To ensure equal expression of X-linked genes between XO males and XX hermaphrodites, a complex of proteins named the dosage compensation complex (DCC) binds along the entire length of both hermaphrodite X chromosomes to down regulate gene expression by half. The DCC is similar to the evolutionary conserved 13S condesin complex required for mitotic and meiotic chromosome condensation. However, dosage compensation homologs of only four of the five condensin subunits have been identified to date in numerous genetic and biochemical screens. In search of the missing subunit and of other proteins that participate in dosage compensation, we undertook both a biochemical approach and a candidate approach to identify factors that physically or functionally interact with the DCC. Our candidates were assayed by RNAi in sensitized genetic backgrounds to evaluate their role in dosage compensation. In the biochemical approach, we immunoprecipitated the DCC and determined the identity of proteins present using tandem mass spectrometry. The discovery of a homolog of the condensin subunit CAP-G, as the fifth component of the DCC validated that approach. In the candidate approach, we studied genes implicated in the regulation of gene expression. Intriguingly, a component of the RNAi machinery was found to influence the dosage compensation process. Loss of function of
alg-2, an Argonaute-like protein involved in miRNA processing, in a sensitized background disrupted localization of the DCC to X, indicating that miRNAs might play a role in dosage compensation (collaboration with A. Pasquinelli). Other components of the RNAi machinery are being evaluated. Our search also implicated proteins involved in chromatin modification and structure.
mys-1, like
alg-2, was found to synergize with other dosage compensation genes and to be involved in X localization of the DCC.
mys-1 encodes a MYST family histone acetyltransferase, a subunit of a putative Tip60/NuA4-like complex involved in many cellular processes, including regulation of gene expression via histone acetylation. R08C7.3, a histone H2A.Z variant, was found to interact physically with the DCC. Inactivation of H2A.Z by RNAi in a sensitized genetic background disrupts dosage compensation with minimal disruption of DCC localization. By immunostaining, H2A.Z appears to be specifically less abundant on dosage compensated X chromosomes than on autosomes. H2A.Z is similarly underrepresented on mammalian dosage compensated X chromosomes, providing a molecular link between mammalian X inactivation and C. elegans dosage compensation.