Multi-subunit condensin complexes are essential for chromosome condensation during both mitosis and meiosis and have also been implicated as having important roles in transcriptional regulation during interphase. Metazoans contain two condensin complexes, I and II, which specifically localize to different chromosome regions where they perform different functions. Caenorhabditis elegans contains a third condensin complex, Condensin DC, whose localization is uniquely restricted to the hermaphrodite X chromosome where it acts as part of the Dosage Compensation Complex (DCC) to repress X-transcription. The regulatory mechanisms by which Condensin DC is targeted specifically to the X chromosome are not yet fully understood.
As part of the DCC, Condensin DC interacts with at least four other non-condensin proteins, including two zinc-finger-containing proteins, SDC-2 and SDC-3, which act to recruit Condensin DC to approximately 100 recruitment sites across the X-chromosome. Evidence suggests that this initial targeting is sequence-dependent. Sites of initial recruitment, termed recruitment elements on X (rex), are enriched for a 10bp recruitment motif. Our analyses indicate that this motif is four times enriched on the X-chromosome as compared to autosomes and is often clustered at rex-sites. However, the motif is not unique to the X-chromosome; both the X chromosome and the autosomes contain many perfect matches that are not bound by the DCC. Further, we show that insertion of a rex-site in single-copy onto an autosome fails to detectably recruit DCC. Increasing the number of inserted rex-site copies overcomes the inability recruit on autosomes. We conclude that motif sequence, while important for DCC recruitment, is not sufficient to recruit the complex on its own. We hypothesize that chromosomal context of the X chromosome facilitates the specificity of DCC recruitment.