| The maternal-effect sterile proteins, MES-2, MES-3, MES-4, and MES-6, are essential for germline development in Caenorhabditis elegans. Homozygous mes mutants from heterozygous mothers produce sterile but otherwise healthy hermaphrodite progeny. To understand the mechanism of MES functions, I performed three types of analyses. First, the immunolocalization pattern of MES-3 was determined. In wild-type worms, MES-3 is localized in the nuclei of embryos and the germline. Its normal localization in late embryos depends on MES-2 and MES-6, but not on MES-4. In the germline, the expression of MES-3 is repressed by GLD-1 in germ cells at the pachytene stage. Second, RNAi analyses were performed to search for enhancers of MES-3. SET-1 was found to enhance the zygotic functions of both MES-3 and MES-4, but not of MES-2 and MES-6. The set-1 encodes two alternative transcripts set-1l and set-1s. Both transcripts are enriched in the C. elegans germline. SET-1L protein is localized in the nuclei of all cells in embryos, and predominantly localized in the germline of adult worms. Finally, a series of biochemical analyses, including immunoprecipitation, sucrose gradient, and gel filtration assays, were performed on C. elegans embryo extracts. MES-2, MES-3, and MES-6 were found to be in a complex of ∼280 kDa. MES-4 is not in this complex. Based on the above results and results of former members of the Strome lab, a model for the mechanism of MES function is proposed. The MES-2/MES-3/MES-6 complex represses gene expression from both autosomes and X chromosomes, and excludes MES-4 from binding to the X chromosomes. MES-4 activates gene expression from autosomes only. In the absence of MES-2, MES-3, and MES-6, MES-4 is localized on the X chromosomes and causes elevated X gene expression, which leads to sterility. In the absence of MES-4, genes that are important for germline development are not expressed properly, which also results in germ cell death. |
