The sensitivity of α-σ1278b to methylammonium was dependent on the nature of the nitrogen source. Yeast strains carrying novel mutations, or , were isolated by selection for methylammonium resistance in media with allantoin or arginine as the sole nitrogen source. The mutations result in a considerable relief from nitrogen metabolite repression of allantoinase synthesis (), or of arginase and ornithine transaminase synthesis ( and, to a lesser extent, ). Both mutations were recessive to the wild type and were neither linked to each other, nor to the structural genes of allantoinase, arginase and ornithine transaminase. Hence they represent regulatory genes without operator characteristics. Experiments with ammoniumlimited continuous cultures indicated that ammonium transport was not affected by the and mutations. Ammonium repression of catabolic NAD-dependent glutamate dehydrogenase was also unaffected. The locus, in cooperation with other loci, appears to control nitrogen metabolite repression of the arginine degradative pathway; it is probably pathway-specific. The locus may have a general function in nitrogen metabolite repression of various degradative pathways since the mutation causes resistance to methylammonium in the presence of several metabolically unrelated nitrogen sources.


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