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Genetic and molecular analysis of the drosophila melanogaster polycomb group gene additional sex combs Milne, Thomas Arthur


Proteins of the trithorax group (trxG) and the Polycomb group (PcG) maintain the spatially restricted active and repressed states respectively of the homeotic genes throughout development. Mutations in PcG genes cause homeotic genes to be ectopically expressed which generally results in posterior homeotic transformations. Mutations in trxG genes have the opposite effect. They reduce homeotic gene expression and generally cause anterior transformations. Mutations in trxG and PcG genes mutually suppress each others homeotic transformations, suggesting that the two groups of proteins either act antagonistically or that they have opposite and independent functions. Additional sex combs (Asx) can enhance both trxG and PcG homeotic mutations, suggesting that Asx is important for both activation and repression of the homeotic loci. In addition, Asx has both tissue and stage specific effects on homeotic gene regulation.To gain further insight into Asx function, I used antibodies to the Asx protein to examine its expression pattern in embryos and to show that Asx protein binds to sites on polytene chromosomes containing other P cG proteins, but that it also binds to some unique target sites. Asx protein is probably part of a large PcG complex that binds to many target sites but the fact that it can bind to some unique target sites also indicates that it may be a member of other non-PcG protein complexes. To identify and clone genes important for Asx activity, interactor cDNAs from a yeast 2 hybrid screen were mapped to specific chromosomal sites. A large deletion (Df[3L)ZN4T) that removes the locus of the z40 interactor genetically interacts with Asx and shows target specific homeotic regulatory defects. Also, the PcG gene super sex combs (sxc) shows a strong genetic interaction with Asx and an attempt was made to transposon tag and clone sxc. These results suggest that Asx is a component of both repression and activation and that its stage and tissue specific activities are modulated by interactions with specific protein subsets such as z40.

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