Two different genes from Schwanniomyces occidentalis determine ribosomal resistance to cycloheximide.

Two genes (SCR1 and SCR2) encoding natural cycloheximide resistance in the budding yeast Schwanniomyces occidentalis have been cloned by expression in Saccharomyces cerevisiae. Both genes determine resistance to the inhibitory action of cycloheximide on the ribosome, SCR1 and SCR2 are present as single copies in Schwanniomyces occidentalis, where they map on chromosomes II and V, respectively. The nucleotide sequence of SCR2 contains an open reading frame of 321 nucleotides which is interrupted by an intron of 452 nucleotides. It encodes a polypeptide of 106 amino acids of molecular mass 12.25 kDa and pI 11.19. The deduced amino acid sequence shows a high degree of similarity to the L41 protein of the 60S ribosomal subunit from several eukaryotic organisms. The intron and the 5' non-coding region of SCR2 possess conserved elements which are typical of yeast ribosomal protein genes. A single amino acid change determines the resistance or sensitive phenotype to cycloheximide of the 80S ribosome since replacement of Gln56 in L41 from Schwanniomyces with Pro, by site-directed mutagenesis, confers cycloheximide sensitivity. SCR2 may serve as a practical yeast cloning marker if integrated in a multicopy plasmid.