title: Enzymatic lesions in methionine mutants of Aspergillus nidulans: role and regulation of an alternative pathway for cysteine and methionine synthesis.
abstract: In Aspergillus nidulans the pathway involving cystathionine formation is the main one for homocysteine synthesis. Mutants lacking cystathionine gamma-synthase or beta-cystathionase are auxotrophs suppressible by: (i) mutations in the main pathway of cysteine synthesis (cysA1, cysB1, and cysC1), (ii) mutations causing stimulation of cysteine catabolism (su101), and (iii) mutations in a presumed regulatory gene (suAmeth). A relative shortage of cysteine in the first group of suppressors causes a derepression of homocysteine synthase, the enzyme involved in the alternative pathway of homocysteine synthesis. A similar derepression is observed in the suAmeth strain. Homocysteine synthesized by this pathway serves as precursor for cysteine and methionine synthesis. A mutant with altered homocysteine synthase is a prototroph, indicating that this enzyme is not essential for the fungus.
mesh_terms: D001233:Aspergillus nidulans / Q000201:enzymology / Q000378:metabolism*; D003540:Cystathionine / Q000096:biosynthesis; D003545:Cysteine / Q000096:biosynthesis*; D005809:Genes, Regulator; D005978:Glutathione / Q000096:biosynthesis; D006710:Homocysteine / Q000096:biosynthesis; D008190:Lyases / Q000096:biosynthesis; D008715:Methionine / Q000096:biosynthesis*; D009154:Mutation; D013489:Suppression, Genetic; D013654:Taurine / Q000096:biosynthesis
chemical_list: D006710:Homocysteine; D013654:Taurine; D003540:Cystathionine; D008715:Methionine; D008190:Lyases; D005978:Glutathione; D003545:Cysteine