Translation

Not all AUG codons serve as the site of initiation. Most AUGs are intended to code for methionines within the polypeptide chain. Therefore, in addition to the methionine-bearing initiator tRNA, another set of methionine-specific tRNAs are used for these internal AUG codons. The ribosome must be able to distinguish between these two kinds of AUG codons. In bacteria, additional information contained within the mRNA sequence immediately before the intended initiating AUG, called a Shine-Dalgarno sequence, helps the ribosome to recognize where it should start translating. Any AUG sequences on the 5′ side of the initiation codon are ignored. In eukaryotic cells, a different strategy is used to recognize the initiating AUG codon. The mRNA contains a special structure at its 5′ end, which helps the ribosome to attach and then to scan down the RNA molecule until it reaches the first AUG triplet. In bacteria and eukaryotes, AUG codons encountered during translation after initiation are recognized by a non-initiator methionine-bearing tRNA.