The worm solution: a chromosome-full of condensin helps gene expression go down.

Dosage compensation in the nematode Caenorhabditis elegans is achieved by the binding of a condensin-like dosage compensation complex (DCC) to both X chromosomes in hermaphrodites to downregulate gene expression two-fold. Condensin I(DC), a sub-part of the DCC, differs from the mitotic condensin I complex by a single subunit, strengthening the connection between dosage compensation and mitotic chromosome condensation. The DCC is targeted to X chromosomes by initial binding to a number of recruiting elements, followed by dispersal or spreading to secondary sites. While the complex is greatly enriched on the X chromosomes, many sites on autosomes also bind the complex. DCC binding does not correlate with DCC-mediated repression, suggesting that the complex acts in a chromosome-wide manner, rather than on a gene-by-gene basis. Worm dosage compensation represents an excellent model system to study how condensin-mediated changes in higher order chromatin organization affect gene expression.