, causing deficiencies in mitochondrial translation and respiration. N1 Methylpseudouridine provides even less innate immune response than Ψ, as well as improving translation capacity. Both Pfizer BioNTech and Moderna mRNA vaccines therefore use N1 Methylpseudouridine rather than Ψ.