More surprises in translation: Initiation without the initiator tRNA

One of the most widespread beliefs in molecular biology is that protein synthesis is initiated with methionine or formylmethionine in all organisms, by using AUG as the initiation codon and a special methionine tRNA called the initiator tRNA (1, 2). Eubacteria, mitochondria, and chloroplasts initiate protein synthesis with formylmethionine, whereas archaebacteria and eukaryotic cytoplasm initiate with methionine. In eubacteria, codons such as GUG and UUG, which are related to AUG by a single base change, are occasionally used for initiation. However, these codons are read by the same initiator tRNA and are, therefore, translated as formylmethionine. In eukaryotic systems, AUG is almost exclusively the codon used for initiation. In the rare cases where ACG, CUG, AUU, and AGG are used, protein synthesis is still thought to be initiated with methionine (3–5). The first surprising exception to the universally accepted notion that protein synthesis is always initiated with methionine or formylmethionine was provided by the demonstration that protein synthesis in Escherichia coli could be initiated with codons other than AUG and amino acids other than methionine (6, 7). This work involved the use of anticodon sequence mutants of the E. coli initiator tRNA (Fig. 1). The anticodon mutation allows the mutant initiator tRNA to initiate protein synthesis by using a codon complementary to the new anticodon. Because the anticodon is, in many cases, an important determinant for recognition of the tRNA by aminoacyl-tRNA synthetases, the mutant tRNAs are often aminoacylated with different amino acids. Thus, protein synthesis could even be initiated with UAG, a termination codon, and with formylglutamine (6). Recent studies have extended some of these findings also to mammalian cells (8). Surprising and unexpected as these results were, translation of the mutant mRNAs still required an initiator tRNA, although in a mutated form, for …