In order for translation to begin, a codon must be selected for initiation. We now know that the codon is AUG, but why AUG? Eukaryotic initiation has been known to begin with methionine and Donahue, Cigan and his partners have shown that the transfer ribonucleic acid that initiates translation carries an unformylated methionine, referred to as initiator tRNA-Met capable of binding to its designated codon, AUG (Cigan et al., 1988).
To discover the role of initiator tRNAs, Cigan, Donahue and colleagues altered the anticodon of one of the yeast tRNAiMets to 3'-UCC-5', which would recognize the codon AGG, not AUG. They then introduced the gene encoding this changed tRNA to histidine negative cells, prior to changing the initiation codon of the histidine gene to several versions, with one being AGG. When the cells were grown without histidine, initiation codons of the AGG type could base-pair with the UCC codon on the initiator tRNA and support growth because of the presence of translation. None of the other codon versions were capable of growth because they could not pair with the UCC anticodon. With these results in hand, Cigan and colleagues further placed a second AGG initiation site upstream of the first AGG, but out-of-frame. No growth occured in this instance, as formation of an initiation complex with the UCC initiator tRNA would occur at the first AGG and lead to premature termination by encountering a stop codon. Therefore, the initiator tRNA selects the appropriate codon on which to begin (Cigan et al., 1988; Weaver 1999). However, how the preinitiation complex finds the AUG start codon is a process of great complexity.