About Start and Stop Codons

The codon AUG is called the START codon as it the first codon in the transcribed mRNA that undergoes translation. AUG is the most common START codon and it codes for the amino acid methionine (Met) in eukaryotes and formyl methionine (fMet) in prokaryotes. During protein synthesis, the tRNA recognizes the START codon AUG with the help of some initiation factors and starts translation of mRNA. Some alternative START codons are found in both eukaryotes and prokaryotes. Alternate codons usually code for amino acids other than methionine, but when they act as START codons they code for Met due to the use of a separate initiator tRNA. Non-AUG START codons are rarely found in eukaryotic genomes. Apart from the usual Met codon, mammalian cells can also START translation with the amino acid leucine with the help of a leucyl-tRNA decoding the CUG codon. Mitochondrial genomes use AUA and AUU in humans and GUG and UUG in prokaryotes as alternate START codons. In prokaryotes, E. coli is found to use AUG 83%, GUG 14%, and UUG 3% as START codons. The lacA and lacI coding regions in the E coli lac operon don’t have AUG START codon and instead use UUG and GUG as initiation codons respectively.