Biosynthesis of the 2‐Pyridone Tenellin in the Insect Pathogenic Fungus Beauveria bassiana

Abstract

Genomic DNA from the insect pathogenic fungus Beauveria bassiana was used as a template in a PCR with degenerate primers designed to amplify a fragment of a C‐methyl transferase (CMeT) domain from a highly reduced fungal polyketide synthase (PKS). The resulting 270‐bp PCR product was homologous to other fungal PKS CMeT domains and was used as a probe to isolate a 7.3‐kb fragment of genomic DNA from a Bam H1 library. Further library probing and TAIL‐PCR then gave a 21.9‐kb contig that encoded a 12.9‐kb fused type I PKS–NRPS ORF together with ORFs encoding other oxidative and reductive enzymes. A directed knockout experiment with a BaR cassette, reported for the first time in B. bassiana, identified the PKS–NRPS as being involved in the biosynthesis of the 2‐pyridone tenellin. Other fungal PKS–NRPS genes are known to be involved in the formation of tetramic acids in fungi, and it thus appears likely that related compounds are precursors of 2‐pyridones in fungi. B. bassiana tenellin KO and WT strains proved to be equally pathogenic towards insect larvae; this indicated that tenellin is not involved in insect pathogenesis.