Cycloheximide resistance can be mediated through either ribosomal subunit.

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Abstract
Two cycloheximide-resistant mutants of Tetrahymena thermophila were analyzed to determine the site of their cycloheximide resistance. Mutations in both strains were previously shown to be genetically dominant and located at separate loci (denoted Chx-A and Chx-B). Strains carrying these mutations were readily distinguished by the extent to which they were resistant to the drug. The homozygous double mutant was more resistant than either single mutant. Cell-free extracts of wild type and of the 3 mutant strains, assayed for protein synthetic activity by runoff of natural mRNA and poly(U)-dependent phenylalanine polymerization, demonstrated that in vitro the mutants were all more resistant than the wild type. Further fractionation of the cell-free systems into ribosomes and supernates localized cycloheximide resistance to the ribosome for both Chx-A and Chx-B homozygotes. Ribosome dissociation and pairwise subunit mixing in the in vitro system indicated that ribosome resistance was conferred by the 60S subunit from 1 strain whereas resistance in the other strain was mediated through the 40S subunit. This was further confirmed by reconstruction of all 4 cycloheximide-resistance phenotypes by mixing ribosomal subunits from appropriate strains. The mechanisms by which these mutations confer resistance to cycloheximide are probably different.