Nitrilase in biosynthesis of the plant hormone indole-3-acetic acid from indole-3-acetonitrile: cloning of the Alcaligenes gene and site-directed mutagenesis of cysteine residues

Abstract

Indole-3-acetic acid is the major auxin in most plants. In Cruciferae, including Brassicaceae, indole-3-acetic acid is synthesized from indole-3-acetonitrile by nitrilase, after indole-3-acetonitrile is formed from tryptophan via indole-3-acetaldoxime or indole glycosinolates as the intermediate. We cloned and sequenced the gene for nitrilase (EC 3.5.5.1), which catalyzes the hydrolysis of indole-3-acetonitrile to indole-3-acetic acid, from Alcaligenes faecalis JM3. The amino acid sequence deduced from the nucleotide sequence of the nitrilase gene shows 34.7% identity with that of Klebsiella ozaenae nitrilase. A DNA clone containing the nitrilase gene expressed the active enzyme in Escherichia coli with excellent yield. Among five cysteine residues (Cys-40, Cys-115, Cys-162, Cys-163, and Cys-218) in the Alcaligenes nitrilase, only Cys-163 was conserved at the corresponding position in the Klebsiella nitrilase. Two mutant enzymes, in which Cys-162 and Cys-163 were replaced with Asn and Ala, respectively, were constructed by site-directed mutagenesis. A 35% increase of the specific activity and a large reduction of the Km for thiophene-2-acetonitrile (which was used as a standard substrate for the nitrilase) were observed in the Cys-162-->Asn mutant enzyme. The Cys-163-->Ala mutation resulted in complete loss of nitrilase activity, clearly indicating that Cys-163 is crucial for the activity and Cys-162 could not provide the catalytic function of Cys-163.