Sweet Silencing

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Science  03 Jul 2009:
Vol. 325, Issue 5936, pp. 45-46
DOI: 10.1126/science.1177264

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Knowledge about protein modifications in gene regulation is rapidly expanding. The various modifications of histones (proteins associated with DNA) alone, including myriad phosphorylations, acetylations, methylations, and ubiquitylations, occur at more than 25 different locations on a nucleosome (the compact structure of histones and DNA that comprise chromatin). That doesn't include the many covalent modifications on nonhistone transcription factors, which can also be phosphorylated, acetylated, or methylated. By contrast, modification by sugar molecules might appear less relevant to regulating chromatin. Most sugar-conjugated proteins accumulate in domains at the cell surface or intracellular membrane compartments, far from the hub of gene transcription activity in the nucleus. A notable exception is modification by O-linked N-acetylglucosamine (O-GlcNAc) on serine and threonine residues of cytoplasmic and nuclear proteins (1). On page 93 in this issue, Gambetta et al. (2) reveal a key role for O-GlcNAc glycosylation in gene silencing by Polycomb group (PcG) proteins.