Bacillus thuringiensis delta-endotoxin Cry1C domain III can function as a specificity determinant for Spodoptera exigua in different, but not all, Cry1-Cry1C hybrids

In order to test our hypothesis that Bacillus thuringiensis delta-endotoxin Cry1Ca domain III functions as a determinant of specificity for Spodaptera exigua, regardless of the origins of domains I and II, we have constructed by cloning and in vivo recombination a collection of hybrid proteins conta ining domains I and II of various Cry1 toxins combined with domain III of C ry1Ca, Cry1Ab, Cry1Ac, Cry1Ba, Cry1Ea, and Cry1Fa all become more active ag ainst S. exigua when their domain III is replaced by (part of) that of Cry1 Ca, This result shows that domain III of Cry1Ca is an important and versati le determinant of S. exigua specificity. The toxicity of the hybrids varied by a factor of 40, indicating that domain I and/or II modulate the activit y as well. Cry1Da-Cry1Ca hybrids were an exception in that they were not si gnificantly active against S. exigua or Manduca sexta, whereas both parenta l proteins were highly toxic. Incidentally, in a Cry1Ba-Cry1Ca hybrid, Cry1 Ca domain III can also strongly increase toxicity for hi. sexta.