Transfer RNA, also known as transfer RNATransfer RNA, commonly referred to as tRNA, is an RNA composed of 76-90 nucleotides. Its 3 'end can attach specific amino acids under the catalysis of aminoacyl tRNA synthetase.During translation, tRNA can recognize the codon on mRNA by its own anti codon, and transfer the amino acid corresponding to the codon to the polypeptide chain in ribosome synthesis.In theory, each tRNA molecule can only attach to one amino acid, but the genetic code has degeneracy, which makes more than one tRNA can attach to one amino acid.
Before tRNA was discovered, Francis Crick assumed that there was a way toRNAMessage converted intoproteinAdaptor of the message exists.In the early 1960s, biologists such as Alexander Ritchie and Donald Kaspar began to study the structure of tRNA. In 1965, Robert W. Hawley first isolated tRNA and clarified its sequence and approximate structure. As a result of his contribution, he obtainedNobel Prize in Physiology or Medicine 。TRNA was first successfully crystallized by Robert M. Bock. Later, some people successively proposed the leaf like secondary structure of tRNA, which was developed by Jin Chenghao and Alexander Richie in 1973X-ray diffraction analysisconfirm.Another British team led by Aaron Kluger released the same discovery of X-ray crystallography in the same year.
definition
Announce
edit
TRNA structure[2]
Most tRNAs are composed of clover shaped short chains formed by folding more than seventy to ninety nucleotides,relative molecular mass25000~30000, and the sedimentation constant is about 4S.Formerly known as linked RNA, soluble RNA, etc.The main function is to carry amino acids into ribosomes and synthesize proteins under the guidance of mRNA, that is, take mRNA as a template to translate the nucleotide sequence with code meaning into the amino acid sequence of proteins.TRNA and mRNA are related through the interaction of anti codon and codon.In the process of peptide chain formation, the first tRNA that enters the ribosome and binds to the mRNA start codon is calledInitial tRNAThe other tRNAs participate in peptide chain extension, which is called extended tRNA. According to the arrangement of the codes on the mRNA, the tRNAs carrying specific amino acids enter the ribosome in turn.After the peptide chain is formed, tRNA is released from the ribosome.The whole process is called tRNA cycle.One tRNA can only carry one amino acid, such asalanine TRNA only carries alanine, but one amino acid can be carried by more than one tRNA.In the same organism, different tRNAs carrying the same amino acid are called "homologous receptor tRNAs".There are 20 amino acids that make up proteins. According to the codon swing theory, at least 31 tRNAs are required, but only 22 tRNAs exist in vertebrates.This is mainly throughCodon-AnticodonThe simplification of pairing enables a tRNA to recognize all four codons of a codon family.The organelle tRNA carrying the same amino acid is also different from the cytoplasm tRNA.After organism mutation, one of the correction mechanisms is to correctGene synthesisOne is to correct tRNA to maintain the relative correctness of translation decoding.Multiple corrected tRNAs can carry the same amino acid.
structure
Announce
edit
Trifolium structure of tRNA
The transport RNA molecule consists of 70~90nucleotideAnd folded into short chains in the shape of clover.There are two different molecules in the figure above,PhenylalanineTRNA (4tna) and aspartate tRNA (2tra).The two ends of the tRNA chain are close to each other at the end of the L-shaped structure indicated at the top of the figure.Amino acids are linked at the position indicated by the arrow.An L-shaped arm is formed in the center of this chain, as shown below the Trifolium Structure of tRNA, revealing the formation ofAnticodonThree nucleotides of.Clover structureThe other two rings are wrapped in an elbow shape, where they provide the entire molecular structure.Four common RNA bases---adenine，Uracil，Guanineandcytosine Obviously, it cannot provide enough space to form a solid structure, because theseBaseMost have been modified to extend their structure.There are two strange examples. Look at No. 37AnticodonAdjacentBase, onmethionineTRNA (1yfg) orPhenylalanineThe starting site of tRNA (4tna and 6tna).
Primary structure
Yeast was first detected by R.W. Holly et al. in 1965alanine The primary structure of tRNA is the nucleotide sequenceBy 1983, there had been more than 200 tRNAs (including tRNAs of different biological sources, different organs, organelles, and calibration tRNAs)Primary structureBe clarified.According to A-U, G-C and G-UBase pairingPrinciples, with some exceptions,
Secondary structure
TRNA molecules can be arranged into clover modelSecondary structure。It consists of three rings, namely D-ringDihydrouridineHigh acid (D) content], anti codon ring (the middle of the ring isAnticodon）And T Ψ C ring [because most tRNAs containThymidylate（T）、Pseudouridine（Ψ）、Cytidine acid(C) Sequence], four stems, namely, D-stem (stem connected with D-ring), anti codon stem (connected with anti codon ring), T Ψ C stem (connected with T Ψ C ring) and amino acid acceptor stem (also called CCA stem), because the molecular ends of all tRNAs contain cytidine acid (C), cytidine acid (C)Adenylate(A) Sequence, CCA is indispensable for connecting amino acids], and between anti codon stem and T Ψ C stemVariable armComposition.The length of variable arms of different tRNAs varies from two to ten nucleotides.Except the variable arm and D-ringnucleotideNumber andBase pairIt is basically constant.The figure "Three dimensional structure of tRNA crystal" also shows the conservative or semi conservative components in tRNA molecules.These components are important for the maintenance of tRNATertiary structureIt is very important.
Three dimensional structure of tRNA crystal
Structural characteristics of tRNA
One of the structural features of tRNA is that it contains more modifier, such as D, T, Ψ, etc. mentioned above;Most of the modified components in nucleic acids are found in tRNA.The distribution of modifier in tRNA molecule is regular, but its function is not clear.
1974 X-rayCrystal diffractionThe three-dimensional structure of the first tRNA, yeast phenylalanine tRNA crystal, was measured by the method. The molecular panorama is like the inverted English letter L, which is flat, 60 angstroms long and 20 angstroms thick. As shown in the Three dimensional Structure of tRNA Crystals, it is located in tRNASecondary structureOn the basis, the amino acid acceptor stem folds into a right-handed antiparallel double helix between the T Ψ C stem and the D stem and the anti codon stem.tRNATertiary structureIt is formed between the conservative or semi conservative components and the nucleotides constituting the secondary structurehydrogen bond(called tertiary structure hydrogen bond) maintenance.The three-dimensional structure of other tRNA crystals is similar to that of yeast phenylalanine tRNA, but some parameters are different.The configuration of tRNA in solution is consistent with its crystal structure.
function
Announce
edit
TRNA cycle
It mainly carries amino acid intoribosomeAnd synthesize proteins under the guidance of mRNA.Namely, take mRNA as the template, and translate the nucleotide sequence with code meaning into the amino acid sequence in the protein (seebiosynthesisRibosome).TRNA and mRNA areAnticodonRelating to the interaction of codons.stayPeptide chainIt is the first to enter ribosome and mRNAInitial codonThe combined tRNA is called the starting tRNA, and the remaining tRNAs participate in the peptide chain extension, called the extension tRNA. According to the arrangement of the codes on the mRNA, the tRNAs carrying specific amino acids enter in turnribosome。After the peptide chain is formed, tRNA is released from the ribosome.The whole process is called tRNA cycle, as shown in tRNA Cycle.TRNA is recognized by anti codon and mRNA, but not one anti codon can only recognize oneCodon。For example, anticodon CIG (I isHypoxanthine nucleotide）Can recognize three kinds of codons.commonlyAnticodonThe rare nucleotides in can recognize multiple codons due to their loose pairing. This phenomenon is called "wobble" in biology.
How tRNA Works[3]
TRNA carries amino acids through CCA at the 3 'end of the molecule.Amino acids are linked toAdenylateThe tRNA carrying amino acids on the 2 'or 3' OH group of amino acid is called glycyl tRNA, for example, the tRNA carrying glycine is called glycyl tRNA.The binding of amino acids to tRNA is determined byAminoacylTRNA synthetase catalysis is carried out in two steps: ① amino acid+ATP → aminoacyl AMP+inorganpyrophosphate；②Aminoacyl AMP+tRNA → aminoacyl tRNA+AMP.There is at least one tRNA and one aminoacyl tRNA synthetase corresponding to one amino acid (seeProtein Biosynthesis）。
TRNA also has some other specific functions, such as carrying amino acids to specificReceptor moleculeTo synthesize cell membrane or cell wall components;AsReverse transcriptaseprimerParticipate in DNA synthesis;As an inhibitor of some enzymes, etc.Some aminoacyl tRNA can also regulate thebiosynthesis。TRNA like RNA is found in many plant virus RNA moleculesClover structureSome can also accept amino acids, whose function is unknown[4]。
The processing of tRNA precursor includes: removing the redundant nucleotides at both ends or inside the precursor molecule;The modified nucleotides that form mature tRNA molecules;If the 3 'end of the precursor molecule lacks the CCA sequence, the CCA end needs to be added.The processing process isEnzyme catalysisUnder.
Artificial synthesis: 1981, Chinese scientistWang DebaoFor the first timetotal synthesisYeastalanine tRNA。It consists of 76nucleotideComposition, including all modified ingredients in natural molecules, and the product is similar to natural moleculesbiological activity(See RNA and nucleic acid synthesis).
