Phosphopantetheinyl Transferase CfwA/NpgA Is Required for Aspergillus nidulans Secondary Metabolism and Asexual Development

Abstract

Polyketide synthases (PKSs) and/or nonribosomal peptide synthetases (NRPSs) are central components of secondary metabolism in bacteria, plants, and fungi. In filamentous fungi, diverse PKSs and NRPSs participate in the biosynthesis of secondary metabolites such as pigments, antibiotics, siderophores, and mycotoxins. However, many secondary metabolites as well as the enzymes involved in their production are yet to be discovered. Both PKSs and NRPSs require activation by enzyme members of the 4-phosphopantetheinyl transferase (PPTase) family. Here, we report the isolation and characterization of Aspergillus nidulans strains carrying conditional (cfwA2) and null (⌬cfwA) mutant alleles of the cfwA gene, encoding an essential PPTase. We identify the polyketides shamixanthone, emericellin, and dehydroaustinol as well as the sterols ergosterol, peroxiergosterol, and cerevisterol in extracts from A. nidulans large-scale cultures. The PPTase CfwA/NpgA was required for the production of these polyketide compounds but dispensable for ergosterol and cerevisterol and for fatty acid biosynthesis. The asexual sporulation defects of cfwA, ⌬fluG, and ⌬tmpA mutants were not rescued by the cfwA-dependent compounds identified here. However, a cfwA2 mutation enhanced the sporulation defects of both ⌬tmpA and ⌬fluG single mutants, suggesting that unidentified CfwA-dependent PKSs and/or NRPSs are involved in the production of hitherto-unknown compounds required for sporulation. Our results expand the number of known and predicted secondary metabolites requiring CfwA/NpgA for their biosynthesis and, together with the phylogenetic analysis of fungal PPTases, suggest that a single PPTase is responsible for the activation of all PKSs and NRPSs in A. nidulans.Filamentous fungi represent a rich and diverse source of bioactive compounds derived from secondary metabolism. Indeed, many positive and negative effects that fungi have on human activity are mediated by secondary metabolites such as antibiotics and other pharmaceutical drugs, mycotoxins, or pathogen virulence factors (22, 30, 54-56, 60, 66). In contrast to primary metabolism, common to most living organisms, secondary metabolism is not essential for the immediate survival of the producing organism. Furthermore, different taxonomic groups produce different types of secondary metabolites.Despite their large chemical diversity, secondary metabolites can be grouped according to their primary metabolism precursors. Acetyl coenzyme A, shikimate, and amino acids are major secondary metabolite building units. Acetyl coenzyme A is used to produce terpenoids, steroids, carotenoids, and polyketides.

Response Regulators SrrA and SskA Are Central Components of a Phosphorelay System Involved in Stress Signal Transduction and Asexual Sporulation in Aspergillus nidulans

“…In this fungus, asexual reproduction (conidiation) is induced by environmental signals such as exposure to air (2,16,69), nutrient starvation (66), and selfgenerated signals (39,43,65,67,70). The mechanisms by which these signals are perceived and transduced are not well understood.…”

Identification and Characterization of the Asperthecin Gene Cluster of Aspergillus nidulans

“…Our own genomic analyses suggest that this number may be a slight overestimate (because more than one polyketide synthase [PKS] and/or nonribosomal peptide synthetase may be involved in a single pathway), but clearly A. nidulans has the ability to synthesize many secondary metabolites. Only a limited number of secondary metabolites in A. nidulans have been identified (aspyridones A and B [from a single pathway], aspoquinolones A to D [probably products of a single pathway], penicillin, asperlin, sterigmatocystin, triacetylfusarinine, ferricrocin, shamixanthone, variecoxanthone, terrequinone A, emericellin, dehydroaustinol, and the sterols, ergosterol, peroxiergosterol, and cerevisterol [3,15,20]). It follows that the majority of A. nidulans secondary metabolites remain to be identified.…”

FlbD, a Myb Transcription Factor of Aspergillus nidulans, Is Uniquely Involved in both Asexual and Sexual Differentiation

“…FlbA is a regulator of a heterotrimeric G protein signaling pathway that stimulates vegetative growth and inhibits conidiation (83). In contrast, FluG is responsible for the production of an extracellular signaling factor required for activation of the flbD-E genes in vegetative cells and to induce conidiation, along with other unidentified compounds (38,39,44,56,64). FlbB is a TF of the bZIP type that unexpectedly also localizes at the hyphal tips of vegetative hyphae, forming a complex with FlbE (17,18,21).…”

“…nidulans asexual reproduction is induced by environmental signals such as exposure to air (2,70) or nutrient starvation (63). It involves the production of chemical signals (9,25,38,40,44,64,72) and depends on activation of the brlA gene (10), which encodes a transcription factor (TF) of the Zn finger family (1,29,48,54). Normally, conidiation involves the production of a mycelial cell compartment, from which a conidiophore stalk develops.…”