Regulatory cascade and biological activity of Beauveria bassiana oosporein that limits bacterial growth after host death

Abstract

The regulatory network and biological functions of the fungal secondary metabolite oosporein have remained obscure. Beauveria bassiana has evolved the ability to parasitize insects and outcompete microbial challengers for assimilation of host nutrients. A novel zinc finger transcription factor, BbSmr1 (B. bassiana secondary metabolite regulator 1), was identified in a screen for oosporein overproduction. Deletion of Bbsmr1 resulted in up-regulation of the oosporein biosynthetic gene cluster (OpS genes) and constitutive oosporein production. Oosporein production was abolished in double mutants of Bbsmr1 and a second transcription factor, OpS3, within the oosporein gene cluster (ΔBbsmr1ΔOpS3), indicating that BbSmr1 acts as a negative regulator of OpS3 expression. Real-time quantitative PCR and a GFP promoter fusion construct of OpS1, the oosporein polyketide synthase, indicated that OpS1 is expressed mainly in insect cadavers at 24-48 h after death. Bacterial colony analysis in B. bassiana-infected insect hosts revealed increasing counts until host death, with a dramatic decrease (∼90%) after death that correlated with oosporein production. In vitro studies verified the inhibitory activity of oosporein against bacteria derived from insect cadavers. These results suggest that oosporein acts as an antimicrobial compound to limit microbial competition on B. bassiana-killed hosts, allowing the fungus to maximally use host nutrients to grow and sporulate on infected cadavers.Beauveria bassiana | oosporein | biological role | transcription factor | fungal-bacterial competition

Cyclosporine Biosynthesis in Tolypocladium inflatum Benefits Fungal Adaptation to the Environment

“…A few secondary metabolites produced by insect-pathogenic fungi have been established to contribute to fungal virulence against insect hosts such as the destruxins produced by Metarhizium species (27) and the beauvericin and oosporein biosynthesized by Beauveria bassiana (28, 29). The oosporein produced by Beauveria could also help the fungus to outcompete bacterial growth during fungal colonization of insect hosts (30, 31). Consistent with the immune inhibition and insecticidal effect of CsA on insects (16, 32), our insect bioassays revealed that abolishment of CSN production impaired fungal virulence against insect hosts.…”

The Thm1 Zn(II)2Cys6 transcription factor contributes to heat, membrane integrity and virulence in the insect pathogenic fungus Beauveria bassiana

“…Given that the ΔBbThm1 showed only mild to moderate effects to other stress phenotypes, that is, oxidative, osmotic and/or general growth, these data suggest that heat and/or detergent susceptibility reflect critical specific factors contributing to infection, a finding that has not been previously reported and/or known to be important for successful mycosis by this pathogen. To the best of our knowledge to date, only one other Gal4-like TF has been substantially characterized in entomopathogenic fungi, namely the Ops3 TF that is part of and a regulator of the oosporein secondary metabolite gene cluster Fan et al, 2017).…”

“…Infection occurs via adhesion of fungal conidia to the cuticle of the insect and subsequent direct penetration of the exoskeleton (Ortiz-Urquiza and . Penetrating hyphae elaborate in vivo hyphal bodies that proliferate within the host hemocoel, ultimately growing from the inside out to sporulate on the host cadaver Wanchoo et al, 2009;Fan et al, 2017). For most susceptible insects, death typically occurs within 3-6 days of post-infection, although significant variation can be seen between different hosts.…”

“…Similarly ablation of the stress response TF, Msn2 or the forkhead TF (Fkh2) resulted in wide ranging effects on conidiation, stress, response, development and cell cycle control, secondary metabolite production and virulence (Luo et al, 2014b;Wang et al, 2015). The OpS3 TF controlling expression of the B. bassiana oosporein biosynthetic cluster as well as a novel upstream zinc finger TF that control OpS3 expression have been characterized, revealing new insights into various aspect of the infection process Fan et al, 2017). In addition, characterization of the transcriptional co-activator, multiprotein bridging factor 1 (MBF-1) has indicated a role for this protein in hyphal morphogenesis, stress tolerance and virulence, as well as interactions with TATAbinding protein and an AP-1 like TF Song et al, 2015;Chu et al, 2018).…”

Precursor‐Directed Diversification of Cyclic Tetrapeptidic Pseudoxylallemycins

“…Fungi are known producers of a broad array of structurally diverse NRPS‐derived natural products, which include immunosuppressant cyclosporine from Tolypocladium inflatum , insecticidal destruxin from Metarhizium anisopliae , xyloallenolide A containing an aromatic allenic ether moiety from a mangrove‐associated Xylaria sp. 2508, and insecticidal and antitumor bassianolide and beauvericin from Beauveria bassiana . We recently reported the isolation of antibacterial tetracyclic peptides named pseudoxylallemycins A–F ( 1 – 6 ) from the stowaway fungus Pseudoxylaria sp.…”