Abstract
In many organisms sexual fate is determined by a chromosome-based method which entails a difference in sex chromosome-linked gene dosage. Consequently, a gene regulatory mechanism called dosage compensation equalizes X-linked gene expression between the sexes. Dosage compensation initiates as cells transition from pluripotency to differentiation. In Caenorhabditis elegans, dosage compensation is achieved by the dosage compensation complex (DCC) binding to both X chromosomes in hermaphrodites to downregulate gene expression by twofold. The DCC contains a subcomplex (condensin IDC) similar to the evolutionarily conserved condensin complexes which play a fundamental role in chromosome dynamics during mitosis. Therefore, mechanisms related to mitotic chromosome condensation are hypothesized to mediate dosage compensation. Consistent with this hypothesis, monomethylation of histone H4 lysine 20 is increased, whereas acetylation of histone H4 lysine 16 is decreased, both on mitotic chromosomes and on interphase dosage compensated X chromosomes in worms. These observations suggest that interphase dosage compensated X chromosomes maintain some characteristics associated with condensed mitotic chromosome. This chromosome state is stably propagated from one cell generation to the next. In this review we will speculate on how the biochemical activities of condensin can achieve both mitotic chromosome compaction and gene repression.
Highlights
Dosage compensation occurs in many species with a difference in sex chromosome number between males (XY or XO) and females (XX)
In C. elegans, repression of gene expression is achieved by the dosage compensation complex (DCC), which binds the Xs in hermaphrodites to downregulate gene expression by half
Condensin II regulates chromosome territory formation in multiple cell types. This conclusion is based on the finding that CAP-H2 promotes axial compaction and proper compartmentalization of the interphase nucleus into chromosome territories in both nurse cells and salivary glands (Bauer et al, 2012). These findings suggest that the interphase function of condensin II is similar to its role in axial compaction of mitotic chromosomes
Summary
Dosage compensation occurs in many species with a difference in sex chromosome number between males (XY or XO) and females (XX) This mechanism equalizes gene expression between the sexes and balances X and autosomal gene expression (Ohno, 1967). Mammals and the nematode, Caenorhabditis elegans, are hypothesized to upregulate X chromosome expression in both sexes (Gupta et al, 2006; Lin et al, 2007; Deng et al, 2011, 2013; Lin et al, 2011) This X upregulation balances male X and autosomal expression, but causes X overexpression in females/hermaphrodites. In C. elegans, repression of gene expression is achieved by the dosage compensation complex (DCC), which binds the Xs in hermaphrodites to downregulate gene expression by half. Unlike condensins I and II, which compact and segregate all mitotic and meiotic chromosomes, condensin IDC is X-specific resulting in gene repression www.frontiersin.org
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