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Two ways to condense

Heterochromatin condensation, shown by histone H3 phosphorylation (black, above dotted line), is disrupted in cells lacking Myb (below). LIPSICK/MACMILLAN

Gene-rich euchromatin and largely untranscribed heterochromatin may follow different rules to prepare for mitosis, according to J. Robert Manak, Joseph Lipsick, and colleagues (Stanford University, Stanford, CA).

From S phase to prophase, chromatin must replicate and then condense to prepare for mitosis. In most eukaryotic cells, euchromatin starts the process by replicating first, but it is the last to condense. Heterochromatin is sandwiched in between, opening for replication and then condensing straightaway.

Proper condensation, the authors find, requires Myb, a fly kinase that regulates S phase. Fly cells lacking Myb divided normally for about a dozen cycles but then began to arrest in prophase with condensation errors. "They start to condense," says Lipsick, "but they can't finish."

The failure, the group found, lay in euchromatin. Condensation was normal in heterochromatin but failed to spread to euchromatin. Myb was normally found on euchromatin, where it may maintain histone modifications that drive condensation either directly or via other proteins.

Lipsick thinks that cells might use a separate mechanism to make sure that heterochromatin condenses immediately after it replicates. "Heterochromatin is mostly composed of transposons," he says. "Like a computer virus, its goal is to make more copies of itself. It might be to a cell's advantage to keep it turned off whenever possible."

Reference:

Manak, J.R., et al. 2007. Nat. Cell Biol. doi:10.1038/ncb1580.

Alla Katsnelson

akatsnelson{at}gmail.com

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This Article Full Text (PDF, 1003K) PPT slides of all figures Alert me when this article is cited Services Email this article Similar articles in this journal Alert me to new content in the JCB Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Katsnelson, A. Search for Related Content PubMed Articles by Katsnelson, A. Social Bookmarking                            What's this?