Increased ribosomal affinity for mRNA causes resistance to edeine in a mutant of Saccharomyces cerevisiae

The effect of edeine on the translation of mRNA or poly(U)‐directed polyphenylalanine synthesis has been studied in an edeine‐resistant mutant of Saccharomyces cerevisiae under three different experimental conditions: in the whole lysate system, in a micrococcal‐nuclease‐treated lysate, and in a high‐salt‐treated lysate. The results indicate that translation of messenger is more resistant to edeine in the whole lysate than in the depleted lysates: these observations suggest that resistance to edeine is associated with the presence of endogenous mRNA. It is shown that 40S mutant subunits have a higher affinity for polysomal RNA than 40S wild‐type subunits. Since the mRNA binding is inhibited by 7‐methylguanosine 5′‐monophosphate, the interaction between polysomal RNA and 40S ribosomes is specific for mRNA. The data demonstrate that in each of the depleted lysates, with edeine initially present, the formation of the 80S initiation complex is inhibited. However, edeine inhibition of [ 3 H]methionine binding to 80S ribosomes is overcome completely in the mutant extract by preincubation of this lysate with polysomal RNA. The results indicate that the mutant may carry a specific change in a messenger‐binding factor or in a ribosomal protein thereby permitting an increased stability of the messenger‐ribosome complex which consequently results in an increased resistance of the mutant lysate to edeine.