ROLE OF DNA IN THE ACTIVATION OF THE CRY1A INSECTICIDAL CRYSTAL PROTEIN FROM BACILLUS-THURINGIENSIS

The Cry1A insecticidal crystal protein (protoxin) from six subspecies of Bacillus thuringiensis as well as the Cry1Aa, Cry1Ab, and Cry1Ac pr oteins cloned in Escherichia coli was found to contain 20-kilobase pai r DNA, Only the N-terminal toxic moiety of the protoxin was found to i nteract with the DNA. Analysis of the crystal gave approximately 3 bas e pairs of DNA per molecule of protoxin, indicating that only a small region of the N-terminal toxic moiety interacts with the DNA, It is pr oposed that the DNA-protoxin complex is virus-like in structure with a central DNA core surrounded by protein interacting with the DNA with the peripheral ends of the C-terminal region extending outward, It is shown that this structure accounts for the unusual proteolysis observe d in the generation of toxin in which it appears that peptides are rem oved by obligatory sequential cleavages starting from the C terminus o f the protoxin. Activation of the protoxin by spruce budworm (Choristo neura fumiferana) gut juice is shown to proceed through intermediates consisting of protein-DNA complexes, Larval trypsin initially converts the 20-kilobase pair DNA-protoxin complex to a 20-kilobase pair DNA-t oxin complex, which is subsequently converted to a 100-base pair DNA-t oxin complex by a gut nuclease and ultimately to the DNA-free toxin.