Immunochemical relatedness of fungal NADPH-cytochrome P-450 reductases and their ability to reconstitute pisatin demethylase activity

Characterization of pisatin-inducible cytochrome p450s in fungal pathogens of pea that detoxify the pea phytoalexin pisatin

Many fungi that are pathogenic on pea have the ability to demethylate and thus detoxify the pea phytoalexin pisatin. This detoxification reaction has been studied most thoroughly in Nectria haematococca MP VI where it functions as a virulence trait. The enzyme catalyzing this reaction [pisatin demethylase (pda)] is a cytochrome P450. In the current study, the induction of whole-cell pda activity and the biochemical properties of pda in microsomal preparations from the pea pathogens Ascochyta pisi, Mycosphaerella pinodes, and Phoma pinodella are compared to the pda produced by N. haematococca. Based on cofactor requirements and their inhibition by carbon monoxide, cytochrome P450 inhibitors, and antibodies to NADPH:cytochrome P450 reductase, we conclude that the pdas from the other pea pathogens also are cytochrome P450s. All of the enzymes show a rather selective induction by pisatin, have a low K(m) toward pisatin, and have a fairly high degree of specificity toward pisatin as a substrate, suggesting that each pathogen may have a specific cytochrome P450 for detoxifying this plant antibiotic. Since the pdas in these fungi differ in their pattern of sensitivity to P450 inhibitors and display other minor biochemical differences, we suggest that these fungi may have independently evolved a specialized cytochrome P450 as a virulence trait for a common host.

Cytochrome P450 enzyme systems in fungi

The involvement of cytochrome P450 enzymes in many complex fungal bioconversion processes has been characterized in recent years. Accordingly, there is now considerable scientific interest in fungal cytochrome P450 enzyme systems. In contrast to S. cerevisiae, where surprisingly few P450 genes have been identified, biochemical data suggest that many fungi possess numerous P450 genes. This review summarizes the current information pertaining to these fungal cytochrome P450 systems, with emphasis on the molecular genetics. The use of molecular techniques to improve cytochrome P450 activities in fungi is also discussed.

Cloning and characterization of the PDA6-1 gene encoding a fungal cytochrome P-450 which detoxifies the phytoalexin pisatin from garden pea

The ability to detoxify pisatin, a phytoalexin produced by garden pea (Pisum sativum), is controlled by a family of PDA (pisatin demethylating ability) genes in the phytopathogenic fungus Nectria haematococa, MP (mating population) VI. Six known PDA genes each encode characteristic levels of inducible enzyme activity and are associated with different degrees of virulence on pea. To elucidate the phenotypic differences associated with these genes, we have cloned and characterized the PDA6-1 gene which encodes a pisatin-detoxifying enzyme and we compare it to another PDA gene, PDAT9. Pisatin demethylation was measured in PDA6-1 transformants of Aspergillus nidulans and shown to be regulated by glucose. The deduced amino acid (aa) sequence of PDA6-1 was 90% identical to that of the cytochrome P-450 encoded by PDAT9, but lacked nine aa at the C terminus, which has been postulated to be a site involved in substrate binding. A 35-bp sequence present upstream of a third PDA gene, PDA1, which appears to be important for induction of PDA1 by pisatin, was conserved in PDAT9, but not in PDA6-1. We conclude that PDA6-1, which does not appear to contribute to the virulence of N. haematococa on pea, differs significantly from PDAT9, which is associated with high virulence.

Isolation of a phytoalexin-detoxification gene from the plant pathogenic fungus Nectria haematococca by detecting its expression in Aspergillus nidulans

Detoxification of the pea phytoalexin pisatin via demethylation, mediated by a cytochrome P-450 monooxygenase, is thought to be important for pathogenicity of the fungus Nectria haematococca on pea. To isolate a fungal gene encoding pisatin demethylating activity (pda), we transformed Aspergillus nidulans with a genomic library of N. haematococca DNA constructed in a cosmid which carried the A. nidulans trpC gene. Transformants were selected for Trp+ and then screened for pda. One transformant among 1250 tested was Pda+ and was less sensitive to pisatin in culture than Pda- A. nidulans. The cosmid containing the gene (PDA) conferring this activity was recovered by phage lambda packaging of transformant genomic DNA. When A. nidulans was transformed with the cloned cosmid, 98% of the Trp+ transformants were Pda+. RNA blots probed with a 3.35 kb subclone carrying PDA indicated that the gene is expressed constitutively in A. nidulans but is inducible by pisatin in N. haematococca.