Yeast ribosomal protein L24 affects the kinetics of protein synthesis and ribosomal protein L39 improves translational accuracy, while mutants lacking both remain viable

Four mutant strains from Saccharomyces cerevisiae were used to study riboso me structure and function. They included a strain carrying deletions of the two genes encoding ribosomal protein L24, a strain carrying a mutation spb 2 in the gene for ribosomal protein L39, a strain carrying a deletion of th e gene for L39, and a mutant lacking both L24 and L39. The mutant lacking o nly L24 showed just 25% of the normal polyphenylalanine-synthesizing activi ty followed by a decrease in P-site binding, suggesting the possibility tha t protein L24 is involved in the kinetics of translation, Each of the two L 39 mutants displayed a 4-fold increase of their error frequencies over the wild type. This was accompanied by a substantial increase in A-site binding , typical of error-prone mutants. The absence of L39 also increased sensiti vity to paromomycin, decreased the ribosomal subunit ratio, and caused a co ld-sensitive phenotype. Mutant cells lacking both ribosomal proteins remain ed viable. Their ribosomes showed reduced initial rates caused by the absen ce of L24 but a normal extent of polyphenylalanine synthesis and a substant ial in vivo reduction in the amount of 80S ribosomes compared to wild type. Moreover, this mutant displayed decreased translational accuracy, hypersen sitivity to the antibiotic paromomycin, and a cold-sensitive phenotype, all caused mainly by the deletion of L39. Protein L39 is the first protein of the 60S ribosomal subunit implicated in translational accuracy.