FIGURE 9-49 Opposing influence of Polycomb and Trithorax complexes on expression of the Hox transcription factor Abd-B in Drosophila embryos. At the stage of Drosophila embryogenesis shown, Abd-B is normally expressed only in posterior segments of the developing embryo, as shown at the top (wt) by immunostaining with a specific anti–Abd-B antibody. In embryos with homozygous mutations of Scm, a Polycomb gene (PcG) encoding a protein associated with the PRC1 complex, Abd-B expression is derepressed in all embryo segments. In contrast, in homozygous mutants of trx, a Trithorax gene (trxG), Abd-B repression is increased so that the protein is expressed at high concentrations only in the most posterior segment.

[From Klymenko, T., and Muller, J., “The histone methyltransferases Trithorax and Ash1 prevent transcriptional silencing by Polycomb group proteins,” EMBO Reports ©2004 John Wiley and Sons. Reproduced with permission of Wiley-VCH.]