Hi Theresa, To add to Anat's comments: Although the AUG codon for the first methionine and all other methionines in a protein coding sequence look the same, they are read in a very different way by the ribosomal machinery. The first AUG is recognized by the initiation complex, which includes the separate small ribosomal subunit (40s), a special tRNA-methionine, and initiation factors (proteins) including eIF2. This leads to assembly of a complete ribosome and initiation of protein synthesis. Subsequently, in the process of elongation, AUG codons are read by a different tRNA, which is brought to the 80s ribosome bound to a protein called elongation factor 1a. This is an oversimplification, of course, but the point is that the initiation codon (=the first amino acid to be incorporated to the protein) is read by a special tRNA, hence the universal use of methionine.
Opher
