THE FUSARIUM-SOLANI GENE ENCODING KIEVITONE HYDRATASE, A SECRETED ENZYME THAT CATALYZES DETOXIFICATION OF A BEAN PHYTOALEXIN

Among the antimicrobial phytoalexins produced by Phaseolus vulgaris (French bean) is the prenylated isoflavonoid, kievitone. The bean pathogen, Fusarium solani f. sp. phaseoli, secretes a glycoenzyme, kievitone hydratase (EC 4.2.1.95), which catalyzes conversion of kievitone to a less toxic metabolite. Among F. solani strains, those that are highly virulent to P. vulgaris also produce kievitone hydratase constitutively, suggesting that the enzyme is a virulence factor. Based on the N-terminal amino acid sequence of purified enzyme, the kievitone hydratase cDNA and gene (khs) were cloned. The identities of khs and the cDNA were confirmed by their expression in transgenic Neurospora crassa and Emericella nidulans. Based on the gene and cDNA sequences, khs is predicted to encode a preprotein of 350 amino acids, from which a 19 amino acid N-terminal transit peptide is removed during maturation and secretion. The predicted mass of the mature polypeptide, 37 kDa, contrasts with the 47 to 49 kDa size estimated by electrophoresis of purified enzyme, confirming that the enzyme is extensively glycosylated. The inferred polypeptide sequence has seven canonical sites for N-glycosylation. Southern blot-hybridization analysis of F. s. f. sp. phaseoli DNA indicates one khs locus and an additonal locus with weak hybridization to the khs probe. Sequences related to khs were also detected in several isolates of F. solani and the related teleomorph, Nectria haematococca. However, strains of F. oxysporum known to exhibit inducible kievitone hydratase activity (but not pathogenic to bean) did not have detectable khs homology. Nevertheless, all isolates known to cause severe disease on bean possessed khs sequence.