Resistance to l-methionine-S-sulfoximine in Chlamydomonas reinhardtii is 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 inhibitor of glutamine synthetase, EC 6.3.1.2) because this strain degraded and utilized this compound as a nitrogen source for growth (A.R. Franco et al., 1996, Plant Physiol 110: 1215–1222). Resistance to MSX has now been characterized in a double mutant of this alga, called MPA1, which 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 MPA1 is altered in the same MSX-resistance locus as mutant ARF1. However, 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 its utilization, and (ii) that LAO activity accounts for the degradation of MSX in mutant ARF1. Data indicate that C. reinhardtii possesses a broad-specificity carrier system responsible for the transport of arginine and other amino acids, including MSX. We propose that the alteration of this carrier confers resistance to MSX in mutants ARF1 and MPA1. Received: 6 April 1998 / Accepted: 8 June 1998