Historically, fly and human Polycomb proteins were considered
textbook exemplars of transcriptional repressors, or proteins
that silence the process by which DNA gives rise to new
proteins. Now, work by a team of researchers at the Stowers
Institute for Medical Research challenges that dogma. In a cover
story in the May 2012 issue of the journal Molecular and
Cellular Biology, Stowers Investigator Ali Shilatifard, Ph.D.,
and his team report that in the fruit fly Drosophila
melanogaster a component of the Polycomb repressive complex 2
(PRC2), which is called Jarid2, occasionally activates gene
expression. Polycomb proteins, a group of chromatin-binding
factors, were initially found to control how developing fly
embryos undergo segmentation. Subsequently, researchers
discovered that humans also express a gaggle of proteins
resembling fly Polycomb factors and that these proteins have
crucially significant roles: under normal conditions PRC2
proteins are required for the differentiation of stem cells, but
when ov