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

Significance Since the discovery of oosporein more than 70 years ago, there have been conflicting reports on its potential antimicrobial and insecticidal activities. Our results indicate that oosporein is unlikely to function as an insect toxin or to be involved in early to mid-infection processes, including penetration and immune evasion. Instead, oosporein most likely functions after death of the host to thwart bacterial competition on a host cadaver, allowing the fungus to maximally use host nutrients and complete its life cycle. Our data also reveal that oosporein production is regulated by a cascade of transcription factors, with BbSmr1 acting as an upstream negative regulator, targeting the expression of OpS3, which in turn acts as a positive regulator of the oosporein biosynthetic gene cluster.