Determination of a transcriptional regulator-like gene involved in biosynthesis of elsinochrome phytotoxin by the citrus scab fungus, Elsinoe fawcettii

Elsinochromes are nonhost-selective, light-activated, polyketide-derived toxins produced by many phytopathogenic Elsinoe species. We recently showed that the polyketide synthase-encoding gene EfPKS1 is essential for elsinochrome biosynthesis in the citrus scab fungus Elsinoe fawcettii. Sequence analysis beyond the EfPKS1 gene identified nine putative ORFs: four genes, designated RDT1, TSFI, PRF1 and ECT1, all encode polypeptides likely to have biosynthetic or efflux functions; five additional genes, OXR1 and EfHP1 to EfHP4, encode hypothetical proteins of unknown function. Northern-blot analysis revealed that expression of these genes in E fawcettii was not completely correlated with accumulation of elsinochromes under nitrogen limitation, alkaline pH or high concentrations of glucose. Targeted disruption of the TSF1 gene, encoding a putative transcriptional activator, yielded fungal mutants unable to produce elsinochromes, and defective in both conidiation and expression of RDT1, EfPKS1, PRF1 and EfHP1, whereas expression of RDT1, TSF1, PRF1 and ECT1 was nearly abolished in EfPKS1-disrupted mutants. By contrast, expression of OXR1, EfHP2 and EfHP3 was not affected by disrupting either EfPKS1 or TSF1. Taken together, the results indicate that in addition to polyketide synthase, the products of TSF1 and other adjacent genes may also play a crucial role in elsinochrome production.