Mosquito carboxylesterase estα2¹ (A₂): Cloning and sequence of the full-length cDNA for a major insecticide resistance gene worldwide in the mosquito Culex quinquefasciatus

Organophosphorus insecticide resistance in Culex mosquitoes is commonly caused by increased activity of one or more esterases. The commonest phenotype involves elevation of the esterases Estα2 (A₂) and Estβ2 (B₂). A cDNA encoding the Estα2 esterase has now been isolated from a Sri Lankan insecticide-resistant mosquito (Culex quinquefasciatus, Say) expression library. In line with a recently suggested nomenclature system (Karunaratne, S. H. P. P. (1994) Characterization of Multiple Variants of Carboxylesterases Which Are Involved in Insecticide Resistance in the Mosquito Culex quinquefasciatus. Ph.D. thesis, University of London), as the first sequenced variant of this esterase, it is now referred to as Estα2¹. The full-length cDNA of estα2¹ codes for a 540-amino acid protein, which has high homology with other esterases and lipases and belongs to the serine or B-esterase enzyme family. The predicted secondary structure of Estα2¹ is similar to the consensus secondary structure of proteins within the esterase/lipase family where the secondary and tertiary structures have been resolved. The level of identity (∼47% at the amino acid level) between the estα2¹ and the various Culex estβ (B₁ and B₂) cDNA alleles that have been cloned and sequenced suggests that the two esterase loci are closely related and arose originally from duplication of a common ancestral gene. The lack of a distinct hydrophobic signal sequence for Estα2¹ and two possible N-linked glycosylation sites, both situated close to the active site serine, suggest that it is a nonglycosylated protein that is not exported from the cell. Southern and dot blot analysis of genomic DNA from various insecticide-resistant and susceptible mosquito strains show that the estα2¹ gene, like estβ2¹, is amplified in resistant strains. The restriction fragment length polymorphism patterns, after probing Southern blots of EcoRI-digested genomic DNA with estα2¹ cDNA, show that the amplified and nonamplified estα alleles differ in the resistant and susceptible Sri Lankan mosquitoes.