During development, cells that constitute somatic tissues must differentiate from the germ cells while the latter are required to remain undifferentiated. How is the distinction between the germ and somatic cells maintained? Early C. elegans embryos keep the germ-soma difference through differential transcription; the germ cell-specific protein PIE-1 suppresses transcription in the germ cells while tissue-specific transcription programs are activated by maternal transcription factors in the soma. We think that transcriptional mechanisms can also account for the germ-soma distinction in later stages and that
eps-1 (ectopic P-granules in the soma) is essential for this process.
eps-1 encodes a ubiquitous nuclear protein related to Drosophila Suppressor of Hairy Wing (Su(HW)). A loss of both maternal and zygotic
eps-1(+) activity (via RNAi or germline mosaicism of the rescued
eps-1 mutant strain [A. Puoti and J. Kimble]) causes numerous somatic cells to adopt germ-like characteristics. For example, both intestinal and hypodermal cells lose their proper cell-cell adhesions and adopt nuclear morphology resembling that of the germ cells. The transformed somatic cells ectopically express germ cell-specific proteins including PGL-1, GLH-2 and GLH-3, which appears to reflect de novo synthesis of the P-granule components.
eps-1(+) is not required for asymmetric segregation of maternal P-granules during embryogenesis, and furthermore, in situ hybridization reveals ectopic accumulation of mRNAs for the P-granule proteins, but not for somatic genes, in the
eps-1 animals. The latter observation is consistent with the hypothesis that EPS-1 functions at the level of transcription. Interestingly, in addition to its suppressive role in the soma,
eps-1 is zygotically required for germ cell development, indicating its dual function in the germ-soma distinction. Su(HW) binds to chromatin insulators and is thought to repress transcription by blocking the enhancer-promoter interaction. Subnuclear distribution of EPS-1 is consistent with its potential involvement in the insulator function. Furthermore,
eps-1 genetically interacts with C. elegans genes proposed to affect higher order chromatin structures. EPS-1 may thus regulate transcription by reorganizing chromatin structure into distinct domains and may function differentially between germ and somatic cells.