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 e xpress many messenger RNAs expressed in somatic cells, In contrast, we find that ribosomal RNAs are expressed in both cell types, We show th at this deficiency in mRNA production correlates with the absence of a specific phosphoepitope on the carboxyterminal domain of RNA polymera se II, In both C. elegans and Drosophila embryos, this phosphoepitope appears in somatic nuclei coincident with the onset of embryonic trans cription, but remains absent from germ cells until these cells associa te with the gut primordium during gastrulation, In contrast, a second distinct RNA polymerase II phosphoepitope is present continuously in b oth somatic and germ cells, The germ-line-specific factor PIE-1 is req uired 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- I is also required for the germ-line-specific pattern of RNA polymeras e II phosphorylation, These observations link inhibition of mRNA produ ction in embryonic germ cells to a specific modification in the phosph orylation 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 embryoge nesis, In addition, these studies also suggest that different phosphor ylated isoforms of RNA polymerase II perform distinct functions.