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 dom estica is associated with reduction in the carboxylesterase activity of a p articular esterase isozyme. This has been attributed to a 'mutant ali-ester ase hypothesis', which invokes a structural mutation to an ali-esterase res ulting in the loss of its carboxylesterase activity but acquisition of OP h ydrolase activity. It has been shown that the mutation in Lucilia cuprina i s a Gly(137)-->Asp substitution in the active site of an esterase encoded b y the Lc alpha E7 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 ins ecticide resistance on a blowfly. Proc. Natl. Acad. Sci. USA 94, 7464-7468) . We now report the cloning and characterisation of the orthologous M. dome stica Md alpha E7 gene, including the sequencing of cDNAs from the OP resis tant Rutgers and OP susceptible sbo and WHO strains. The Md alpha E7 gene h as the same intron structure as Lc alpha E7 and encodes a protein with 76% amino acid identity to Lc alpha E7. Comparisons between susceptible and res istance alleles show resistance in M. domestica is associated with the same Gly(137)-->Asp mutation as in L. cuprina. Bacterial expression of the Rutg ers allele shows its product has OP hydrolase activity. The data indicate i dentical catalytic mechanisms have evolved in orthologous Md alpha E7 and L c alpha E7 molecules to endow diazinon-type resistance on the two species o f higher Diptera. (C) 1999 Elsevier Science Ltd. All rights reserved.