Abstract: A high level of protease activities were expressed both by Zoophthora radicans original strain (R₀) and host-adapted strains (R₁ and R₅) when they grew in MS medium containing insect cuticle or gelatin. On the contrary, very low level of protease activities were detected in extremely poor nutrient medium (MS) or rich nutrient medium (Sabouraud's dexteose broth, SDB). A 37 kD band belonging to serine protease was found to be induced in any cultural condition, suggesting it is a conservative sequence coding protease for three strains. On the other hand, a 46 kD band belonging to metalloprotease was expressed only in MS medium containing gelatin; however, its expression could be inhibited by MS medium containing glucose. Interestingly, a protease band with molecular weight of 67 kD disappeared, whereas the bands of 46 kD and 117 kD became more obvious for isolates after several infection cycles through Plutella xylostella when isolates grew in SDB medium. The results indicated that inducible high substrate-specific extracellular protease was greatly expressed to adapt to new host during passages of subculture through P. xylostella.

Key words: Zoophthora radicans, passages of reisolate, extracellular proteases, substrate-specific proteases, inducible expression