Kozak consensus sequence

Jump to: navigation, search

The Kozak consensus sequence, Kozak consensus or Kozak sequence, is a sequence which occurs on eukaryotic mRNA and has the consensus (gcc)gccRccAUGG, where R is a purine (adenine or guanine) three bases upstream of the start codon (AUG), which is followed by another 'G' (Kozak, 1987).

This sequence on an mRNA molecule is recognized by the ribosome as the translational start site, from which point a protein is coded by that mRNA molecule. The ribosome requires this sequence, or a possible variation (see below) to initiate translation. The Kozak sequence is not to be confused with the ribosomal binding site (RBS), that being either the 5' cap of a messenger RNA or an Internal Ribosome Entry Site (IRES).

In vivo, this site is often not matched exactly on different mRNAs and the amount of protein synthesized from a given mRNA is dependent on the strength of the Kozak sequence. Some nucleotides in this sequence are more important than others: the AUG is essential since it is the actual initiation codon encoding a methionine amino acid at the N-terminus of the protein. The A nucleotide of the "AUG" is referred to as number 1. For a 'strong' consensus, the nucleotides at positions +4 (i.e. G in the consensus) and -3 (i.e. either A or G in the consensus) relative to the number 1 nucleotide must both match the consensus (there is no number 0 position). An 'adequate' consensus has only 1 of these sites, while a 'weak' consensus has neither. The cc at -5 and -2 are not as conserved, but contribute to the overall strength.

There are examples in vivo of each of these types of Kozak consensus, and they probably evolved as yet another mechanism of gene regulation. Lmx1b is an example of a gene with a weak Kozak consensus sequence. For initiation of translation from such a site, other features are required in the mRNA sequence in order for the ribosome to recognize the initiation codon.

File:KozakConsensus.jpg
A sequence logo showing the most conserved bases around the initiation codon from all human mRNAs. Note: The -4 in the image should read -3.

References

  • Kozak, M. (1984) Nature, 308:241-246. [1] PMID 6700727
  • Kozak, M. (1986) Point mutations define a sequence flanking the AUG initiator codon that modulates translation by eukaryotic ribosomes. Cell. 44(2):283-92. [2] PMID 3943125
  • Kozak, M. (1987) An analysis of 5'-noncoding sequences from 699 vertebrate messenger RNAs. Nucleic Acids Res. 15(20):8125-48 [3] PMID 3313277
  • Kozak, M. (1990) Downstream secondary structure facilitates recognition of initiator codons by eukaryotic ribosomes.
  • Kozak, M. (1991) An analysis of vertebrate mRNA sequences: Intimations of translational control, J. Cell. Biol. 115, 887–903. [[4]]
  • Dunston, JA et al. (2004) The human LMX1B gene: transcription unit, promoter, and pathogenic mutations. Genomics. 2004 Sep;84(3):565-76.

See also

it:Sequenza di Kozak



Linked-in.jpg