Start codon

The Central dogma of molecular biology describes the process of translation of a gene to a protein. Specific sequences of DNA act as a template to synthesize mRNA.

The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and a modified Met (fMet) in prokaryotes. The most common start codon is AUG.

The start codon is often preceded by an untranslated region 5' UTR. In prokaryotes this includes the ribosome binding site.

Alternate start codons (non AUG) are very rare in eukaryotic genomes. Mitochondrial genomes and prokaryotes use alternate start codons more significantly (mainly GUG and UUG). For example E. coli uses 83% AUG (3542/4284), 14% (612) GUG, 3% (103) UUG ^[1] and one or two others (e.g., an AUU and possibly a CUG).^[2][3] Bioinformatics programs usually allow for alternate start codons when searching for protein coding genes.^{[citation needed]}

Note that these alternate start codons are still translated as Met when they are at the start of a protein (even if the codon encodes a different amino acid otherwise). This is because a separate transfer RNA (tRNA) is used for initiation.

Well-known coding regions that do not have AUG initiation codons are those of lacI (GUG)^[4][5] and lacA (UUG)^[6] in the E. coli lac operon.

^A The codon AUG both codes for methionine and serves as an initiation site: the first AUG in an mRNA's coding region is where translation into protein begins.^[7]

See also

• Stop codon
• Codon
• Transfer RNA
• Central Dogma of Molecular Biology

External links

• The Genetic Codes. Compiled by Andrzej (Anjay) Elzanowski and Jim Ostell, National Center for Biotechnology Information (NCBI), Bethesda, Maryland, U.S.A.

References

1. ↑ Blattner, F. R.; Plunkett g, G.; Bloch, C. A.; Perna, N. T.; Burland, V.; Riley, M.; Collado-Vides, J.; Glasner, J. D.; Rode, C. K.; Mayhew, G. F.; Gregor, J.; Davis, N. W.; Kirkpatrick, H. A.; Goeden, M. A.; Rose, D. J.; Mau, B.; Shao, Y. (1997). "The Complete Genome Sequence of Escherichia coli K-12". Science 277 (5331): 1453–1462. doi:10.1126/science.277.5331.1453. PMID 9278503. 
2. ↑ Sacerdot, C.; Fayat, G.; Dessen, P.; Springer, M.; Plumbridge, J. A.; Grunberg-Manago, M.; Blanquet, S. (1982). "Sequence of a 1.26-kb DNA fragment containing the structural gene for E.coli initiation factor IF3: Presence of an AUU initiator codon". The EMBO journal 1 (3): 311–315. PMC 553041. PMID 6325158. 
3. ↑ Missiakas, D.; Georgopoulos, C.; Raina, S. (1993). "The Escherichia coli heat shock gene htpY: Mutational analysis, cloning, sequencing, and transcriptional regulation". Journal of bacteriology 175 (9): 2613–2624. PMC 204563. PMID 8478327. 
4. ↑ E.coli lactose operon with lacI, lacZ, lacY and lacA genes GenBank: J01636.1
5. ↑ Farabaugh, P. J. (1978). "Sequence of the lacI gene". Nature 274 (5673): 765–769. doi:10.1038/274765a0. PMID 355891. 
6. ↑ NCBI Sequence Viewer v2.0
7. ↑ Nakamoto T (March 2009). "Evolution and the universality of the mechanism of initiation of protein synthesis". Gene 432 (1–2): 1–6. doi:10.1016/j.gene.2008.11.001. PMID 19056476.