Search

Beauveria bassiana is a globally distributed entomopathogenic fungus that produces various secondary metabolites to support its pathogenesis in insects. Two polyketide synthase genes, pks14 and pks15, are highly conserved in entomopathogenic fungi and are important for insect virulence. However, understanding of their mechanisms in insect pathogenicity is still limited. Here, we overexpressed these two genes in B. bassiana and compared the metabolite profiles of pks14 and pks15 overexpression strains to those of their respective knockout strains in culture and in vivo using tandem liquid chromatography-mass spectrometry (LC-MS/MS) with Global Natural Products Social Molecular Networking (GNPS). The pks14 and pks15 clusters exhibited crosstalk with biosynthetic clusters encoding insect-virulent metabolites, including beauvericins, bassianolide, enniatin A, and the intracellular siderophore ferricrocin under certain conditions. These secondary metabolites were upregulated in the pks14-overexpressing strain in culture and the pks15-overexpressing strain in vivo. These data suggest that pks14 and pks15, their proteins or their cluster components might be directly or indirectly associated with key pathways in insect pathogenesis of B. bassiana, particularly those related to secondary metabolism. Information about interactions between the polyketide clusters and other biosynthetic clusters improves scientific understanding about crosstalk among biosynthetic pathways and mechanisms of pathogenesis. [ABSTRACT FROM AUTHOR]

Entomopathogenic fungi utilize specific secondary metabolites to defend against insect immunity, thereby enabling colonization of their specific hosts. We are particularly interested in the polyketide synthesis gene pks15, which is involved in metabolite production, and its role in fungal virulence. Targeted disruption of pks15 followed by genetic complementation with a functional copy of the gene would allow for functional characterization of this secondary metabolite biosynthesis gene. Using a Beauveria bassiana ∆pks15 mutant previously disrupted by a bialophos-resistance (bar) cassette, we report here an in-cis complementation at bar cassette using CRISPR/Cas9 gene editing. A bar-specific short guide RNA was used to target and cause a double-strand break in bar, and a donor DNA carrying a wild-type copy of pks15 was co-transformed with the guide RNA. Isolate G6 of ∆pks15 complemented with pks15 was obtained and verified by PCR, Southern analyses and DNA sequencing. Compared to ∆pks15 which showed a marked reduction in sporulation and insect virulence, the complementation in G6 restored with insect virulence, sporulation and conidial germination to wild-type levels. Atomic force and scanning electron microscopy revealed that G6 and wild-type conidial wall surfaces possessed the characteristic rodlet bundles and rough surface while ∆pks15 walls lacked the bundles and were relatively smoother. Conidia of ∆pks15 were larger and more elongated than that of G6 and the wild type, indicating changes in their cell wall organization. Our data indicate that PKS15 and its metabolite are likely not only important for fungal virulence and asexual reproduction, but also cell wall formation. [ABSTRACT FROM AUTHOR]

Naphthoquinones are deep red polyketide pigments produced by the ant-pathogenic fungus Ophiocordyceps sp. BCC1869. In culture, biosynthesis of these naphthoquinones remains at a low level during the first 20 days and reaches its maximum production level at approximately 50 days. The MFS transporter gene MFS1 was previously identified in Ophiocordyceps sp. BCC1869 from a subtractive EST library between the fungus grown under naphthoquinone-inductive and naphthoquinone-repressive conditions. We cloned and sequenced this transporter gene, which has an open reading frame of 1505 bp and three introns (48, 52, and 58 bp). Phylogenetic analysis showed this MFS transporter was tightly clustered with fungal riboflavin transporters. Functional analysis of this gene was performed by overexpression of MFS1 under the control of a strong, constitutive promoter. We successfully transformed the fungus with this overexpression plasmid using PEG-protoplast transformation, which generated nine transformants per µg of plasmid. RT-PCR indicated that the MFS1 expression level in the overexpressing strains increased 3- to 10-fold compared to the wild type. HPLC analysis of crude extracts of mutants and wild type demonstrated that four naphthoquinone derivatives, erythrostominone, epierythrostominol, deoxyerythrostominone, and deoxyerythrostominol, were the major naphthoquinones produced and excreted in staggering quantities (20- to 2300-fold) in 7-day old liquid cultures by the mutant C7, compared to the wild type. High resolution electrospray ionization mass spectrometry verified mass spectra of these purified metabolites. Three other naphthoquinone derivatives, whose structures have not been identified, were also detected in high amount in the mutant liquid cultures. [ABSTRACT FROM AUTHOR]

Iron is an essential element for life. However, the iron overload can be toxic. Here, we investigated the significant increase of tenellin and iron-tenellin complex production in ferricrocin-deficient mutants of Beauveria bassiana. Our chemical analysis indicated that the ferricrocin-deficient mutants T1, T3 and T5 nearly abolished ferricrocin production. In turn, these mutants had significant accumulation of iron-tenellin complex in their mycelia at 247-289 mg g-1 cell dry weight under iron-replete condition. Both tenellin and iron-tenellin complex were not detected in the wild-type under such condition. Mass analysis of the mutants' crude extracts demonstrated that tenellin formed a 3:1 complex with iron in the absence of ferricrocin. The unexpected link between ferricrocin and tenellin biosynthesis in ferricrocin-deficient mutants could be a survival strategy during iron-mediated oxidative stress. [ABSTRACT FROM AUTHOR]

The ant-pathogenic fungus BCC1869 produces six naphthoquinone (NQ) derivatives. These NQs can be found in fungal-infected ants or produced in culture. Also, the NQs have antibacterial, anticancer, and antimalarial activities and are red pigments with potential for use as natural colorants. Suppressive subtractive hybridization identified genes that were expressed under NQ-producing conditions but not under nonproducing conditions. On potato dextrose agar, the mycelia produced red pigments and secreted them into the medium and as droplets on top of the colony. High-performance liquid chromatography analysis indicated that the red pigment was predominantly erythrostominone with small amounts of its derivatives. For suppressive subtractive hybridization, the cDNA from O. unilateralis cultures on complete medium agar cultures (lacking NQs) were subtracted from those on potato dextrose agar (which produce and secrete NQs). Sixty-six unique expressed sequence tags (ESTs) were identified and include five transporter genes, two transcriptional regulator genes, and several genes in secondary metabolism and biodegradation. The transporter genes include an ATP-binding cassette transporter gene OuAtr1 and a major facilitator superfamily transporter gene OuMfs1. Expression of selected ESTs was further validated using quantitative reverse transcription PCR. Gene expression result indicates that OuAtr1 and OuMfs1 were dramatically upregulated (136- and 29-fold increase, respectively) during the NQ-producing stage compared with the NQ-nonproducing stage. Upregulation of other genes was also detected. This EST collection represents the first group of genes identified from this potential biocontrol agent and includes candidate genes for production and secretion of the red NQs. Roles of these genes could be further determined using a functional analysis. [ABSTRACT FROM AUTHOR]

Polyketides draw much attention because of their potential use in pharmaceutical and biotechnological applications. This study identifies an abundant pool of polyketide synthase (PKS) genes from local isolates of tropical fungi found in Thailand in three different ecological niches: insect pathogens, marine inhabitants, and lichen mutualists. We detected 149 PKS genes from 48 fungi using PCR with PKS-specific degenerate primers. We identified and classified 283 additional PKS genes from 13 fungal genomes. Phylogenetic analysis of all these PKS sequences the comprising ketosynthase (KS) conserved region and the KS-acyltransferase inter-domain region yielded results very similar to those for phylogenies of the KS domain and suggested a number of remarkable points. (i) Twelve PKS genes amplified from 12 different insect-pathogenic fungi form a tight cluster, although along with two PKS genes extracted from genomes of Aspergillus niger and Aspergillus terreus, in reducing clade III. Some of these insect-specific fungal PKSs are nearly identical. (ii) We identified 38 new PKS-nonribosomal peptide synthetase hybrid genes in reducing clade II. (iii) Four distinct clades were discovered with more than 75% bootstrap support. We propose to designate the novel clade D1 with 100% bootstrap support "reducing clade V." The newly cloned PKS genes from these tropical fungi should provide useful and diverse genetic resources for future research on the characterization of polyketide compounds synthesized by these enzymes. [ABSTRACT FROM AUTHOR]

The reducing clade IIb polyketide synthase gene, pks14 , is preserved throughout the evolution of entomopathogenic fungi. We examined the functions of pks14 in Beauveria bassiana using targeted gene disruption, and pks14 disruption was verified by Southern blot and PCR analyses. The radial growth, cell dry weight and conidial germination of Δpks14 were comparable to that of the wild type. Our sequence and gene expression analyses of the pks14 biosynthetic cluster demonstrated: (i) cotranscription and constitutive expression of nearly all the genes of the aforementioned cluster including the C2H2 zinc finger transcription regulator gene, but not pks14 and the cytochrome P450 gene; (ii) expression of the pks14 gene in the insect-containing culture condition only; and (iii) a KAR9-like gene in direct proximity with pks14 is the only gene showing co-regulation. The Δ pks14 -infected Spodoptera exigua larvae survived significantly longer than those infected by the wild type, indicating a marked reduction in the virulence of Δpks14 against the insect. LT50 of Δpks14 was increased by 1.55 days. Hyphal body formation was decreased in the hemolymph of insects infected by Δpks14 as compared with those inoculated by the wild type. Our results suggest that PKS14-catalyzed polyketide enhances virulence and pathogenicity of B. bassiana on insects. [ABSTRACT FROM AUTHOR]

The quality of Beauveria bassiana conidia directly affects the virulence against insects. In this study, continuous subculturing of B. bassiana on both rice grains and potato dextrose agar (PDA) resulted in 55 and 49 % conidial yield reduction after 12 passages and 68 and 60 % virulence reduction after 20 and 12 passages at four d post-inoculation, respectively. The passage through Tenebrio molitor and Spodoptera exigua restored the virulence of rice and PDA subcultures, respectively. To explore the molecular mechanisms underlying the conidial quality and the decline of virulence after multiple subculturing, we investigated the conidial proteomic changes. Successive subculturing markedly increased the protein levels in oxidative stress response, autophagy, amino acid homeostasis, and apoptosis, but decreased the protein levels in DNA repair, ribosome biogenesis, energy metabolism, and virulence. The nitro blue tetrazolium assay verified that the late subculture's colony and conidia had a higher oxidative stress level than the early subculture. A 2A-type protein phosphatase and a Pleckstrin homology domain protein Slm1, effector proteins of the target of rapamycin (TOR) complex 1 and 2, respectively, were dramatically increased in the late subculture. These results suggest that TOR signalling might be associated with ageing in B. bassiana late subculture, in turn affecting its physiological characteristics and virulence. [ABSTRACT FROM AUTHOR]

The reducing clade III polyketide synthase genes, including pks15 , are highly conserved among entomopathogenic fungi. To examine the function of pks15 , we used targeted disruption to investigate the impact of Beauveria bassiana pks15 on insect pathogenesis. Southern analysis verified that the Δ pks15 mutant was disrupted by a single integration of the transformation cassette at the pks15 locus. The Δ pks15 mutant had a slight reduction in radial growth, and it produced fewer spores. Our insect bioassays indicated the Δ pks15 mutant to be significantly reduced in virulence against beet armyworms compared to wild type (WT), which could be partially accounted for by its markedly decreased ability to survive phagocytosis. Total haemocyte count decreased sharply by 50-fold from days 1–3 post-inoculation in insects infected with WT, compared to a 5-fold decrease in the Δ pks15 mutant. The mutant also produced fewer hemolymph hyphal bodies than WT by 3-fold. In co-culture studies with amoebae that have phagocytic ability similar to that of insect haemocytes, at 48 h the mortality rate of amoebae engulfing Δ pks15 decreased by 72 %, and Δ pks15 CFU decreased by 83 % compared to co-culture with WT. Thus, the Δ pks15 mutant had a reduced ability to cope with phagocytosis and highly reduced virulence in an insect host. These data elucidate a mechanism of insect pathogenesis associated with polyketide biosynthesis. [ABSTRACT FROM AUTHOR]

Entomopathogenic fungi are able to invade and kill insects. Various secondary metabolites can mediate the interaction of a fungal pathogen with an insect host and also help the fungus compete with other microbes. Here we screened 23 isolates of entomopathogenic fungi for polyketide synthase (PKS) genes and amplified 72 PKS gene fragments using degenerate PCR. We performed a phylogenetic analysis of conserved ketosynthase and acyltransferase regions in these 72 sequences and 72 PKSs identified from four insect fungal genome sequences. The resulting genealogy indicated 47 orthologous groups with 99–100 % bootstrap support, suggesting shared biosynthesis of identical or closely related compounds from different fungi. Three insect-specific groups were identified among the PKSs in reducing clades IIa, IIb, and III, which comprised PKSs from 12, 9, and 30 fungal isolates, respectively. A IIa-IIb pair could be found in seven fungi. Expression analyses revealed that eleven out of twelve PKS genes identified in Beauveria bassiana BCC 2660 were expressed in culture. PKS genes from insect-specific clades IIa and IIb were expressed only in insect-containing medium, while others were expressed only in PDB or in CYB, PDB and SDY. The data suggest the potential production of several polyketides in culture. [ABSTRACT FROM AUTHOR]