Sequence-context dependency shown by eIF5^G31Rmutant for UUG start codon selection inSaccharomyces cerevisiae

Abstract

In higher eukaryotes, the efficiency of start codon selection depends upon its sequence context A/GxxAUGG, especially at the -3 and +4 position. However, S. cerevisiae prefers AAA/UAUGU sequence context surrounding the AUG start codon. Furthermore, it was demonstrated that the first AUG codon on the mRNA serves as a signal for ribosomal recognition of the initiation site selection. In yeast, the G31R mutation in eIF5 shows Suppressor of initiation codon (Sui) phenotype enabling 40S ribosome to initiate at UUG start codon. To understand whether the UUG codon recognition by the eIF5G31R mutant is sensitive to the sequence context, we made different HIS4-LacZ reporter constructs with UUG as start codon and changed the sequence context at -3 and -2 positions. The HIS4UUG-LacZ transcripts that carry purine (A/G) at the -3-position showed better β-Galactosidase activity than pyrimidine (U/C). Furthermore, on changing the -1 position for the AAA context, we discovered purines are more preferred at the -1 position for the UUG start codon selection.