Analysis of determinants of binding and transcriptional activation of the pisatin-responsive DNA binding factor of Nectria haematococca

A binuclear zinc transcription factor binds the host isoflavonoid-responsive element in a fungal cytochrome p450 gene responsible for detoxification

The PDA1 gene of the filamentous fungus Nectria haematococca MPVI (anamorph: Fusarium solani) encodes pisatin demethylase, a cytochrome P450. Pisatin is a fungistatic isoflavonoid produced by garden pea (Pisum sativum), a host for this fungus. Pisatin demethylase detoxifies pisatin and functions as a virulence factor for this fungus. Pisatin induces PDA1 expression both in cultured mycelia as well as during pathogenesis on pea. The regulatory element within PDA1 that provides pisatin-responsive expression was identified using a combination of in vivo functional analysis and in vitro binding analysis. The 40 bp pisatin-responsive element is located 635 bp upstream of the PDA1 transcription start site. This element was sufficient to provide strong pisatin-induced expression to a minimal promoter in vivo and was required for pisatin regulation of the PDA1 promoter. A gene encoding a DNA-binding protein specific to this 40 bp element was isolated from a N. haematococca cDNA library using the yeast one-hybrid screen. The cloned gene possesses sequence motifs found in the binuclear zinc (Cys 6-Zn 2) family of transcription factors unique to fungi. The results suggest that it is a regulator of this fungal cytochrome P450 gene and may provide pisatin-responsive regulation.

Flavonoids and isoflavonoids - a gold mine for metabolic engineering

Flavonoid-derived plant natural products have long been known to function as floral pigments for the attraction of insect pollinators, as signal molecules for beneficial microorganisms in the rhizosphere, and as antimicrobial defense compounds. New functions for flavonoid compounds continue to be found, particularly in plant-microorganism signaling, and there has been an explosion of interest in flavonoids and isoflavonoids as health-promoting components of the human diet. The flavonoid and isoflavonoid pathways are probably the best characterized natural product pathway in plants, and are therefore excellent targets for metabolic engineering. Manipulation of flavonoid biosynthesis can be approached via several strategies, including sense or antisense manipulation of pathway genes, modification of the expression of regulatory genes, or generation of novel enzymatic specificities by ra-tional approaches based on emerging protein structure data. In addition, activation tagging provides a novel approach for the discovery of uncharacterized structural and regulatory genes of flavonoid biosynthesis.

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.