We report the cloning and sequencing of a glutathione S-transferase (GST) gene from the housefly Musca domestica. A cDNA lambda gt11 library was prepared from the organophosphate insecticide-resistant housefly strain Cornell-R--a variant that has elevated GST activity. The lambda phage GST clone was identified on the basis of its ability to cross-hybridize to a GST DNA probe from Drosophila melanogaster. Based on amino acid homology to other GSTs and expression of GST activity in Escherichia coli, the Musca GST gene (MdGST-1) belongs to the GST gene family. Although organophosphate resistance in Cornell-R is largely due to one of the GSTs, MdGST-1 is probably not the enzyme responsible for resistance. The mutation that controls resistance to organophosphate insecticides in Cornell-R is highly unstable and we isolated spontaneous variants to both insecticide sensitivity and to even higher levels of resistance. This provided us with an isogenic set of three strains. We found that MdGST-1 transcript levels as measured by Northern assays are higher in all three Cornell-R strains relative to the sensitive wild type, but that the sensitive Cornell-R strain has more MdGST-1 transcript than does the highly resistant Cornell-R strain. These data as well as Southern analysis of genomic DNA allow us to conclude: (1) there are multiple GST genes in M. domestica; (2) the natural variant Cornell-R overproduces excess transcript from two and probably more of these genes; and (3) the unstable mutation in Cornell-R influences the levels of multiple GSTs.

ISSOR is an insertion sequence associated with the transposon Tn5. IS50R carries the structural genes for two proteins; one (Pl) is the Tn5 transposase, and the other (P2) is an inhibitor of transposition. These two proteins are translated from two different transcripts, ml and m2. When bacteriophage X::ISSOR DNA was introduced into a bacterial cell, ml and m2 were initially at relative levels of about 1 to 2. As time progressed the amount of ml fell, whereas the amount of m2 continued to increase, until after about 3 h the ratio of ml to m2 was about 1 to 80. The temporal changes in the levels of these transcripts correlated with temporal changes in P1 and P2 levels and Tn5 transposition that have been documented in other studies. We measured the stability of the messages and showed that the differences in the levels of ml and m2 must reflect real differences in the strengths of their promoters and that the changes in transcription kinetics are mediated by the dam methylation system of the cell and are not determined by IS50R products. Our results show that the 5' end of m2 is about twice as stable as that of ml, which raises the possibility that differential message stability does, in part, influence the ratio of inhibitor to transposase.