Start codon

The start codon is generally defined as the point, sequence, at which a ribosome begins to translate a sequence of RNA into amino acids.

When an RNA transcript is "read" from the 5' carbon to the 3' carbon by the ribosome the start codon (AUG) is the first codon on which the tRNA bound to Met, methionine, and ribosomal subunits attach. ATG and AUG denote sequences of DNA and RNA, respectively, that are the start codon or initiation codon encoding the amino acid methionine (Met) in eukaryotes and a modified Met (fMet) in prokaryotes.

The principle called 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 in a process termed "transcription" in the nucleus. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called "translation."

Three nucleotide bases specify one amino acid in the genetic code, a mapping encoded in the tRNA of the organism. The first three bases of the coding sequence (CDS) of mRNA to be translated into protein are called a start codon or initiation codon. The start codon is almost always preceded by an untranslated region 5' UTR.

The start codon is typically AUG (or ATG in DNA; this also encodes methionine). Very rarely in higher organisms (eukaryotes) are non AUG start codons used.

In addition to AUG, alternative start codons, mainly GUG and UUG are used in prokaryotes. For example E. coli uses 83% ATG (AUG), 14% GTG (GUG), 3% TTG (UUG) and one or two others (e.g., ATT and CTG).

Interestingly when an alternative start codon is used, it gets translated as methionine - even if the codon might normally encode a different amino acid. This is because a separate tRNA is used for initiation.

Well known coding regions that do not have ATG initiation codons are those of lacI (GTG) and lacA (TTG) in the E. coli lac operon.