![]() While the +4 and the −3 positions in the Kozak sequence have the greatest relative importance in the establishing a favorable initiation context a CC or AA motif at −2 and −1 were found to be important in the initiation of translation in tobacco and maize plants. There is also evidence that a G in the -6 position is important in the initiation of translation. The cc at −1 and −2 are not as conserved, but contribute to the overall strength. An 'adequate' consensus has only 1 of these sites, while a 'weak' consensus has neither. either A or G in the consensus) relative to the +1 nucleotide must both match the consensus (there is no 0 position). For a 'strong' consensus, the nucleotides at positions +4 (i.e. The A nucleotide of the "AUG" is delineated as +1 in mRNA sequences with the preceding base being labeled as −1. Kozak sequence strength refers to the favorability of initiation, affecting how much protein is synthesized from a given mRNA. Variation within the Kozak sequence alters the "strength" thereof. (Rarely, GUG is used as an initiation codon, but methionine is still the first amino acid as it is the met-tRNA in the initiation complex that binds to the mRNA). The AUG is the initiation codon encoding a methionine amino acid at the N-terminus of the protein. the sequence in parentheses (gcc) is of uncertain significance.a lower-case letter denotes the most common base at a position where the base can nevertheless vary.'R' indicates that a purine ( adenine or guanine) is always observed at this position (with adenine being more frequent according to Kozak).the 'AUGG' sequence is constant or rarely, if ever, changes. upper-case letters indicate highly conserved bases, i.e.The underlined nucleotides indicate the translation start codon, coding for Methionine.The sequence was defined as 5'- (gcc)gccRcc AUGG-3' (IUPAC nucleobase notation summarized here) where: human, cow, cat, dog, chicken, guinea pig, hamster, mouse, pig, rabbit, sheep, and Xenopus), subsequent studies confirmed its conservation in higher eukaryotes generally. While initially limited to a subset of vertebrates ( i.e. The Kozak sequence was determined by sequencing of 699 vertebrate mRNAs and verified by site-directed mutagenesis. 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). Kozak discovered the sequence through a detailed analysis of DNA genomic sequences. The sequence was named after the scientist who discovered it, Marilyn Kozak. As it has become more studied, expansions of the nucleotide sequence, bases of importance, and notable exceptions have arisen. A wrong start site can result in non-functional proteins. It ensures that a protein is correctly translated from the genetic message, mediating ribosome assembly and translation initiation. Regarded as the optimum sequence for initiating translation in eukaryotes, the sequence is an integral aspect of protein regulation and overall cellular health as well as having implications in human disease. Furthermore, our results provide an experimental approach to testing and optimizing an important tool for molecular biology.The Kozak consensus sequence ( Kozak consensus or Kozak sequence) is a nucleic acid motif that functions as the protein translation initiation site in most eukaryotic mRNA transcripts. We conclude that the canonical Kozak sequence is a poor predictor of translation efficiency in different model organisms. Rather, the most frequent natural variation of the Kozak sequence is almost twice as efficient. Our data reveal that while the canonical Kozak sequence is efficient, in zebrafish it is neither the most common nor the most efficient translation initiation sequence. Combining bioinformatics analysis with molecular biology techniques, and using zebrafish as a test case, we identify Kozak sequences based on their natural variance and characterize their function in vivo. Despite the critical importance of the Kozak sequence to regulation of gene expression, there have been no correlation studies between its natural variance and efficiency of translation. In eukaryotes, targeting the small ribosomal subunit to the mRNA transcript requires a Kozak sequence at the translation initiation site. ![]()
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