Deletion of aprA and nprA genes for alkaline protease A and neutral protease A from Bacillus thuringiensis: effect on insecticidal crystal proteins

The aprA gene encoding alkaline protease A (AprA) was cloned from Bacillus thuringiensis subsp. kurstaki, and the cloned gene was used to construct aprA-deleted ( aprA1) strains of B. thuringiensis. An aprA1 strain of B. thuringiensis that contained the wild-type gene for neutral protease A ( nprA + ) displayed levels of extracellular proteolytic activity that were similar to those of an aprA + nprA + strain. However, when EDTA was included in the protease assay to inhibit NprA activity the aprA1nprA + strain displayed only 2% of the extracellular proteolytic activity of the aprA + nprA + strain. A strain that was deleted for both aprA and nprA ( aprA1nprA3 strain) failed to produce detectable levels of proteolytic activity either in the presence or absence of EDTA in the assay. Compared with the aprA + nprA + strain the aprA1nprA + strain yielded 10% more full-length Cry1Bb crystal protein and the aprA1nprA3 strain yielded 25% more full-length Cry1Bb protein. No significant differences were seen in the 50% lethal dose of Cry1Bb protein from aprA + nprA + and aprA1nprA3 strains against three species of lepidopteran insects. These results suggest that enhanced yield of certain crystal proteins can be obtained by deletion of the genes aprA and nprA which are the major extracellular proteases of B. thuringiensis.