Comparison of midgut proteinases in Bacillus thuringiensis-susceptible and-resistant European corn borer, Ostrinia nubilalis (Lepidoptera : Pyralidae)

The midgut proteinases from a Bacillus thuringiensis-susceptible (IA-S) and four laboratory-selected resistant strains (KS-SC, KS-NE, IA-1, and IA-3) of European corn borer (Ostrinia nubilalis) were characterized using three synthetic substrates, N alpha-benzoyl-L-arginine p-nitroanilide (BApNA) for trypsin-like, N-succinyl-ala-ala-pro-phe p-nitroanilide (SAAPFpNA) for chy motrypsin-like, and N-succinyl-ala-ala-pro-leu p-nitroanilide (SAAPLpNA) fo r elastase-like proteinase activities. The hydrolyzing efficiency of trypsi nlike proteinases, determined by V-max, decreased 35% in the KS-SC resistan t strain compared with the susceptible strain. There were no significant di fferences in the Michaelis constant (K-m) among the five strains for the sa me substrate. When the purified B, thuringiensis Cry1Ab protoxin was used a s the substrate, there was a detectable reduction in the hydrolysis of prot oxin that was mediated by midgut proteinases from the KS-SC strain compared with the IA-S strain. Thus, the reduced trypsin-like proteinase activity a ppeared to lead to the reduced activation of the B. thuringiensis protoxins . This may confer or contribute to B. thuringiensis resistance in this stra in. However, no significant difference was found in trypsin activity betwee n the LA-S strain and the three other resistant strains (i.e., KS-NE, IA-1, and IA-3) and in chymotrypsin activity among all strains examined. These r esults suggest that other resistance mechanisms are responsible for the B. thuringiensis resistance in the KS-NE, IA-1, IA-2, and IA-3 strains of Euro pean corn borer. (C) 1999 Academic Press.