Mutations in Chlamydomonas reinhardtii Conferring Resistance to the Herbicide Sulfometuron Methyl

Chlamydomonas reinhardtii mutants resistant to the herbicide sulfometuron methyl (SM) were isolated and characterized. Growth of C. reinhardtii is sensitive to inhibition by SM at a concentration of 1 micromolar. Four mutants resistant to 10- to 100-fold higher concentrations were isolated. All possess a form of acetolactate synthase (ALS) whose specific activity in cell extracts is 100- to 1000-fold more resistant to SM than is the specific activity of wild-type enzyme. Only one mutant had abnormally low ALS specific activity in the absence of SM. All mutations were inherited as single lesions in the nuclear genome and were expressed in heterozygous diploids. The mutations in two strains mapped to linkage group IX about 30 centimorgans from streptomycin resistance and on the same side of the centromere, and in genetic crosses between mutants no segregation was observed. Accordingly, all mutations are tentatively assigned to gene smr-1. Herbicide resistance appears to be suitable as a selectable marker for molecular transformation in this organism.