PNAS Plus Significance Statements

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 midinfection 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. (See pp. E1578–E1586.)