Transcriptionally repressed germ cells lack a subpopulation of phosphorylated RNA polymerase II in early embryos of Caenorhabditis elegans and Drosophila melanogaster

Early embryonic germ cells in C. elegans and D. melanogaster fail to express many messenger RNAs expressed in somatic cells. In contrast, we find that ribosomal RNAs are expressed in both cell types. We show that this deficiency in mRNA production correlates with the absence of a specific phosphoepitope on the carboxy-terminal domain of RNA polymerase II. In both C. elegans and Drosophila embryos, this phosphoepitope appears in somatic nuclei coincident with the onset of embryonic transcription, but remains absent from germ cells until these cells associate with the gut primordium during gastrulation. In contrast, a second distinct RNA polymerase II phosphoepitope is present continuously in both somatic and germ cells. The germ-line-specific factor PIE-1 is required to block mRNA production in the germ lineage of early C. elegans embryos (Seydoux, G., Mello, C. C., Pettitt, J., Wood, W. B., Priess, J. R. and Fire, A. (1996) Nature 382, 713-716). We show here that PIE-1 is also required for the germ-line-specific pattern of RNA polymerase II phosphorylation. These observations link inhibition of mRNA production in embryonic germ cells to a specific modification in the phosphorylation pattern of RNA polymerase II and suggest that repression of RNA polymerase II activity may be part of an evolutionarily conserved mechanism that distinguishes germ line from soma during early embryogenesis. In addition, these studies also suggest that different phosphorylated isoforms of RNA polymerase II perform distinct functions.