Aspartic Hydroxamate Resistance and Asparaginase Regulation in the Fungus Aspergillus nidulans

Abstract

Eleven mutants resistant to a toxic analogue of asparagine, aspartic hydroxamate, were isolated; they are allelic and map at the ahrA locus. These mutations result in low or non-detectable asparaginase activity. ahrA mutations are recessive for asparaginase activity and aspartic hydroxamate resistance. The ahrA locus is in linkage group VIII and is loosely linked with abaA, palB, uZ9 and chaA. Asparaginase activity was measured by the formation of aspartic hydroxamate from asparagine and hydroxylamine. The Km values of asparaginase for asparagine and hydroxylamine are 0.6 and 8.3 mM, respectively. Minimum asparaginase activity is present in cells grown on ammonium or glutamine. Maximum asparaginase activity is present in wild-type cells grown on ammonium and then held in N-free medium for 3 h. Derepression from this ammonium repression requires protein synthesis. A number of different types of ammonium-repressed and of ammonium-derepressed mutants have abnormal regulation of asparaginase activity.