RESISTANCE TO L-METHIONINE-S-SULFOXIMINE IN CHLAMYDOMONAS-REINHARDTIIIS DUE TO AN ALTERATION IN A GENERAL AMINO-ACID-TRANSPORT SYSTEM

It was suggested that the mutant ARF1 of Chlamydomonas reinhardtii is resistant to L-methionine-S-sulfoximine (MSX, an irreversible inhibito r of glutamine synthetase, EC 6.3.1.2) because this strain degraded an d utilized this compound as a nitrogen source for growth (A.R. France et al., 1996, Plant Physiol 110: 1215-1222). Resistance to MSX has now been characterized in a double mutant of this alga, called MPA1, whic h is resistant to MSX and lacks L-amino acid oxidase (LAO activity, EC 1.4.3.2). Biochemical and genetic evidence indicate that the mutant M PA1 is altered in the same MSX-resistance locus as mutant ARF1. Howeve r, mutant MPA1 neither degraded nor utilized MSX as a nitrogen source. This led us to conclude that (i) resistance to MSX is not linked to i ts utilization, and (ii) that LAO activity accounts for the degradatio n of MSX in mutant ARF1. Data indicate that C. reinhardtii possesses a broad-specificity carrier system responsible for the transport of arg inine and other amino acids, including MSX. We propose that the altera tion of this carrier confers resistance to MSX in mutants ARF1 and MPA 1.