Fungal biosynthesis of the bibenzoquinone oosporein to evade insect immunity

Abstract

Quinones are widely distributed in nature and exhibit diverse biological or pharmacological activities; however, their biosynthetic machineries are largely unknown. The bibenzoquinone oosporein was first identified from the ascomycete insect pathogen Beauveria bassiana >50 y ago. The toxin can also be produced by different plant pathogenic and endophytic fungi with an array of biological activities. Here, we report the oosporein biosynthetic machinery in fungi, a polyketide synthase (PKS) pathway including seven genes for quinone biosynthesis. The PKS oosporein synthase 1 (OpS1) produces orsellinic acid that is hydroxylated to benzenetriol by the hydroxylase OpS4. The intermediate is oxidized either nonenzymatically to 5,5′-dideoxy-oosporein or enzymatically to benzenetetrol by the putative dioxygenase OpS7. The latter is further dimerized to oosporein by the catalase OpS5. The transcription factor OpS3 regulates intrapathway gene expression. Insect bioassays revealed that oosporein is required for fungal virulence and acts by evading host immunity to facilitate fungal multiplication in insects. These results contribute to the known mechanisms of quinone biosynthesis and the understanding of small molecules deployed by fungi that interact with their hosts.

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

“…The oosporein biosynthetic gene cluster was partially characterized recently (22,23). The cluster contains 14 putative ORFs (GenBank accession nos.…”

“…EJP62787-EJP62800, antiSMASH predicted) and includes a gene for a (type I) nonreducing polyketide synthase gene (OpS1; GenBank accession no. EJP62792) that produces the orsellinic acid precursor to oosporein (23) (Fig. 2A).…”

“…The latter gene cluster was later verified experimentally as being responsible for oosporein synthesis via both genetic and biochemical characterization (23). The oosporein biosynthetic cluster comprises at least seven ORFs, including a nonreducing polyketide synthase [oosporein synthase 1 (OpS1)], a membrane transporter (OpS2), a transcription factor (TF) (OpS3), and four additional enzymes involved in oosporein biosynthesis (OpS4-OpS7).…”

“…system (23). The mechanism behind OpS3 activation is unknown; however, it was previously shown that the B. bassiana ortholog of the Msn2 stress response TF acts as a pH-dependent negative regulator of oosporein biosynthesis (24).…”

“…The mechanism behind OpS3 activation is unknown; however, it was previously shown that the B. bassiana ortholog of the Msn2 stress response TF acts as a pH-dependent negative regulator of oosporein biosynthesis (24). Based on characterization of the various OpS gene deletion mutants, a putative role for oosporein in hyphal body development and immune evasion has been suggested (23).…”