Identification of a delta-Endotoxin Gene Product Specifically Active against Spodoptera littoralis Bdv. among Proteolysed Fractions of the Insecticidal Crystals of Bacillus thuringiensis subsp. aizawai 7.29

At least three different insecticidal crystal protein genes were shown to be expressed in Bacillus thuringiensis subsp. aizawai 7.29, a strain that is potentially active against the cotton leafworm Spodoptera littoralis Bdv. Among crude K-60 fractions (60- to 70-kilodalton kDa] molecules) that were products of proteolysed crystals containing the active domains of the protoxin molecules, we were able to distinguish several distinct components on the basis of their antigenic relationship and their larvicidal properties. A purified fraction designated SF2 was a 61-kDa component specifically active against Pieris brassicae L. and homologous to the B. thuringiensis subsp. berliner 1715 plasmid-encoded crystal protein. A second fraction designated SF1 was composed of 63- and 65-kDa polypeptides and was specifically active against S. littoralis. The SF1 fraction and particularly the 65-kDa component were not antigenically related to the 61-kDa component. The purified fractions were compared with the products of three different crystal protein genes we previously cloned from total DNA of B. thuringiensis subsp. aizawai, among them a new type of crystal protein gene encoding a protein that is specifically active against S. littoralis and other insects of the Noctuidae family. This approach led us to consider the 65-kDa component as a minimum active part of a delta-endotoxin that is encoded by this new gene. Products of the two other cloned genes can be correlated with the 61- and 63-kDa components, respectively. Thus, in B. thuringiensis subsp. aizawai 7.29, multiple delta-endotoxin genes of different structural types direct the synthesis of several delta-endotoxins with different host specificities which were identified as components of the insecticidal crystals.