Abstract
A strain of Chlamydomonas reinhardtii, named ARF-1, which grows with the glutamine synthetase (GS) inhibitor L-methionine-S-sulfoximine (MSX), has been isolated and characterized. Mutant ARF-1 is affected at a single and dominant gene, tentatively assigned to the allele msr-1-2. Neither the uptake of ammonia nor the two GS isoenzyme activities of the mutant were affected by MSX in vivo. GS activities, however, were fully abolished in vitro, thus suggesting that neither GS isoform was an altered enzyme resistant to the inhibitor. Resistance to MSX does not seem to be due to either a defect in a permease responsible for the transport of MSX or over-expression of GS activity, nor did we find an alternative enzymatic pathway for the assimilation of ammonium. Resistance was independent of the nitrogen source used and was strongly enhanced by the addition of acetate. Unlike the parental strain, mutant ARF-1 can degrade and utilize MSX as the sole nitrogen source for growth, which could account for the observed resistance. Thus, this mutant can be classified as a novel type of MSX-resistant mutant. This mutant can also use phosphinothricin, methionine sulfone, or methionine sulfoxide as the sole sources of nitrogen. This capability cosegregated in the genetic crosses and was also observed in all the diploids isolated. An MSX/[alpha]-ketoglutarate aminotransferase activity, not present in the parental strain 305, was detected in mutant ARF-1 cells. Therefore, we propose that the locus msr-1-2 either codes for this transaminase activity or its product gene is necessary to express this transaminase activity.