Three new glutathione transferase (GST) genes from the housefly Musca
domestica are described. These genes, identified as MdGST-2, -3, and -
4, were from cDNA clones obtained from a cDNA bank in phage lambda. Th
e bank was prepared using poly(A)(+) RNA from a housefly that is highl
y resistant to organophosphate insecticides because of enhanced expres
sion of multiple members of the glutathione transferase gene family. T
he DNA sequence of each is reported and has a complete open reading fr
ame that specified an amino acid sequence similar to other dipteran gl
utathione transferases. Based on phylogenetic analysis, we can conclud
e that the insect glutathione transferase gene family falls into two g
roups, each of which evolves at a different rate, presumably due to di
fferences in functional constraints. We show that MdGST-1 (and their h
omologues from Drosophila and Lucilia) evolve at a significantly slowe
r rate than the other members of the gene family. Each house fly GST c
DNA was inserted into a bacterial plasmid expression system and a glut
athione transferase activity was expressed in Escherichia coli. The tr
anscription pattern of each of these glutathione transferases was exam
ined in a variety of different housefly strains that are known to diff
er in their resistance to organophosphate insecticides due to differen
t patterns of glutathione transferase expression. We found that the le
vel of transcription for two of our clones was positively correlated w
ith the level of organophosphate resistance.