SPECIFIC INDUCTION OF A CUTICLE-DEGRADING PROTEASE OF THE INSECT PATHOGENIC FUNGUS METARHIZIUM-ANISOPLIAE

The insect pathogenic fungus Metarhizium anisopliae produces several e xtracellular cuticle-degrading proteases and evidence is consistent th at one of these, a chymoelastase PR1, is a determinant of pathogenicit y. We have shown previously that the wide-domain regulatory circuits o f carbon and nitrogen derepression regulate PR1 production. In the pre sent work we have established in addition that PR1 is specifically ind uced by insect cuticle, but not by other soluble or insoluble proteina ceous substrates. The feeding of elastin or collagen to derepressed es tablished mycelium (starved for carbon and nitrogen) did not enhance P R1 production significantly and the soluble proteins BSA and gelatin r apidly and completely repressed PR1. The carbohydrate polymers cellulo se and xylan gave derepressed basal levels of PR1. However, addition o f locust cuticle enhanced PR1 production to a level approximately 10-f old that of derepressed mycelium. In order to establish if the enhanci ng effect of insect cuticle on PR1 production was due to specific indu ction or merely a reflection of enhanced growth on this insoluble dual carbon and nitrogen source, ergosterol was used as a measure of funga l growth. Expressing enzyme activity per mg dry weight showed that PR1 production in cuticle cultures increased approximately five- and nine fold after 12 and 24 h growth compared with elastin-grown cultures. Th us, the substantial increase in PR1 production on cuticle was shown no t to be a function of fungal growth and this confirms that PR1 is indu ced by a component of insect cuticle; we believe this is the first rep ort of induction by a specific substrate for any microbial protease.