Differential Stress-Induced Regulation of Two Quinone Reductases in the Brown Rot BasidiomyceteGloeophyllum trabeum

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Abstract
Quinone reductases (QRDs) have two important functions in the basidiomyceteGloeophyllum trabeum, which causes brown rot of wood. First, a QRD is required to generate biodegradative hydroxyl radicals via redox cycling between twoG. trabeumextracellular metabolites, 2,5-dimethoxyhydroquinone (2,5-DMHQ) and 2,5-dimethoxy-1,4-benzoquinone (2,5-DMBQ). Second, because 2,5-DMBQ is cytotoxic and 2,5-DMHQ is not, a QRD is needed to maintain the intracellular pool of these metabolites in the reduced form. Given their importance inG. trabeummetabolism, QRDs could prove useful targets for new wood preservatives. We have identified twoG. trabeumgenes, each existing in two closely related, perhaps allelic variants, that encode QRDs in the flavodoxin family. Past work with QRD1 and heterologous expression of QRD2 in this study confirmed that both genes encode NADH-dependent, flavin-containing QRDs. Real-time reverse transcription PCR analyses of liquid- and wood-grown cultures showed thatqrd1expression was maximal during secondary metabolism, coincided with the production of 2,5-DMBQ, and was moderately up-regulated by chemical stressors such as quinones. By contrast,qrd2expression was maximal during fungal growth when 2,5-DMBQ levels were low, yet was markedly up-regulated by chemical stress or heat shock. The total QRD activity in lysates ofG. trabeummycelium was significantly enhanced by induction beforehand with a cytotoxic quinone. The promoter ofqrd2contains likely antioxidant, xenobiotic, and heat shock elements, absent inqrd1, that probably explain the greater response ofqrd2transcription to stress. We conclude from these results that QRD1 is the enzymeG. trabeumroutinely uses to detoxify quinones during incipient wood decay and that it could also drive the biodegradative quinone redox cycle. However, QRD2 assumes a more important role when the mycelium is stressed.