Methionine-Independent Translation Initiation from Naturally Occurring Non-AUG Codons

Peter      Medveczky; Attila      Nemeth; Laszlo      Graf; Laszlo      Szilagyi

Abstract

Translational initiation sites in eukaryotic mRNAs are reached via a scanning mechanism which predicts that translation should start at the AUG codon nearest the 5 end of the mRNA. During the translational initiation in eukaryotic cell, the small (40S) ribosomal subunit binds to the 5 end of the mRNA. The 40S ribosomal subunit, carrying MettRNAi* eIF2*GTP complex and other factors, then migrates through the 5 UTR until it encounters the first AUG codon, which is recognized by base pairing with the anticodon in Met-tRNAi. Recently interest has focused on mechanisms that occasionally allow escape from this first-AUG rule. Approximately 10% of eukaryotic mRNAs skip this first AUG if it is in an unfavorable sequence context, and translation starts from the second or any downstream AUG codon. Furthermore, there is an increasing number of evidence to show that scanning translational initiation may start from a non-AUG codon that precedes or follow the first AUG codon in the mRNA. The initiation from non-AUG codons was discovered among viral genes, but newly reported examples include eukaryotic genes like those for some proto-oncogenes, transcription factor kinases and growth factors. Previously it was shown that regardless of the initiation codon used, methionine appears to be the initiating residue in all examples that have been investigated in eukaryotes. Recently few published results and our experiments showed that leucine can be translated as an initiator amino acid using CUG codon. Genome analysis generally considers the first AUG triplet in the open reading frame as the translation initiation codon. However the recent results indicate that non-AUG translation initiation may be operable more often than has been anticipated. This may have a great impact on the analysis of gene function based on genome information.

Keywords: Translation initiation, scanning ribosome, IRES, CUG codon, leucine, human trypsin 4