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 (F rench bean) is the prenylated isoflavonoid, kievitone. The bean pathog en, 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 constitutive ly, suggesting that the enzyme is a virulence factor. Based on the N-t erminal amino acid sequence of purified enzyme, the kievitone hydratas e 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 pre dicted to encode a preprotein of 350 amino acids, from which a 19 amin o acid N-terminal transit peptide is removed during maturation and sec retion. The predicted mass of the mature polypeptide, 37 kDa, contrast s with the 47 to 49 kDa size estimated by electrophoresis of purified enzyme, confirming that the enzyme is extensively glycosylated. The in ferred polypeptide sequence has seven canonical sites for N-glycosylat ion. Southern blot-hybridization analysis of F. s. f. sp. phaseoli DNA indicates one khs locus and an additonal locus with weak hybridizatio n to the khs probe. Sequences related to khs were also detected in sev eral isolates of F. solani and the related teleomorph, Nectria haemato cocca. However, strains of F. oxysporum known to exhibit inducible kie vitone hydratase activity (but not pathogenic to bean) did not have de tectable khs homology. Nevertheless, all isolates known to cause sever e disease on bean possessed khs sequence.