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Transfer RNA
 
 
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Transfer RNA (abbreviated tRNA) is a small RNA
RNA

Ribonucleic acid is a type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate....
 (usually about 74-95 nucleotides) that transfers a specific active amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has a 3' terminal
Directionality (molecular biology)

Directionality, in molecular biology, refers to the end-to-end chemical orientation of a single strand of nucleic acid. The chemical convention of naming carbon atoms in the nucleotide Furanose numerically gives rise to a 5' end and a 3' end ....
 site for amino acid attachment. This covalent linkage is catalyzed by an aminoacyl tRNA synthetase
Aminoacyl tRNA synthetase

An aminoacyl tRNA synthetase is an enzyme that catalyzes the esterification of a specific amino acid or its precursor to one of all its compatible cognate tRNAs to form an aminoacyl-tRNA....
. It also contains a three base
Nucleotide

Nucleotides are molecules that comprise the structural units of RNA and DNA. Additionally, nucleotides play central roles in metabolism. In that capacity, they serve as sources of chemical energy , participate in cell signaling , and are incorporated into important cofactors of enzymatic reactions ....
 region called the anticodon that can base pair to the corresponding three base codon region on mRNA.






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Transfer RNA (abbreviated tRNA) is a small RNA
RNA

Ribonucleic acid is a type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate....
 (usually about 74-95 nucleotides) that transfers a specific active amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has a 3' terminal
Directionality (molecular biology)

Directionality, in molecular biology, refers to the end-to-end chemical orientation of a single strand of nucleic acid. The chemical convention of naming carbon atoms in the nucleotide Furanose numerically gives rise to a 5' end and a 3' end ....
 site for amino acid attachment. This covalent linkage is catalyzed by an aminoacyl tRNA synthetase
Aminoacyl tRNA synthetase

An aminoacyl tRNA synthetase is an enzyme that catalyzes the esterification of a specific amino acid or its precursor to one of all its compatible cognate tRNAs to form an aminoacyl-tRNA....
. It also contains a three base
Nucleotide

Nucleotides are molecules that comprise the structural units of RNA and DNA. Additionally, nucleotides play central roles in metabolism. In that capacity, they serve as sources of chemical energy , participate in cell signaling , and are incorporated into important cofactors of enzymatic reactions ....
 region called the anticodon that can base pair to the corresponding three base codon region on mRNA. Each type of tRNA molecule can be attached to only one type of amino acid, but because the genetic code
Genetic code

The genetic code is the set of rules by which information encoded in genetic material is Translation into proteins by living cell s. The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids....
 contains multiple codons that specify the same amino acid, tRNA molecules bearing different anticodons may also carry the same amino acid.

Structure

3d Trna
tRNA has primary structure
Primary structure

In biochemistry, the primary structure of a biological molecule is the exact specification of its atomic composition and the chemical bonds connecting those atoms ....
, secondary structure
Secondary structure

In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids ....
 (usually visualized as the cloverleaf structure), and tertiary structure
Tertiary structure

In biochemistry and chemistry, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates....
 (all tRNAs have a similar L-shaped 3D structure that allows them to fit into the P and A sites of the ribosome
Ribosome

Ribosomes are complexes of RNA and protein that are found in all cell s. Ribosomes from bacteria, archaea and eukaryotes, the three domains of life on Earth, have significantly different structure and RNA....
).
  1. The 5'-terminal phosphate
    Phosphate

    A phosphate, an inorganic chemical, is a Salt of phosphoric acid. Inorganic phosphates are mining to obtain phosphorus for use in agriculture and industry....
     group.
  2. The acceptor stem is a 7-bp stem made by the base pairing of the 5'-terminal nucleotide with the 3'-terminal nucleotide (which contains the CCA 3'-terminal group used to attach the amino acid). The acceptor stem may contain non-Watson-Crick base pairs.
  3. The CCA tail is a CCA sequence at the 3' end of the tRNA molecule. This sequence is important for the recognition of tRNA by enzymes critical in translation. In prokaryotes, the CCA sequence is transcribed. In eukaryotes, the CCA sequence is added during processing and therefore does not appear in the tRNA gene.
  4. The D arm
    D arm

    The D arm is a feature in the tertiary structure of transfer RNA. It is composed of the two D stems and the D loop. The D loop contains the base dihydrouracil....
     is a 4 bp stem ending in a loop that often contains dihydrouridine
    Dihydrouridine

    Dihydrouridine is a pyrimidine which is the result of adding two hydrogen atoms to a uridine, making it a fully saturated pyrimidine ring with no remaining double bonds....
    .
  5. The anticodon arm is a 5-bp stem whose loop contains the anticodon
    Transfer RNA

    Transfer RNA is a small RNA that transfers a specific active amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation ....
    . It also contains a Y that stands for a modified purine
    Purine

    Purine is a heterocyclic compound aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
     nucleotide
    Nucleotide

    Nucleotides are molecules that comprise the structural units of RNA and DNA. Additionally, nucleotides play central roles in metabolism. In that capacity, they serve as sources of chemical energy , participate in cell signaling , and are incorporated into important cofactors of enzymatic reactions ....
    .
  6. The T arm
    T arm

    The T-arm or T-loop is a specialized region on the tRNA molecule which acts as a special recognition site for the ribosome to allow a tRNA-ribosome complex to form during the process of protein biosynthesis....
     is a 5 bp stem containing the sequence T?C where ? is a pseudouridine
    Pseudouridine

    Pseudouridine is the C-glycoside isomer of the nucleoside uridine, and it is the most prevalent of the over one hundred different modified nucleosides found in RNA....
    .
  7. Bases that have been modified, especially by methylation
    Methylation

    Methylation in the chemical sciences denotes the attachment or substitution of a methyl on various Substrate . This term is commonly used in chemistry, biochemistry, soil science and the biological sciences....
    , occur in several positions outside the anticodon. The first anticodon base is sometimes modified to inosine
    Inosine

    Inosine is a nucleoside that is formed when hypoxanthine is attached to a ribose ring via a ?-N9-glycosidic bond.Inosine is commonly found in tRNAs and is essential for proper translation of the genetic code in wobble base pairs....
     (derived from adenine) or pseudouridine
    Pseudouridine

    Pseudouridine is the C-glycoside isomer of the nucleoside uridine, and it is the most prevalent of the over one hundred different modified nucleosides found in RNA....
     (derived from uracil).


Anticodon

An anticodon is a unit made up of three nucleotides that correspond to the three bases of the codon on the mRNA. Each tRNA contains a specific anticodon triplet sequence that can base-pair to one or more codons for an amino acid. For example, one codon for lysine
Lysine

Lysine is an a-amino acid with the chemical formula HO2CCH4NH2. This amino acid is an essential amino acid, which means that humans cannot synthesize it....
 is AAA; the anticodon of a lysine tRNA might be UUU. Some anticodons can pair with more than one codon due to a phenomenon known as wobble base pair
Wobble base pair

A wobble base pair is a G-U and I-U / I-A / I-C pair fundamental in RNA secondary structure. Its thermodynamic stability is comparable to that of the Base pair....
ing. Frequently, the first nucleotide of the anticodon is one of two not found on mRNA: inosine
Inosine

Inosine is a nucleoside that is formed when hypoxanthine is attached to a ribose ring via a ?-N9-glycosidic bond.Inosine is commonly found in tRNAs and is essential for proper translation of the genetic code in wobble base pairs....
 and pseudouridine
Pseudouridine

Pseudouridine is the C-glycoside isomer of the nucleoside uridine, and it is the most prevalent of the over one hundred different modified nucleosides found in RNA....
, which can hydrogen bond
Hydrogen bond

A hydrogen bond is the attractive force between one electronegative atom and a hydrogen covalently bonded to another electronegative atom. It results from a dipole-dipole force with a hydrogen atom bonded to nitrogen, oxygen or fluorine ....
 to more than one base in the corresponding codon position. In the genetic code
Genetic code

The genetic code is the set of rules by which information encoded in genetic material is Translation into proteins by living cell s. The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids....
, it is common for a single amino acid to be specified by all four third-position possibilities; for example, the amino acid glycine
Glycine

Glycine is the organic compound with the chemical formula NH2CH2COOH. It is the smallest of the 20 amino acids commonly found in proteins, coded by codons GGU, GGC, GGA and GGG....
 is coded for by the codon sequences GGU, GGC, GGA, and GGG.

To provide a one-to-one correspondence between tRNA molecules and codons that specify amino acids, 61 tRNA molecules would be required per cell. However, many cells contain fewer than 61 types of tRNAs because the wobble base is capable of binding to several, though not necessarily all, of the codons that specify a particular amino acid.

Aminoacylation

Aminoacylation
Aminoacylation

Aminoacylation is the process of adding an aminoacyl group to a compound.See also*Acylation*Transfer RNA#Aminoacylation...
 is the process of adding an aminoacyl group to a compound. It produces tRNA molecules with their CCA 3' ends covalently linked to an amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
.

Each tRNA is aminoacylated
Aminoacylation

Aminoacylation is the process of adding an aminoacyl group to a compound.See also*Acylation*Transfer RNA#Aminoacylation...
 (or charged) with a specific amino acid by an aminoacyl tRNA synthetase
Aminoacyl tRNA synthetase

An aminoacyl tRNA synthetase is an enzyme that catalyzes the esterification of a specific amino acid or its precursor to one of all its compatible cognate tRNAs to form an aminoacyl-tRNA....
. There is normally a single aminoacyl tRNA synthetase for each amino acid, despite the fact that there can be more than one tRNA, and more than one anticodon, for an amino acid. Recognition of the appropriate tRNA by the synthetases is not mediated solely by the anticodon, and the acceptor stem often plays a prominent role.

Reaction:
  1. amino acid + ATP ? aminoacyl-AMP + PPi
  2. aminoacyl-AMP + tRNA ? aminoacyl-tRNA + AMP


Binding to ribosome

The ribosome has three binding sites for tRNA molecules: the A, P, and E sites. During translation the A site binds an incoming aminoacyl-tRNA as directed by the codon currently occupying this site. This codon specifies the next amino acid to be added to the growing peptide chain. The A site only works after the first aminoacyl-tRNA has attached to the P site. The P-site codon is occupied by peptidyl-tRNA that is a tRNA with multiple amino acids attached as a long chain. The P site is actually the first to bind to aminoacyl tRNA. This tRNA in the P site carries the chain of amino acids that has already been synthesized. The E site is occupied by the empty tRNA as it is about to exit the ribosome.

tRNA genes


Organisms vary in the number of tRNA genes
Gênes

G?nes is the name of a d?partement in France of the First French Empire in present Italy. It was named after the city Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa....
 in their genome
Genome

In classical genetics, the genome of a diploid organism including eukarya refers to a full set of chromosomes or genes in a gamete; thereby, a regular somatic cell contains two full sets of genomes....
. The nematode
Nematode

The "roundworms" or "nematodes" are the most diverse phylum of body cavity, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasite....
 worm C. elegans
Caenorhabditis elegans

'Caenorhabditis elegans' is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular biology and developmental biology of C....
, a commonly used model organism in genetics
Genetics

Genetics , a discipline of biology, is the science of heredity and Genetic variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding....
 studies, has 29,647 genes in its nuclear
Cell nucleus

In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in all eukaryote cell ....
 genome, of which 620 code for tRNA. The budding yeast Saccharomyces cerevisiae
Saccharomyces cerevisiae

Saccharomyces cerevisiae is a species of budding yeast. It is perhaps the most useful yeast owing to its use since ancient times in baking and brewing....
 has 275 tRNA genes in its genome. In the human genome, which according to current estimates has about 27,161 genes in total, there are about 4,421 non-coding RNA
Non-coding RNA

A non-coding RNA is a functional RNA molecule that is not Translation into a protein. Less-frequently used synonyms are non-protein-coding RNA , non-messenger RNA , small non-messenger RNA , functional RNA ....
 genes, which include tRNA genes. There are 22 mitochondrial tRNA genes; 497 nuclear genes encoding cytoplasmic tRNA molecules and there are 324 tRNA-derived putative pseudogenes.

Cytoplasmic tRNA genes can be grouped into 49 families according to their anticodon features. These genes are found on all chromosomes, except 22 and Y chromosome. High clustering on 6p is observed (140 tRNA genes), as well on 1 chromosome.

tRNA molecules are transcribed (in eukaryotic cells) by RNA polymerase III
RNA polymerase III

RNA polymerase III Transcription DNA to synthesize ribosomal 5S rRNA, tRNA and other small RNAs. The genes transcribed by RNA Pol III fall in the category of "housekeeping" genes whose expression is required in all cell types and most environmental conditions....
, unlike messenger RNA
Messenger RNA

Messenger ribonucleic acid is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcription from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes....
 which is transcribed by RNA polymerase II
RNA polymerase II

RNA polymerase II is an enzyme found in eukaryotic cells. It catalyzes the Transcription of DNA to synthesize precursors of mRNA and most snRNA and microRNA....
. pre-tRNAs contain introns; in bacteria these self-splice
Splicing (genetics)

In molecular biology, splicing is a modification of an RNA after transcription , in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation ....
, whereas in eukaryotes and archaea
Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
 they are removed by tRNA splicing endonuclease
Endonuclease

Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain, in contrast to exonucleases, which cleave phosphodiester bonds at the end of a polynucleotide chain....
.

History

The existence of tRNA was first hypothesized by Francis Crick
Francis Crick

Francis Harry Compton Crick Order of Merit Royal Society , Ph.D., was a British molecular biology, physics, and neuroscience, and most noted for being one of the co-discoverers of the structure of the DNA molecule in 1953....
, based on the assumption that there must exist an adapter molecule capable of mediating the translation of the RNA alphabet into the protein alphabet. Significant research on structure was conducted in the early 1960s by Alex Rich and Don Caspar, two researchers in Boston, the Jacques Fresco group in Princeton University
Princeton University

Princeton University is a private university university located in Princeton, New Jersey, New Jersey, United States. The school is one of the eight universities of the Ivy League and has the largest per-student Financial endowment in the world....
 and a United Kingdom
United Kingdom

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom , the UK or Britain,is a sovereign state located off the northwestern coast of continental Europe....
 group at King's College London
King's College London

King's College London is a United Kingdom higher education institution and co-founding constituent college of the University of London. Founded by George IV of the United Kingdom and the Arthur Wellesley, 1st Duke of Wellington in 1829, its royal charter is predated, in England, only by those of the Universities of University of Oxford and Un...
. A later publication reported the primary structure in 1965 by Robert W. Holley
Robert W. Holley

Robert William Holley was an American biochemist, he was awarded the Nobel Prize in Physiology or Medicine in 1968 for describing the structure of alanine tRNA, linking DNA and protein synthesis....
. The secondary and tertiary structures were derived from X-ray crystallography
X-ray crystallography

X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and scatters into many different directions....
 studies reported independently in 1974 by American
Kim Sung-Hou

Kim Sung-Hou Ph.D. is a Korean-born American X-ray crystallography and protein biophysicist. Dr. Kim reported first 3D structure of tRNA with Alexander Rich in 1973....
 and British research groups headed, respectively, by Alexander Rich and Aaron Klug
Aaron Klug

Sir Aaron Klug, Order of Merit, President of the Royal Society is a Lithuanian-born United Kingdom chemist and biophysicist, and winner of the 1982 Nobel Prize in Chemistry for his development of electron crystallography and his structural elucidation of biologically important nucleic acid-protein complexes....
.

See also

  • mRNA
  • tmRNA
    TmRNA

    tmRNA stands for transfer-messenger-RNA. The gene encoding the tmRNA is ssrA. It is found in all bacterial genomes that have been sequenced, and is an important part of translation regulation....
  • non-coding RNA
    Non-coding RNA

    A non-coding RNA is a functional RNA molecule that is not Translation into a protein. Less-frequently used synonyms are non-protein-coding RNA , non-messenger RNA , small non-messenger RNA , functional RNA ....
     and intron
    Intron

    Introns, derived from the term "intragenic regions" and also called intervening sequence , are DNA regions in a gene that are not translated into proteins....
    s
  • translation
    Translation (genetics)

    Translation is the first stage of protein biosynthesis . Translation is the production of proteins by decoding mRNA produced in Transcription ....
  • Kim Sung-Hou
    Kim Sung-Hou

    Kim Sung-Hou Ph.D. is a Korean-born American X-ray crystallography and protein biophysicist. Dr. Kim reported first 3D structure of tRNA with Alexander Rich in 1973....
  • wobble hypothesis


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