Development of an insulated reporter system to search for cis-acting DNA sequences required for dosage compensation in Drosophila

Dosage compensation (equalisation of X-linked gene products) occurs in Dros ophila melanogaster by a two-fold transcriptional increase of X-linked gene expression in the male. The cis-acting X-linked DNA sequences required for dosage compensation (called DCREs) remain elusive, despite numerous attemp ts to identify them. We have developed an insulated reporter system to mini mise problems previously encountered with identifying these elements. The s ystem consists of the constitutive autosomal armadillo promoter fused to th e lacZ reporter gene (called arm-lacZ) which was flanked by SCS insulator e lements to block potential repressive effects of an autosomal chromatin env ironment. Seven X-linked DNA fragments, totaling 62.7 kb, were each inserte d between the SCS element and the armadillo promoter. If an X-linked fragme nt contains a DCRE, then transgenic males carrying an autosomal insert of t he construct should produce twice the ss-galactosidase activity of females. However, in all cases, males and females expressed the same level of lacZ. Thus, it's likely that none of the X-linked fragments contained a DCRE, su ggesting these elements may be rarer than previously thought. The insulated reporter system was also used to test the hypothesis that some genes may b e dosage compensated due to repression by Sex lethal (Sxl) in females. A fr agment from the runt gene containing three Sxl binding sites was inserted i nto the 3' untranslated region of arm-lacZ. Transgenic males carrying an au tosomal insert of the construct had on average 1.31-1.46 times the level of ss-galactosidase than females, suggesting that some genes could be compens ated, at least partially, by Sxl repression in females.