Larvicidal activity of chimeric Bacillus thuringiensis protoxins.

Bacillus thuringiensis subspecies kurstaki (Btk) and subspecies berliner (Btb) both produce lepidopteran-specific larvicidal protoxins with different activities against the same insect species. Toxic activity resides in the amino-terminal half of both protoxins, whereas the carboxy-terminal half of the molecules is not required for toxicity. The protoxins are 90% homologous, with a major cluster of differences in the amino-terminal half, and a 26 consecutive amino-acid insertion within the carboxy-terminal half of the Btk protoxin. Protoxin chimeras composed of the amino-terminal half of one subspecies and the carboxy-terminal half of the other were generated. Wild-type and chimeric protoxins were compared in bioassays against tobacco hornworm larvae. The amino-terminal half, the toxin itself, dictates specific larvicidal activity.