Mode of action of Bacillus thuringiensis PS86Q3 strain in hymenopteran forest pests

The mode of action of Cry toxins has been described principally in lepidopt eran insects as a multistep process. In this work we describe the mode of a ction of a Cry toxin active in the common pine sawfly Diprion pini (Hymenop tera, Diprionidae). considered a major forest pest in Europe. Strain PS86Q3 contains a long bipyramidal crystal composed of five major proteins. The N -terminal sequence shows that the 155 kDa protein corresponds to Cry5B toxi n and the other proteins belong to the Cry5A subgroup. PCR analysis indicat es the presence of cry5Ac and cry5Ba genes, suggesting that Cry5A protein s hould be Cry5Ac. Activation of protoxins with trypsin or with midgut conten t from D. pini and Cephacia abietis (Hymenoptera, Pamphiliidae) (spruce web spinning sawfly), another important hymenopteran forest pest, produced a si ngle 75 kDa toxin that corresponded to Cry5A by N-terminal sequence and is responsible for the insecticidal activity. Homologous competition experimen ts with D. pini and C. abietis brush border membrane vesicles (BBMV) showed that the binding interaction of Cry5A is specific. Membrane potential meas urements using a fluorescent dye indicate that Cry5A toxin at nM concentrat ion caused immediate permeability changes in the BBMV isolated from both hy menopteran larvae. The initial response and the sustained permeability chan ge are cationic as previously shown for Cry1 toxins. These results indicate that the hymenopteran specific Cry5A toxin exerts toxicity by a similar me chanism as Cry1 toxins. (C) 2001 Elsevier Science Ltd. All rights reserved.