THE PROTOXIN COMPOSITION OF BACILLUS-THURINGIENSIS INSECTICIDAL INCLUSIONS AFFECTS SOLUBILITY AND TOXICITY

Most Bacillus thuringiensis strains producing toxins active on lepidop tera contain several plasmid encoded F-endotoxin genes and package rel ated protoxins into a single inclusion, It was previously found that i n B. thuringiensis subsp, aizawai HD133, which produces an inclusion c omprising the CryIAb, CryIC, and CryID protoxins, there is a spontaneo us loss in about 1% of the cells of a 45-mDa plasmid containing the cr yIAb gene, As a result, inclusions produced by the cured strain were l ess readily solubilized at pH 9.2 or 9.5 and had a decreased toxicity for Plodia interpunctella, despite the presence of the CryIC protoxin, which was active when solubilized, These results suggested that proto xin composition was a factor in inclusion solubility and toxicity and that the cryIAb gene, which is also present on an unstable plasmid in several other subspecies, may have a unique role in inclusion solubili ty and toxicity, Introduction of a cloned copy of this gene into the p lasmid-cured derivative of B. thuringiensis subsp. aizawai HD133 resul ted in an increase in the solubility at pH 9.2 of all of the inclusion proteins from less than 20% to greater than 45% and a lowering of the 50% lethal concentration (LC(50), in micrograms [dry weight] per squa re centimeter) of inclusions for Spodoptera frugiperda from 35 to 10, These values are the same as those found with inclusions from B. thuri ngiensis subsp, aizawai ai HD133, and in all cases, the LC(50) of the solubilized protoxins was 10, Transformants containing related cryIA g enes produced inclusions which were more than 95% solubilized at pH 9. 2 but also had LC(50) of 10. The presence or absence of a particular C ryIA protoxin is thus a major factor in inclusion solubility and toxic ity, Since there is instability of a plasmid containing the cryIAb gen e in many of these subspecies, the inclusions produced by a population of these cells must be heterogeneous in terms of solubility and thus toxicity. Such flexibility may be of adaptive value.