Heterologous expression of four glutathione transferase genes genetically linked to a major insecticide-resistance locus from the malaria vector Anopheles gambiae

A cluster of eight genes encoding glutathione transferases (GSTs) are located on division 33B of polytene chromosome arm 3R of the African malaria mosquito, Anopheles gambiae. This region of the genome contains a major 1,1,1-trichloro-2,2bis-(p-chlorophenyl)ethane (DDT)-resistance locus, rtd1. These GSTs belong to the insect-specific Epsilon class and share between 22.6 and 65.2 % identity at the amino acid level. Two distinct allelic variants of the Epsilon GST, GSTe1, differing at 12 out of 224 amino acid residues, are present in laboratory and field populations of A. gambiae. To investigate the possible role of these GSTs in conferring resistance to the insecticide DDT, both GSTe1 alleles, plus three additional members of this gene cluster, were expressed in Escherichia coli and the recombinant proteins biochemically characterized. The five putative glutathione transferases encoded catalytically active subunits with variable biochemical properties. For example, the two allelic variants of GSTE1-1 encoded proteins with over 100-fold variation in peroxidase activity, while the three remaining GSTs had no detectable peroxidase activity. Only GSTE2-2 was able to metabolize DDT. Western blots using antibodies raised against these GSTs indicated that the expression of GSTE2-2 is elevated in a DDT-resistant strain of A. gambiae.