A Chromodomain Mutation Identifies Separable Roles for C. elegans MRG-1 in Germline and Somatic Development

The packaging of DNA into chromatin strongly influences gene regulation. Post-translational modifications of histones, and the proteins that bind to them, alter the accessibility of chromatin and contribute to the activation and repression of genes. The human MRG15 (MORF4- Related Gene on chromosome 15) protein is a conserved chromodomain-containing protein that binds to methylated lysine 36 on histone H3 (H3K36me) and plays important roles in development, genome integrity, and gene regulation. MRG15 affects transcriptional regulation through its interactions with both histone acetyltransferase (HAT) and histone deacetylase (HDAC) complexes. MRG-1, its C. elegans homolog, has similarly been shown to have important roles in genomic integrity and development, and has also been shown to co- purify with HDAC complexes. Like MRG15, MRG-1 is predicted to bind to H3K36me through its chromodomain, yet despite mrg-1 mutants displaying developmental and germline phenotypes that overlap with H3K36 methyltransferase mutants, the role of the MRG-1 chromodomain has never been characterized. In this study, we examined meiotic cells lacking H3K36me3 to compare to mrg-1 mutant germ cell phenotypes, and mutated key residues in the MRG-1 chromodomain (CD) to assess its function. The CD mutations cause embryonic lethality but few post-embryonic germline defects, in contrast to mrg-1 deletion mutants which are viable but sterile. The CD mutations therefore disrupt somatic development despite the apparent absence of a requirement for MRG-1 protein in embryogenesis. Furthermore, the CD mutants exhibit a dominant RNAi resistance phenotype that is not seen in other mrg-1 mutant alleles. This suggests that the function of MRG-1, and the chromatin modifying complexes with which it interacts, includes tissue-specific interactions involving different requirements for a functional chromodomain. We propose that the CD mutation disrupts proper guidance of complexes within which it acts, and this guidance defect results in improper HDAC and/or HAT regulation causing an indirect defect in RNAi machinery expression or targeting.