Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 190

Translation initiation in the archaea is similar to that in the bacteria. Like bacteria, archaea use well-defined TIRs with leader sequences and formylmethionine for initiation of translation. In contrast, eukaryotes do not seem to use special sequence elements and instead usually use the first AUG from the 5′ end of the mRNA as the initiation codon. Sequences around this initiator AUG may also be important for its recognition, and secondary structure in the mRNA may mask other AUG sequences that could potentially be used as initiator codons. Although eukaryotes have a special methionyl tRNA that responds to the first AUG codon, called Met-tRNA_i, the methionine attached to the eukaryotic initiator tRNA is never formylated. As in bacteria, however, the first methionine is usually removed by an aminopeptidase after the protein is synthesized. Eukaryotes and archaea also seem to use many more initiation factors and elongation factors than do bacteria. The archaea use formylated methionine and S-D sequences like bacteria, but their initiation factors are more similar to those in eukaryotes. Although the exact roles of most of these initiation factors are unknown in archaea, many are obviously related to the initiation and translation factors of bacteria. It therefore appears that the mechanism of translation initiation in the archaea is a sort of hybrid between that in bacteria and that in eukaryotes.