Dear Theresa,
a nicely written explanation from Wikipedia : The Central dogma of molecular biology<http://en.wikipedia.org/wiki/Central_dogma_of_molecular_biology> describes the process of translation<http://en.wikipedia.org/wiki/Translation_(biology)> of a gene<http://en.wikipedia.org/wiki/Gene> to a protein<http://en.wikipedia.org/wiki/Protein>. Specific sequences of DNA<http://en.wikipedia.org/wiki/DNA> act as a template to synthesize mRNA. The start codon<http://en.wikipedia.org/wiki/Codon> is the first codon of a messenger RNA<http://en.wikipedia.org/wiki/Messenger_RNA> (mRNA) transcript translated by a ribosome<http://en.wikipedia.org/wiki/Ribosome>. The start codon always codes for methionine<http://en.wikipedia.org/wiki/Methionine> in eukaryotes<http://en.wikipedia.org/wiki/Eukaryote> and a modified Met (fMet) in prokaryotes<http://en.wikipedia.org/wiki/Prokaryotes>. The most common start codon is AUG. The start codon is almost always preceded by an untranslated region 5' UTR<http://en.wikipedia.org/wiki/5%27_UTR>. In prokaryotes<http://en.wikipedia.org/wiki/Prokaryotes> this includes the ribosome binding site. Alternate start codons (non ATG) are very rare in eukaryotic genomes. Mitochondrial genomes and prokaryotes use alternate start codons more significantly (mainly GUG and UUG). For example E. coli<http://en.wikipedia.org/wiki/E._coli> uses 83% ATG (AUG) (3542/4284), 14% (612) GTG (GUG), 3% (103) TTG (UUG) [1]<http://en.wikipedia.org/wiki/Start_codon#cite_note-1> and one or two others (e.g., an ATT and possibly a CTG).[2]<http://en.wikipedia.org/wiki/Start_codon#cite_note-Sequence_of_a_1.26-kb_DNA_fragment_containing_the_structural_gene_for_E.coli_initiation_factor_IF3:_presence_of_an_AUU_initiator_codon-2>[3]<http://en.wikipedia.org/wiki/Start_codon#cite_note-The_Escherichia_coli_heat_shock_gene_htpY:_mutational_analysis.2C_cloning.2C_sequencing.2C_and_transcriptional_regulation.-3> Bioinformatics programs usually allow for alternate start codons when searching for protein coding genes. 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<http://en.wikipedia.org/wiki/Transfer_RNA> (tRNA) is used for initiation. Well-known coding regions that do not have ATG initiation codons are those of lacI (GTG)[4]<http://en.wikipedia.org/wiki/Start_codon#cite_note-4>[5]<http://en.wikipedia.org/wiki/Start_codon#cite_note-Sequence_of_the_lacI_gene.-5> and lacA (TTG)[6]<http://en.wikipedia.org/wiki/Start_codon#cite_note-6> in the E. coli<http://en.wikipedia.org/wiki/E._coli> lac operon<http://en.wikipedia.org/wiki/Lac_operon>. Hope this helps, Anat. ========================================================================= Anat Bashan , Ph.D Tel:972-8-9344289 @The Ribosome Group The Weizmann Institute of Science Fax:972-8-9344154 The Department of Structural Biology Mobile:972-52-3347229 Rehovot 76100 e-mail: [email protected]<mailto:[email protected]> Israel ========================================================================= Dear all I have a somewhat philosophical question. Why do all protein sequences start with a methionine (not referring to mature/processed form)? What is so special about methionine and cannot be replaced by other amino acids? Second, how does the ribosome know the first start codon is for methionine when the codon is not AUG? This is about the alternative start codons like GUG. Thank you. Theresa
