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

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Abstract
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. Taken together, the results suggest that overexpression of GST-2 in the GG strain of Aedes aegypti is due to allelic segregation at a trans-acting regulatory locus, most likely a repressor, with additional effects due to other modifying loci.

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