Repression of gene expression in the embryonic germ lineage of C. elegans

Abstract

The distinction between soma and germline was recognized more than a century ago: somatic cells form the body of an organism, whereas germ cells serve to produce future generations. In Caenorhabditis elegans, the separation of some and germline occurs through a series of asymmetrical divisions, in which embryonic germline blastomeres divide unequally to produce one somatic daughter and one germline daughter. Here we show that after each asymmetrical division, embryonically transcribed RNAs are detected in somatic, but not germline, blastomeres. This asymmetry depends on the activity of the germline specific factor, PIE-1. In the absence of PIE-1, embryonically transcribed RNAs are detected in both somatic and germline blastomeres. Furthermore, ectopic expression of PIE-1 in somatic blastomeres can significantly reduce the accumulation of new transcripts in these cells. Taken together, these results suggest that germ-cell fate depends on an inhibitory mechanism that blocks new gene expression in the early embryonic germ lineage.

Primordial Germ Cells in Mice

“…One such property is the repression in PGCs of the somatic differentiation program robustly activated in surrounding somatic cells. In C. elegans and D. melanogaster, this is achieved through transient global repression of the RNA polymerase II (Pol II) activity by PIE-1 and the polar granule component, respectively (Nakamura et al 1996;Seydoux et al 1996). PIE-1, bearing two CCCH RNA-binding domains but showing no similarity to known transcriptional repressors, inhibits the transcriptional initiation and elongation activity of RNAPII by distinct mechanisms (Ghosh and Seydoux 2008).…”

Eukaryotic regulatory RNAs: an answer to the ‘genome complexity’ conundrum

“…During early embryogenesis, germ cells avoid differentiation by transiently and globally silencing mRNA transcription (Leatherman and Jongens 2003). In Caenorhabditis elegans early germ cells, an RNA-binding protein, PIE-1, is responsible for transcriptional inhibition by interfering with transcription elongation or associated RNA processing steps Seydoux et al 1996;Zhang et al 2003). Similarly, in Drosophila, a cytoplasmic ncRNA (a major transcript of 0.7 kb and a minor transcript of 1.3 kb) polar granule component (pgc) is involved in germ cell transcriptional inhibition Deshpande et al 2004;Martinho et al 2004).…”

end-1 encodes an apparent GATA factor that specifies the endoderm precursor in Caenorhabditis elegans embryos

“…The pie-1 and pop-1 maternal genes appear to prevent blastomeres other than E from producing endoderm (Mello et al 1992;Lin et al 1995). pie-1 encodes an apparent zinc finger protein that prevents the sister of EMS from adopting an EMS-like fate (Mello et al 1992) by causing general repression of zygotic gene expression in that cell Seydoux et al 1996). In pop-1 mutant embryos, MS adopts the fate of its sister, E, and makes endoderm instead of mesoderm (Lin et al 1995).…”