Biosynthesis of non-melanin pigment by a divergent polyketide synthase in Metarhizium robertsii

Fungal polyketide synthases (PKSs) and their related gene clusters are highly diversified at both inter- and intra-specific levels. The most well characterized PKS enzymes include those responsible for the biosynthesis of polyketide pigments such as melanins. The genome of the insect pathogenic fungus Metarhizium robertsii contains 20 type I PKSs but none has been functionally characterized. In this study, two PKS genes (designated as MrPks1 and MrPKs2) showing homologies to those counterparts for the biosynthesis of heptaketide pigments and dihydroxynaphthalene (DHN)-melanins, respectively, were deleted in two different strains of M. robertsii. The results indicated that disruption of MrPks1 but not MrPks2 impaired fungal culture pigmentation and cell wall structure. In addition to the negative effect of the DHN-melanin pathway inhibitor, it was postulated that DHN-melanin would not be produced by M. robertsii. Various assays revealed that the stress resistance abilities against ultraviolet radiation, heat shock and oxidants, as well as virulence against insects were not impaired in ΔMrPks1 and ΔMrPks2 isolates when compared with the wild-type strain. Thus, the non-melanin pigment(s) produced by the fungus do not contribute to cell damage protection and pathogenicity in M. robertsii. Physiological differences were evident in the two examined wild-type strains. The results from this study advance the understanding of functional divergence of fungal PKSs.