A novel mechanism for the very first step in polypeptide
elongation on the ribosome has recently been proposed by Simonson
and Lake (2002). According to their transorientation hypothesis,
the incoming aminoacyl-transfer RNA (aa-tRNA) initially binds
to the messenger RNA (mRNA) with the anticodon stacked on the
5′ side of the anticodon loop; then, following GTP
hydrolysis, the tRNA swings into the A-site by switching from
the 5′-stacked conformation to the classic 3′-stacked
geometry while maintaining Watson–Crick base pairing with
the mRNA. There are several serious problems with this proposal.