Molecular cloning of a glutathione S-transferase overproduced in an insecticide-resistant strain of the housefly (Musca domestica)

Glutathione transferase gene family from the housefly Musca domestica

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. The bank was prepared using poly(A)+ RNA from a housefly that is highly resistant to organophosphate insecticides because of enhanced expression of multiple members of the glutathione transferase gene family. The DNA sequence of each is reported and has a complete open reading frame that specified an amino acid sequence similar to other dipteran glutathione transferases. Based on phylogenetic analysis, we can conclude that the insect glutathione transferase gene family falls into two groups, each of which evolves at a different rate, presumably due to differences in functional constraints. We show that MdGST-1 (and their homologues from Drosophila and Lucilia) evolve at a significantly slower rate than the other members of the gene family. Each housefly GST cDNA was inserted into a bacterial plasmid expression system and a glutathione transferase activity was expressed in Escherichia coli. The transcription pattern of each of these glutathione transferases was examined in a variety of different housefly strains that are known to differ in their resistance to organophosphate insecticides due to different patterns of glutathione transferase expression. We found that the level of transcription for two of our clones was positively correlated with the level of organophosphate resistance.

Resistance-associated point mutations in insecticide-insensitive acetylcholinesterase

Extensive utilization of pesticides against insects provides us with a good model for studying the adaptation of a eukaryotic genome to a strong selective pressure. One mechanism of resistance is the alteration of acetylcholinesterase (EC 3.1.1.7), the molecular target for organophosphates and carbamates. Here, we report the sequence analysis of the Ace gene in several resistant field strains of Drosophila melanogaster. This analysis resulted in the identification of five point mutations associated with reduced sensitivities to insecticides. In some cases, several of these mutations were found to be combined in the same protein, leading to different resistance patterns. Our results suggest that recombination between resistant alleles preexisting in natural populations is a mechanism by which insects rapidly adapt to new selective pressures.

Immunological comparison of cytosolic glutathione S-transferases between rat and two strains of houseflies

Five different antisera, which include three antisera raised against rat liver glutathione S-transferases (GST), one antiserum raised against human pi GST, and one antiserum raised against housefly GST1, were used to examine their cross-reactivity with different classes of GST subunits isolated from rat liver and the housefly. Two classes of rat liver GSTs, alpha and mu, were isolated from rat liver and two classes of housefly GSTs, GST1 and GST2, were isolated from both CSMA and Cornell-R strains. Antiserum against GST 3-3 was the most reactive antiserum and reacted not only with the mu class of GSTs but also with the GST1 class from both CSMA and Cornell-R strains. Antiserum against human pi GST and antiserum against housefly GST1 had weak immunological reactivity toward the GST1 class from both strains of housefly. Antiserum against GST 4-4 and antiserum against GST 1-1 had no immunological reactivity toward any class of GSTs from housefly. None of the five antisera had any immunological cross-reactivity toward subunit 2 of the alpha class of rat GST and the GST2 class of housefly GSTs from both strains.

Purification, molecular cloning and heterologous expression of a glutathione S-transferase from the Australian sheep blowfly (Lucilia cuprina)

Three glutathione S-transferases from Lucilia cuprina (Australian sheep blowfly) pupae were purified by affinity chromatography and anion-exchange chromatography. One isoenzyme was composed of M(r)-24,800 subunits, and two isoenzymes had subunits of M(r) 23,900. The M(r)-23,900 subunits showed immunological identity and were immunologically distinct from the M(r)-24,800 subunits. All three enzymes were active with the substrate 1-chloro-2,4-dinitrobenzene and had low activity with 1,2-dichloro-4-nitrobenzene. A cDNA clone encoding a M(r)-23,900 subunit (LuGST1) was isolated and sequenced. The sequence has close similarities (> 81%) to that of GSTs from the fruitfly Drosophila melanogaster and Musca domestica (housefly). The deduced amino acid sequence of the Lu GST1 subunit showed no significant similarity to that of the mammalian GSTs to the Alpha, Mu and Pi classes, but shows some similarity (33%) over the first 100 residues with the rat subunit 12 Theta-class GST. Southern blots of genomic DNA hybridized with the LuGST1 cDNA identified many hybridizing fragments. Taken together, these data indicated that the L. cuprina genome contains multiple glutathione S-transferase genes.

Constitutive overexpression of the cytochrome P450 gene CYP6A1 in a house fly strain with metabolic resistance to insecticides

Messenger RNA levels of the cytochrome P450 gene CYP6A1 were measured in the insecticide resistant Diazinon-R 'Rutgers' strain and in the susceptible strain sbo of the house fly with a cloned cDNA probe. The constitutive expression of the CYP6A1 gene was at least 10 times higher in the Rutgers strain than in the sbo strain. In both strains, CYP6A1 was inducible by phenobarbital treatment of the flies. Analysis of genomic DNA from the two strains indicated that there was no amplification of the CYP6A1 gene in the Rutgers strain. A developmental analysis revealed that CYP6A1 is expressed larvae and adults, and very low levels of CYP6A1 mRNA were detected in eggs and pupae. The constitutive overexpression of CYP6A1 in the Rutgers strain was observed in both larvae and adults. Crosses between the Rutgers strain and the multiply marked sbo strain revealed that the high constitutive expression of CYP6A1 in the Rutgers strain is controlled by one or more loci located on chromosome II.

A glutathione S-transferase gene of the vector mosquito, Anopheles gambiae

A cDNA (D7-8B) 1456 nucleotides in length was isolated from an adult, female-specific Anopheles gambiae library and identified as a member of the glutathione S-transferase (GST) gene family by virtue of the inferred amino acid sequence. The gene, AgGST2-1, specifies a protein that is 57% identical to the Drosophila melanogaster gene, DmGST-2. The conceptual translation product also shows similarity to the pi family of vertebrate GSTs. Northern analysis reveals multiple and abundant transcription products in larval, pupal and adult RNA preparations. Southern analyses of genomic DNA and hybridizations in situ to polytene chromosomes both suggest that there are multiple members of the GST gene family in Anopheles gambiae.

Genetic and molecular evidence for a trans-acting regulatory locus controlling glutathione S-transferase-2 expression in Aedes aegypti

The amount of glutathione S-transferase-2 (GST-2) protein and enzyme activity in a mutant strain (strain GG) of the yellow fever mosquito (Aedes aegypti) is approximately 25-fold higher than in the wild-type (++) strain. The mode of inheritance of the GG phenotype was studied in F1 and backcross progeny using GST enzyme assays, isozyme-specific antisera, and Northern blot analysis. Enzyme assay of parental and F1 progeny showed that the ++ phenotype was dominant to the GG phenotype. This was true for larvae as well as for all tissues examined in adults in both sexes. Immunoblotting experiments showed that, like the ++ strain, F1 larvae and adults express very low levels of GST-2 protein compared with the GG strain. Northern blotting experiments showed that the steady-state levels of GST-2 mRNA in parental and F1 hybrid larvae closely matched the enzyme activity and immunological data. These results suggest the existence of a trans-acting regulatory locus that acts to repress GST-2 mRNA transcription and/or decrease GST-2 mRNA stability in ++ and F1 hybrids. GST enzyme activity in backcross progeny, however, did not segregate into the two distinct phenotypes (low and high) predicted for a single locus, dominant allele model. Backcross progeny expressed a wide range of GST activity and GST-2 protein amount with no apparent fit to simple Mendelian ratios. These backcross data suggest that additional loci are also involved in regulating GST-2 isozyme expression.(ABSTRACT TRUNCATED AT 250 WORDS)