germ cell-less Acts to Repress Transcription during the Establishment of the Drosophila Germ Cell Lineage

Abstract

Previously, it has been shown that, during early Drosophila and C. elegans development, the germ cell precursors undergo a period of transcriptional quiescence. Here, we report that Germ cell-less (GCL), a germ plasm component necessary for the proper formation of "pole cells," the germ cell precursors in Drosophila, is required for the establishment of this transcriptional quiescence. While control embryos silence transcription prior to pole cell formation in the pole cell-destined nuclei, this silencing does not occur in embryos that lack GCL activity. The failure to establish quiescence is tightly correlated with failure to form the pole cells. Furthermore, we show that GCL can repress transcription of at least a subset of genes in an ectopic context, independent of other germ plasm components. Our results place GCL as the earliest gene known to act in the transcriptional repression of the germline. GCL's subcellular distribution on the nucleoplasmic surface of the nuclear envelope and its effect on transcription suggest that it may act to repress transcription in a manner similar to that proposed for transcriptional silencing of telomeric regions.

The Development of Germline Stem Cells in Drosophila

“…This correlates with the misexpression of somatic genes, such as sisterless-a and scute, in the pole buds of gcl mutant embryos (17). Conversely, ectopic gcl expression at the anterior of an otherwise wild-type embryo is sufficient to downregulate somatic expression of sisterless-a and tailless in anterior cells (17). Thus, gcl is implicated in establishing transcriptional quiescence in newly formed pole buds.…”

“…The level of RNAP II CTD phosphorylation is increased in pole buds (precursors to PGCs) of embryos derived from gcl mutant mothers, bringing it to a level similar to that of the surrounding somatic nuclei. This correlates with the misexpression of somatic genes, such as sisterless-a and scute, in the pole buds of gcl mutant embryos (17). Conversely, ectopic gcl expression at the anterior of an otherwise wild-type embryo is sufficient to downregulate somatic expression of sisterless-a and tailless in anterior cells (17).…”

“…PGC transcriptional quiescence correlates with reduced levels of active RNA polymerase II (RNAP II) that is phosphorylated on its C-terminal domain (CTD), suggesting that transcriptional repression might be established through regulating RNA Pol II function (16,17). The maternally supplied mRNAs gcl, nos, and pgc contribute to the transcriptional quiescence of PGCs, apparently through independent mechanisms that repress the transcription of different sets of genes (18).…”

2′-O-methylation stabilizes Piwi-associated small RNAs and ensures DNA elimination in Tetrahymena

“…In all metazoans studied to date, piRNAs are 29-O-methylated (Vagin et al 2006;Horwich et al 2007;Houwing et al 2007;Kirino and Mourelatos 2007a;Ohara et al 2007;Saito et al 2007) and Piwi proteins function primarily in germline cells (Cox et al 1998;Deng and Lin 2002;Das et al 2008). Since germline cells are transcriptionally inactive at certain developmental stages in many animals (Zalokar 1976;Newport and Kirschner 1982;Seydoux et al 1996;Leatherman et al 2002), piRNAs may have to be 29-O-methylated to increase their turnover time and ensure transposon silencing, even in the absence of their de novo production. Also, scnRNAs in Tetrahymena must be stable because they are produced at early stages of conjugation (Malone et al 2005;Mochizuki and Gorovsky 2005) but they play a role in DNA elimination during late stages of conjugation.…”

Drosophila valois encodes a divergent WD protein that is required for Vasa localization and Oskar protein accumulation

“…germline establishment and abdominal patterning activities (Golumbeski et al, 1991). Pole cell formation depends on the localization of germ cell less (gcl) mRNA (Leatherman et al, 2002) and mitochondrial large ribosomal RNA (Iida and Kobayashi, 1998). Abdominal patterning relies on the vasdependent translation of nanos (nos) mRNA at the posterior pole.…”