Anticodon

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Base pairing of the anticodon (red) of a tRNA Ala with the codon of GCCan mRNA that codes for an alanine
A tRNA Ala from S. cerevisiae -
highlighted in red, the triplet inosine - guanosine - cytidine in the 5 ′ → 3 ′ noted nucleotide sequence (besides I at position 34 of the anticodon also methylated inosine (m 1 I) at position 37)
A human tRNA i Met -
the anticodon cytosine - adenine - uracil of the 5 ′ → 3 ′ noted base sequence highlighted in red ; it pairs with the codon AUGas the start codon

An anticodon consists of the three nucleotides of a tRNA which, as a counterpart, correspond to the three nucleobases of the codon of an mRNA .

Exposed on the short RNA section of the anticodon loop of a tRNA molecule are three consecutive nucleotides, the base sequence of which represents the characteristic anticodon. With this triplet, the tRNA attaches base-pairing to the base triplet of a corresponding codon of the mRNA during translation on the ribosome in protein biosynthesis .

If, for example, the triplet is present GCCas a codon on the mRNA , a tRNA can bind to it via three complementary base pairs with the triplet CGG , as an anticodon (noted in 5 '→ 3' direction: G -G- C ). If this tRNA is loaded with alanine , this amino acid is linked to the peptide chain that forms on the ribosome .

A specific amino acid is assigned to a codon of the mRNA via the anticodon of a tRNA. The assignment between codon and amino acid is called the genetic code . The intermediaries here are the tRNA molecules, which have a specific anticodon on the one hand and a specific amino acid on the other. By means of the base pairing between anticodon and codon, the coding base sequence of a polynucleotide such as mRNA is read and translated into the amino acid sequence of a polypeptide , the primary structure of a protein .

Different tRNA molecules are necessary for the synthesis of proteins in a cell. Differences between the tRNA species consist on the one hand in the specific amino acid with which they are loaded by specific enzymes - the aminoacyl-tRNA synthetases - and on the other hand in their anticodon. This recognition region represents the matching counterpart to a codon as a sequence of three bases. But it is not always necessary that all three bases each form complementary pairs with those of a codon triplet; Occasionally two pairings are enough for the correct assignment of the respective amino acid to its codon.

For example, alanine encode four codons ( GCU, GCC, GCA, GCG), all and in the first and the second position is equal at the same time distinguished from all other 60 codons. This case is not unusual, but is similar for other amino acids (glycine, proline, threonine, valine, etc.). Under these circumstances, the appropriate assignment is made even if the pairing is only complementary for the first two bases, but somewhat wobbly for the 3rd position of the codon.

So some anticodons can recognize more than just one codon, e.g. B. the anticodon 3'-CGG-5 ' as GCCwell as GCU. After deducting the three stop codons, the standard genetic code contains 4 3 - 3 = 61 different codons. The number of tRNA types in a cell is often significantly lower. From this, Francis Crick concluded as early as 1966 that certain inaccurate matches of codon and anticodon must be sufficient for the function of the tRNA in protein synthesis, and called his conjecture the wobble hypothesis (from English wobble , `` wiggle '').

All organisms contain tRNA genes ( DNA ) whose transcripts (RNA) are transformed into mature tRNA molecules through certain modifications ( post-transcriptional modification ). If the bases of the anticodon loop are also modified, this changes the base pairing potential of their anticodon. For example, in many eukaryotic and prokaryotic cells nucleobase adenine by special enzymes deaminated will hypoxanthine , making the nucleoside adenosine (A) to inosine is converted to (I) ( RNA-editing ). If this concerns the anticodon 3'-CGA-5 'of a tRNA Ala [AGC], which is GCUcomplementary to the codon , an anticodon 3'-CGI-5' is created , which can now form wobble pairings with GCU, GCCand also GCA.

In this way, the possible base pairings of the anticodon of an aminoacyl-tRNA with regard to the third base of a codon can be extended to those which code for the same amino acid. However, restrictions are also necessary to ensure correct reading of the other two codon bases and to exclude mismatches. In particular, the bases of the anticodon loop flanking the anticodon triplet contribute to fine-tuning the translational accuracy through certain sequence elements, which can also be understood as a development process.

The genome in the nucleus of a human cell contains tRNA genes in more than 600 locations , distributed over almost all chromosomes (for example, over thirty partly different ones are known for a tRNA Ala [AGC] and 1 for a tRNA Ala [GGC]). With regard to the respective tRNA anticodon, these genomic base sequences represent almost all - with the exception of six - variants of possible triplets (from A, G, C and T ). The DNA base triplet CAT is of particular importance here, which is transcribed into the anticodon CAU of a tRNA, which can pair 3'-UAC-5 ' with the codon AUG. This is because this codes for the amino acid methionine and, by binding a special tRNA i Met as initiator tRNA, can serve as a start codon with which translation then begins.

literature

B. Alberts, A. Johnson, J. Lewis et al .: Molecular Biology of the Cell. 4th edition. Garland Science, New York 2002, Chapter: From RNA to Protein . Online on the NCBI bookshelf

Individual evidence

  1. ↑ B. Mims, N. Prather, E. Murgola: Isolation and nucleotide sequence analysis of tRNAAlaGGC from Escherichia coli K-12. In: Journal of Bacteriology. 162 (2), May 1985, p. 837; PMC 218931 (free full text).
  2. ^ FHC Crick: Codon-anticodon pairing: the wobble hypothesis. In: J. Mol. Biol. Volume 19, Number 2, 1966, pp. 548-555 ( PMID 5969078 ; PDF ).
  3. ^ W. Zhou, D. Karcher, R. Bock: Importance of adenosine-to-inosine editing adjacent to the anticodon in an Arabidopsis alanine tRNA under environmental stress. In: Nucleic Acids Research. 41 (5), January 2013, p. 3363; PMC 3597679 (free full text).
  4. ↑ S. Ledoux, M. Olejniczak, O. Uhlenbeck: A sequence element that tunes Escherichia coli tRNA (Ala) (GGC) to ensure accurate decoding. In: Nature Structural & Molecular Biology. 16 (4), April 2009, pp. 359-364; PMC 2769084 (free full text).
  5. ↑ I. Shepotinovskaya, O. Uhlenbeck: tRNA residues evolved to promote translational accuracy. In: RNA. 19 (4), April 2013, pp. 510-516; PMC 3677261 (free full text).
  6. ↑ see entries for Homo sapiens in the genomic tRNA database (GtRNAdb).