Cloning of the nprA gene for neutral protease A of Bacillus thuringiensis and effect of in vivo deletion of nprA on insecticidal crystal protein.

The nprA gene, encoding Bacillus thuringiensis neutral protease A, was cloned by the use of gene-specific oligonucleotides. The size of neutral protease A deduced from the nprA sequence was 566 amino acids (60,982 Da). The cloned nprA gene was partially deleted in vitro, and the deleted allele, designated nprA3, was used to construct an nprA3 strain (neutral protease A-deficient strain) of B. thuringiensis. Growth and sporulation of the nprA3 strain were similar to those of an isogenic nprA+ strain, although the extracellular proteolytic activity of the nprA3 strain was significantly less than that of the nprA+ strain. The nprA3 strain produced insecticidal crystal proteins that were more stable than those of the isogenic nprA+ strain after solubilization in vitro, and sporulated cultures of the nprA3 strain contained higher concentrations of full-length insecticidal crystal proteins than did those of its isogenic counterpart. The absence of neutral protease A did not affect the insecticidal activity of a lepidopteran-specific crystal protein of B. thuringiensis. These results indicate that crystal protein stability and yield may be improved by deletion of specific proteases from B. thuringiensis.