Fidelity of HIS4 start codon selection influences 3-amino-1,2,4-triazole sensitivity in GTPase activating protein function defective eIF5.

The eIF5 protein plays an important role in the fidelity of AUG start codon selection. However, the hyper GTPase eIF5G31R mutation in yeast causes preferential utilization of UUG as initiation codon and is termed as suppressor of initiation codon (Sui-) phenotype. The eIF5G31R mutant recognizes upUUG initiation codon from the 5' regulatory leader region of GCN4 transcript and dominantly represses GCN4 expression thereby conferring sensitivity to 3-amino-1,2,4-triazole (3AT)-induced starvation. The 3AT sensitivity was rescued by supplementing HIS4UUG allele. The eIF5G31R mutant has a better efficiency of UUG codon recognition from the HIS4UUG allele under starvation conditions. Moreover, the expression of HIS4UUG allele was significantly lower than the critical level causing additional derepression of GCN4 expression in eIF5G31R mutant to rescue its 3AT sensitivity. The overexpression of eIF1 improved expression of HIS4AUG allele and GCN4 transcript causing 3AT resistance, whereas overexpression of eIF1 resulted in diminished UUG codon recognition of HIS4UUG allele causing 3AT sensitivity, despite having higher GCN4 expression. This paper reports the critical role of HIS4 expression necessary in response to 3AT-induced starvation in the eIF5G31R mutant which is ostensibly not a direct target of 3AT inhibition.