Targeting X Repression: Recruitment, Binding and Function of the C. elegans Dosage Compensation Complex
In organisms that use chromosome-counting mechanisms to determine sex, the chromosome-wide regulatory mechanism of dosage compensation equalizes X-linked gene products between males (1X) and females (2X). In C. elegans, dosage compensation is carried out by a multi-subunit dosage compensation complex (DCC) that resembles condensin, a conserved complex involved in chromosome compaction, resolution and segregation. This DCC binds both X chromosomes of XX animals to decrease X transcript levels by about two-fold. The DCC binds to at least two classes of cis-acting regulatory elements on X. rex sites autonomously recruit the DCC, even when detached from X, where as dox sites are fully bound only when attached to X. Here, I report important advances in our understanding of how DCC binding is targeted to the X chromosome, and how it regulates X-linked gene expression once bound.
In chapter 2, I describe experiments that redefine our understanding of DCC composition, binding and function. DPY-30 is identified as a bona-fide component of both the repressive DCC and the activating MLL/COMPASS complex, which localizes to DCC binding sites across the genome. I define fundamental differences in the molecular requirements for DCC binding to rex and dox sites, and describe the relationship between DCC binding at these two classes of sites. Finally, I show for the first time that C. elegans dosage compensation acts at least at the level of transcription.
In chapter 3, I report the discovery of the first post-translational modification involved in C. elegans dosage compensation. The small ubiquitin-like molecule SUMO is required for proper targeting of the DCC to the X chromosome and for repression of dosage compensated genes on X. I show that multiple components of the DCC are modified by SUMO in a manner dependent on the proteins that recruit the DCC to X. I propose that SUMOylation of the DCC is essential for sex-specific assembly of the complex on X and thus, for repression of X-linked genes.
Finally, in chapter 4, I describe a role for members of the putative TIP60 histone acetyltransferase complex in C. elegans dosage compensation. TIP60 complex components are required for repression of dosage compensated gene expression on X, and cause synergistic lethality when combined with weak dosage compensation mutations. The precise mechanism of TIP60 activity in dosage compensation is still unclear, however, as DCC localization to the X chromosome in genome-wide binding assays is only mildly perturbed. The involvement of a histone acetyltransferase complex suggests that histone modification may be one of the regulatory mechanisms controlling C. elegans dosage compensation.