Osa-containing Brahma chromatin remodeling complexes are required for the repression of Wingless target genes

The Wingless signaling pathway directs many developmental processes in Dros ophila by regulating the expression of specific downstream target genes. We report here that the product of the trithorax group gene osa is required t o repress such genes in the absence of the Wingless signal. The Wingless-re gulated genes nubbin, Distal-less, and decapentaplegic and a minimal enhanc er from the Ultrabithorax gene are misexpressed in osa mutants and represse d by ectopic Osa. Osa-mediated repression occurs downstream of the up-regul ation of Armadillo but is sensitive both to the relative levels of activati ng Armadillo/Pangolin and repressing Groucho/Pangolin complexes present and to the responsiveness of the promoter to Wingless. Osa functions as a comp onent of the Brahma chromatin-remodeling complex; other components of this complex are likewise required to repress Wingless target genes. These resul ts suggest that altering the conformation of chromatin is an important mech anism by which Wingless signaling activates gene expression.