Glutathione S-transferases and insecticide resistance of diamondback moth

The role of glutathione S-transferase (GST, EC 2.5.1.18) in the degradation of and resistance to some insecticides of diamondback moth (DBM) larvae has been confirmed through the purification and characterization of its four isozymes from this notorious insect pest of cruciferous vegetables. Two immuno-related isozymes, GST-3 and GST-4, exhibit apparent substrate preference for 1,2-dichloro-4nitrobenzene, a model substrate, and several organophosphorus (OP) insecticides, i.e., parathion, methyl parathion and paraoxon. GST-3 exists in higher proportion in DBM strains showing OP resistance while GST-4 is observed only in DBM selected with teflubenzuron, a chitin synthesis inhibitor. With degenerate primers based on the N-terminal amino acid sequences of GST-3 and a CNBr-cleaved fragment of this protein, we have obtained a 128 bp DNA product from reverse transcription polymerase chain reaction (PCR). Screening a cDNA library prepared from a methyl parathion-resistant DBM strain with this PCR product as probe has yielded two clones. These two clones are proved to be the same with the longer one of 809 pb encoding a protein, pxGST3, of 216 amino acids with a predicted molecular mass of 24 kDa and a pI of 9.37. An alignment with the nine insect GSTs already cloned shows that PxGST3 shares the highest (46.3%) amino acid sequence identity to one of the two GSTs (MsGST1) of Manduca sexta. PxGST3 is believed to be the first cloned insect GST with a well-defined role in insecticide resistance.