ISOLATION AND CHARACTERIZATION OF AN UNAMPLIFIED ESTERASE B3 GENE FROM MALATHION-RESISTANT CULEX TARSALIS

A malathion-resistant strain of Culex tarsalis has a malathion carboxy lesterase which rapidly hydrolyzes the insecticide. This is in contras t to organophosphate-resistant strains of C. quinquefasciatus and C. p ipiens, which have elevated levels of general B esterases due to ampli fication of the corresponding genes, producing increased amounts of en zyme which appear to protect the insects by sequestering the insectici de. The contribution to resistance of the homologous esterase B3 (Est beta 3) gene (est beta 3) in C. tarsalis was investigated by cloning a nd characterizing sequences from resistant and susceptible strains. es t beta 3 is similar to est beta 1, both structurally and in sequence. The first intron of est beta 3, however has a region of extensive repe ats which may be responsible for the inefficient processing of the tra nscript. Southern blots indicate that the gene is single copy in. both strains, and northern blots show that it is not greatly overexpressed in the resistant insects. est beta 3 cDNAs from resistant and suscept ible strains have 98% amino acid identity. It appears that, in contras t to other studies, est beta 3 does not play a significant role in ins ecticide resistance in our strains of C. tarsalis, and the molecular r esponses of pest insects to organophosphates may be more diverse than has been suggested.