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Transfer RNA
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==Overview== The process of [[Translation (biology)|translation]] starts with the information stored in the nucleotide sequence of [[DNA]]. This is first transformed into mRNA, then tRNA specifies which three-nucleotide codon from the genetic code corresponds to which amino acid.<ref name="crick">{{cite journal | vauthors = Crick FH | title = The origin of the genetic code | journal = Journal of Molecular Biology | volume = 38 | issue = 3 | pages = 367β379 | date = December 1968 | pmid = 4887876 | doi = 10.1016/0022-2836(68)90392-6 | s2cid = 4144681 }}</ref> Each mRNA codon is recognized by a particular type of tRNA, which docks to it along a three-nucleotide [[#Anticodon|anticodon]], and together they form three [[Complementarity (molecular biology)|complementary]] [[base pair]]s. On the other end of the tRNA is a covalent attachment to the amino acid corresponding to the anticodon sequence, with each type of tRNA attaching to a specific amino acid. Because the genetic code contains multiple codons that specify the same amino acid, there are several tRNA molecules bearing different anticodons which carry the same amino acid. The covalent attachment to the tRNA [[Directionality (molecular biology)|3' end]] is catalysed by enzymes called [[aminoacyl tRNA synthetase]]s. During protein synthesis, tRNAs with attached amino acids are delivered to the [[ribosome]] by proteins called [[elongation factor]]s, which aid in association of the tRNA with the ribosome, synthesis of the new polypeptide, and translocation (movement) of the ribosome along the mRNA. If the tRNA's anticodon matches the mRNA, another tRNA already [[#Binding to ribosome|bound to the ribosome]] transfers the growing polypeptide chain from its 3' end to the amino acid attached to the 3' end of the newly delivered tRNA, a reaction catalyzed by the ribosome. A large number of the individual nucleotides in a tRNA molecule may be [[Chemical modification#Chemical modification in biochemistry|chemically modified]], often by [[methylation]] or [[deamidation]]. These unusual bases sometimes affect the tRNA's interaction with [[ribosome]]s and sometimes occur in the [[anticodon]] to alter base-pairing properties.<ref name="Stryer2002">{{cite book |vauthors=Stryer L, Berg JM, Tymoczko JL | title = Biochemistry | publisher = W. H. Freeman | location = San Francisco | year = 2002 | edition = 5th | isbn = 978-0-7167-4955-4 | url = https://www.ncbi.nlm.nih.gov/books/NBK21154/ }}</ref> The addition of a guanine nucleotide at the -1 position (G-1) to the 5β² end of tRNA-His, catalyzed by [[TRNA(His) guanylyltransferase|tRNA-His guanylyltransferase (Thg1)]] and Thg1-like proteins (TLPs) is particularly notable as it proceeds in the 3β² to 5β² direction, which is opposite to the canonical 5β² to 3β² nucleotide addition used by all other known nucleic acid polymerases.<ref>{{Cite journal |last1=Jackman |first1=Jane E. |last2=Gott |first2=Jonatha M. |last3=Gray |first3=Michael W. |date=May 2012 |title=Doing it in reverse: 3'-to-5' polymerization by the Thg1 superfamily |journal=RNA |volume=18 |issue=5 |pages=886β899 |doi=10.1261/rna.032300.112 |issn=1469-9001 |pmc=3334698 |pmid=22456265}}</ref> This reverse polymerization mechanism is biochemically unique and evolutionarily conserved, highlighting its fundamental importance in tRNA maturation.<ref>{{Cite journal |last1=Rao |first1=Bhalchandra S. |last2=Maris |first2=Emily L. |last3=Jackman |first3=Jane E. |date=2011-03-01 |title=tRNA 5β²-end repair activities of tRNA His guanylyltransferase (Thg1)-like proteins from Bacteria and Archaea |url=https://academic.oup.com/nar/article/39/5/1833/2409527 |journal=Nucleic Acids Research |volume=39 |issue=5 |pages=1833β1842 |doi=10.1093/nar/gkq976 |pmid=21051361 |pmc=3061083 |issn=0305-1048}}</ref> Homologs of Thg1 are found in all domains of life, where they can also participate in tRNA repair and quality control.<ref>{{Cite journal |last1=Chen |first1=Allan W. |last2=Jayasinghe |first2=Malithi I. |last3=Chung |first3=Christina Z. |last4=Rao |first4=Bhalchandra S. |last5=Kenana |first5=Rosan |last6=Heinemann |first6=Ilka U. |last7=Jackman |first7=Jane E. |date=2019-03-26 |title=The Role of 3β² to 5β² Reverse RNA Polymerization in tRNA Fidelity and Repair |journal=Genes |language=en |volume=10 |issue=3 |pages=250 |doi=10.3390/genes10030250 |doi-access=free |issn=2073-4425 |pmc=6471195 |pmid=30917604}}</ref> The presence of G-1 is a key identity element for tRNA-His, and its absence severely impairs histidylation efficiency and tRNA function.
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