Degradation of Carboxanilide Fungicides by aNocardiaSpecies

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Abstract
A Nocardia sp. with the ability to utilize the systemic fungicides o-toluanilide, 2,5-dimethyl-3-furancarboxanilide and Carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide) as the only sources of C and N was isolated from soil. The degradation of these carboxanilide fungicides is initiated by hydrolytic cleavage of the amide bond, catalyzed by an aryl acylamidase. Activation energy, pH optimum and regulation of this enzyme were determined. By mutagenic treatment 89 mutants, defective in the degradation of the fungicides, were produced. The mutants can be characterized by their ability to grow on certain aromatic compounds, by enzyme studies and by the detection of accumulation products. Mutants of the first group still grow at the expense of aniline, phenol, catechol and 3,4-dihydroxybenzoate; activities of mutant amidases were shown to be considerably reduced. Growth of the second group of mutants was supported by all the above compounds with the exception of aniline. These mutants are assumed to be defective in the conversion of aniline to catechol, and the accumulation of aniline by a representative of this group was demonstrated. Mutants of the third group only grow on 3,4-dihydroxybenzoate and they accumulate catechol. They were shown to be defective in catechol 1,2-dioxygenase or in muconate cycloisomerase, or in both enzymes. Three mutants, which failed to grow in the presence of any of the aromatic compounds tested, are presumed to be blocked in the last steps of the degradation pathway. A similar mutational sequence was demonstrated by cross-feeding tests.