Fig. 9.

Specific domains or residues in eIF1, eIF1A and eIF2β discriminate against both poor AUG context and non-AUG start codons to maintain optimal initiation accuracy. In the WT, these initiation factors promote recognition of AUG start codons in optimal context (⁻³AAA) and impede recognition of initiation sites with poor context, such as the ⁻³CGU sequence at SUI1, or with a non-AUG start codon such as UUG. Sui⁻ mutations affecting any of these factors render initiation less accurate by diminishing antagonism of start sites with either poor context or a non-AUG start codon. Ssu⁻ mutations affecting eIF1 or eIF1A render initiation hyperaccurate by increasing antagonism of start sites with either poor context or a non-AUG start codon.