Factors on the third chromosome affect the level of Cyp6a2 and Cyp6a8 expression in Drosophila melanogaster

The expression of two second chromosome-linked cytochrome P450 genes, Cyp6a 2 and Cyp6a8, of Drosophila melanogaster was measured in various strains. S ix different strains, including ry(506) and 91-C, showed low or undetectabl e levels of CYP6A2 and CYP6A8 mRNAs, suggesting that low expression is the wild-type phenotype of Cyp6a2 and Cyp6a8 genes. In the 91-R and MHIII-D23 s trains, however, both these genes are overexpressed. In order to examine th e genetic basis of Cyp6a2 and Cyp6a8 expression, CYP6A2 and CYP6A8 RNA leve ls were measured in the Fl hybrids of overproducer (91-R and MHIII-D23) and underproducer (rq(506) and 91-C) strains. Results showed that the total am ounts of CYP6A2 and CYP6A8 mRNAs in the Fl hybrids were lower than half the amounts of these RNAs found in the overproducer parental strains. This sug gested that the underproducer strains carry loci which downregulate Cyp6a2 and Cyp6a8 gene expression. To determine the chromosome linkage of these lo ci, several stocks homozygous for the second chromosome of overproducer 91- R strain and, therefore, homozygous for the Cyp6a2-91R and Cyp6a8-91R allel es were synthesized. The third chromosomes in all these stocks were from th e underproducer ry(506) strain. The levels of expression of both Cyp6a2-91R and Cyp6a8-91R genes in these three stocks were significantly lower than t hat observed in the 91-R strain. One of these stocks, named iso-2, showing reduced expression, was used to synthesize two new isogenic stocks by resub stituting the third chromosome of ry(506) origin with third chromosomes of the 91-R strain. Expression of both Cyp6a2-91R and Cyp6a8-91R alleles was f ound to be much higher in these two resubstituted isogenic stocks than in t he progenitor iso-2 stock. Taken together, these results suggest that the s econd chromosome-linked Cyp6a2 and Cypa8 genes are regulated by loci presen t on the third chromosome, and the wild-type function of these loci is to r epress these two Cyp genes. The data also suggest that Cyp6a2 and Cyp6a8 ov erexpression in the 91-R and MHIII-D23 strains is more likely due to mutati on in the repressor locus (or loci) rather than in the cis-regulatory seque nces of the Cyp6a-2 and Cypda8 genes. (C) 2000 Elsevier Science B.V. All ri ghts reserved.