1. Howard Hughes Medical Institute and,
2. Department of Molecular and Cell Biology, University of California – Berkeley, 16 Barker Hall, Berkeley, California 94720-3204, USA
3. These authors contributed equally to this work.

Correspondence to: Barbara J. Meyer^1,2 Correspondence and requests for materials should be addressed to B.J.M. (Email: bjmeyer@berkeley.edu).

Gene expression in metazoans is regulated not only at the level of individual genes but also in a coordinated manner across large chromosomal domains (for example centromeres, telomeres and imprinted gene clusters^{1, 2, 3}) and along entire chromosomes (for example X-chromosome dosage compensation^{4, 5, 6}). The primary DNA sequence usually specifies the regulation of individual genes, but the nature of cis-acting information that controls genes over large regions has been elusive: higher-order DNA structure, specific histone modifications, subnuclear compartmentalization and primary DNA sequence are possibilities. One paradigm of chromosome-wide gene regulation is Caenorhabditis elegans dosage compensation in which a large dosage compensation complex (DCC) is targeted to both X chromosomes of hermaphrodites to repress transcript levels by half⁶. This essential process equalizes X-linked gene expression between the sexes (XO males and XX hermaphrodites). Here we report the discovery and dissection of cis-acting sites that mark nematode X chromosomes as targets for gene repression by the DCC. These rex (recruitment element on X) sites are widely dispersed along X and reside in promoters, exons and intergenic regions. rex sites share at least two distinct motifs that act in combination to recruit the DCC. Mutating these motifs severely reduces or abolishes DCC binding in vivo, demonstrating the importance of primary DNA sequence in chromosome-wide regulation. Unexpectedly, the motifs are not enriched on X, but altering motif numbers within rex sites demonstrates that motif co-occurrence in unusually high densities is essential for optimal DCC recruitment. Thus, X-specific repression is established through sequences not specific to X. The distribution of common motifs provides the foundation for repression along an entire chromosome.

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