The same amino acid substitution in orthologous esterases confers organophosphate resistance on the house fly and a blowfly

Organophosphate (OP) insecticide resistance in certain strains of Musca domestica is associated with reduction in the carboxylesterase activity of a particular esterase isozyme. This has been attributed to a 'mutant ali- esterase hypothesis', which invokes a structural mutation to an ali-esterase resulting in the loss of its carboxylesterase activity but acquisition of OP hydrolase activity. It has been shown that the mutation in Lucilia cuprina is a Gly¹³⁷ → Asp substitution in the active site of an esterase encoded by the LcαE7 gene (Newcomb, R.D., Campbell, P.M., Ollis, D.L., Cheah, E., Russell, R.J., Oakeshott, J.G., 1997. A single amino acid substitution converts a carboxylesterase to an organophosphate hydrolase and confers insecticide resistance on a blowfly. Proc. Natl. Acad. Sci. USA 94, 7464- 7468). We now report the cloning and characterisation of the orthologous M. domestica MdαE7 gene, including the sequencing of cDNAs from the OP resistant Rutgers and OP susceptible sbo and WHO strains. The MdαE7 gene has the same intron structure as LcαE7 and encodes a protein with 76% amino acid identity to LcαE7. Comparisons between susceptible and resistance alleles show resistance in M. domestica is associated with the same Gly¹³⁷ → Asp mutation as in L. cuprina. Bacterial expression of the Rutgers allele shows its product has OP hydrolase activity. The data indicate identical catalytic mechanisms have evolved in orthologous MdαE7 and LcαE7 molecules to endow diazinon-type resistance on the two species of higher Diptera.