The C. elegans Dosage Compensation Complex Propagates Dynamically and Independently of X Chromosome Sequence

Abstract

The C. elegans dosage compensation complex (DCC) associates with both X chromosomes of XX animals to reduce X-linked transcript levels. Five DCC members are homologous to subunits of the evolutionarily conserved condensin complex, and two noncondensin subunits are required for DCC recruitment to X. We investigated the molecular mechanism of DCC recruitment and spreading along X by examining gene expression and the binding patterns of DCC subunits in different stages of development, and in strains harboring X;autosome (X;A) fusions. We show that DCC binding is dynamically specified according to gene activity during development and that the mechanism of DCC spreading is independent of X chromosome DNA sequence. Accordingly, in X;A fusion strains, DCC binding propagates from X-linked recruitment sites onto autosomal promoters as a function of distance. Quantitative analysis of spreading suggests that the condensin-like subunits spread from recruitment sites to promoters more readily than subunits involved in initial X targeting. A highly conserved chromatin complex is appropriated to accomplish domain-scale transcriptional regulation during C. elegans development. Unlike X recognition, which is specified partly by DNA sequence, spreading is sequence independent and coupled to transcriptional activity. Similarities to the X recognition and spreading strategies used by the Drosophila DCC suggest mechanisms fundamental to chromosome-scale gene regulation.