Start codon

from Wikipedia, the free encyclopedia

The first codon of an open reading frame (ORF) on the mRNA is referred to as the start codon or initiator codon . In general, this is the RNA - base triplet of the codon , which in eukaryotes for the amino acid methionine encoded (Met) or a modified methionine (N-formylmethionine, fMet) in bacteria . The start codon is almost always preceded by an untranslated region ( 5'-UTR ). AUG

In the genetic code , the specific sequence of three nucleobases codes for an amino acid. Usually, the first base triplet of the coding sequence of the mRNA that is translated into the amino acid of a peptide is . This triple codes for methionine, and therefore the nascent polypeptide chain of a native protein begins with Met or fMet as the first amino acid. For the initiation of translation is a special initiator tRNA used as a tRNA i Met in archaea and eukaryotes or a tRNA i fMet in mitochondria and bacteria. AUG

After translation , a protein can be modified ( post-translational modification ). For example, it can be a precursor protein , part of which is subsequently split off enzymatically so that the activated form of a protein is created, for example in the case of insulin . The polypeptide chain of a modified protein does not then have to begin with the amino acid methionine. It is also possible to split off only the methionine at the N terminus or its formyl radical .

Alternative start codons

In eukaryotes, the start codon used almost exclusively is the codon AUGthat codes for the amino acid methionine. In addition, in some cases, the codons ACG, CUGor GUGbe interpreted as a start codon, unless special secondary structures in the sequence context of the mRNA permit.

In prokaryotes, AUGalternative start codons, mainly GUGand UUG, can code for the first amino acid formylmethionine. For example, Escherichia coli uses the codons AUG(77%), GUG(14%), UUG(8%) and a few others as start codons by binding a special initiator fMet-tRNA, although in the further course of elongation , GUGfor valine and UUGfor Encode leucine. Well-known examples of genes that do not have AUGa start codon are lacI ( GUG) and lacA ( UUG) in the lac operon of E. coli .

literature

  • Rolf Knippers: Molecular Genetics. 8th revised edition. Georg Thieme Verlag, Stuttgart et al. 2001, ISBN 3-13-477008-3 .

Individual evidence

  1. ↑ C. Touriol, S. Bornes, S. Bonnal, S. Audigier, H. Prats, AC Prats, S. Vagner: Generation of protein isoform diversity by alternative initiation of translation at non-AUG codons. In: Biology of the cell. Volume 95, Numbers 3-4, 2003 May-Jun, pp. 169-178, PMID 12867081 (Review).