Flexibility in the protoxin composition of Bacillus thuringiensis.

In at least three Bacillus thuringiensis subspecies, multiple protoxin genes are confined to just a few of the many plasmids with two or more on one of > 100 mDa and a particular gene, cryIA(b), on a 40-50 mDa plasmid. The latter is unstable but can be maintained in the population by cell mating. Cells which had lost this plasmid compensated by increasing transcription of the remaining protoxin genes resulting in the formation of inclusions which differed from those in the parental strains in their toxicity profiles for selected insects as well as their solubility. Instability of a particular protoxin-encoding plasmid appears to be a mechanism for rapidly shifting the protoxin gene complement and thus the toxicity profiles of these bacteria.