Cycloheximide resistance conferred by novel mutations in ribosomal proteinL41 of Chlamydomonas reinhardtii

Although most eukaryotic cells are sensitive to the 80S ribosome inhibitor cycloheximide (CYH), naturally occurring CYH resistance is widespread among st yeast species. The primary determinant of resistance appears to be a sin gle residue within ribosomal protein L41; resistance is acquired by the sub stitution of a conserved proline (P-56) by a glutamate residue. We have iso lated the L41 gene (RPL41) from the green alga Chlamydomonas and investigat ed the molecular basis of CYH resistance in various mutant strains. In both the wild-type strain and the mutant act-1, a proline is found at the key p osition in L41.; However, analysis of six independently isolated act-2 muta nts reveals that all have point mutations that replace the proline with eit her leucine or serine. Of the two changes, the leucine mutation confers sig nificantly higher levels of CYH resistance. This work identifies the ACT-2 locus as RPL41 and provides a possible dominant marker for nuclear transfor mation of C. reinhardtii.