Two components of the germ-line-specific P granules of the nematode Caenorhabditis elegans have been identified using polyclonal antibodies specific for each. Both components are putative germ-line RNA helicases (GLHs) that contain CCHC zinc fingers of the type found in the RNA-binding nucleocapsid proteins of retroviruses. The predicted GLH-1 protein has four CCHC fingers; GLH-2 has six. Both GLH proteins localize in the P granules at all stages of germ-line development. However, the two glh genes display different patterns of RNA and protein accumulation in the germ lines of hermaphrodites and males. Injection of antisense glh-1 or glh-2 RNA into wild-type worms causes some offspring to develop into sterile adults, suggesting that either or both genes are required for normal germ-line development. As these very similar glh genes physically map within several hundred kilobases of one another, it seems likely that they represent a fairly recent gene duplication event.Embryos of the free-living soil nematode Caenorhabditis elegans generate distinct founder cells via a series of asymmetric cell divisions. At each division, the germ-line daughter cell inherits distinctive non-membrane-bound particles, called P granules (1-3). P granules are partitioned to the primordial germ cell P 4 of the 16-to 24-cell embryo and become perinuclear. P granules persist around the nuclei of all germ cells, until gametogenesis, at which point they are excluded from mature sperm and become dispersed within the cytoplasm of mature oocytes in preparation for cytoplasmic localization in the embryo. Although the distribution pattern of nematode P granules has been well-studied, the identity and function of P-granule components have yet to be determined.Germ granules are found in many species (4, 5). The germ-line-specific polar granules of Drosophila melanogaster have been well-studied, with a number of different genes identified that are required for polar granule assembly and germ-cell formation, including vasa, staufen, valois, oskar, tudor, mago nashi, and germ-cell-less (6-15). With the exception of vasa, these genes encode novel proteins. Vasa, however, is a member of a family of proteins with recognizable motifs and predictable function. Vasa is an RNA helicase of the DEAD-box family (8, 9) whose ATP-dependent RNA helicase activity has been demonstrated in vitro (16). As polar granules contain RNA as well as protein (11,12,15,17), a germ-linespecific RNA helicase may function to bind and unwind RNAs necessary for germ-line development. Several potential vasa homologues have been cloned, including glh-1 (germ-line helicase 1) from Caenorhabditis, Xvh (Xenopus vasa homologue), mvh (mouse vasa homologue), and rvh (rat vasa homologue) (18)(19)(20)(21). glh-1 in C. elegans is unique among RNA helicase genes reported, including vasa, in that its predicted product contains four retroviral-like zinc fingers (18). We have identified a second C. elegans germ-line RNA helicase gene, glh-2, that also encodes zinc fingers. Immunolocali...

During organogenesis, pluripotent precursor cells acquire a defined identity such as muscle or nerve. The transition from naïve precursor towards the differentiated state is characterized by sequential waves of gene expression that are determined by regulatory transcription factors. A key question is how transcriptional circuitry dictates the succession of events that accompanies developmental competence, cell fate specification and differentiation. To address this question, we have examined how anterior muscles are established within the Caenorhabditis elegans foregut (pharynx). We find that the T-box transcription factor tbx-2 is essential to form anterior pharyngeal muscles from the ABa blastomere. In the absence of tbx-2 function, ABa-derived cells initiate development normally: they receive glp-1/Notch signaling cues, activate the T-box gene TBX-38 and express the organ selector gene PHA-4/FoxA. However, these cells subsequently arrest development, extinguish PHA-4 and fail to activate PHA-4 target genes. tbx-2 mutant cells do not undergo apoptosis and there is no evidence for adoption of an alternative fate. TBX-2 is expressed in ABa descendants and depends on activation by pha-4 and repression by components of glp-1/Notch signaling. Our analysis suggests that a positive feedback loop between tbx-2 and pha-4 is required for ABa-derived precursors to commit to pharyngeal muscle fate.

A key question in development is how pluripotent progenitors are progressively restricted to acquire specific cell fates. Here we investigate how embryonic blastomeres in C. elegans develop into foregut (pharynx) cells in response to the selector gene PHA-4/FoxA. When pha-4 is removed from pharyngeal precursors, they exhibit two alternative responses. Before late-gastrulation (8E stage), these cells lose their pharyngeal identity and acquire an alternative fate such as ectoderm (Specification stage). After the Specification stage, mutant cells develop into aberrant pharyngeal cells (Morphogenesis/Differentiation stage). Two lines of evidence suggest that the Specification stage depends on transcriptional repression of ectodermal genes by pha-4. First, pha-4 exhibits strong synthetic phenotypes with the B class synMuv gene tam-1 (Tandam Array expression Modifier 1) and with a mediator of transcriptional repression, the NuRD complex (NUcleosome Remodeling and histone Deacetylase). Second, pha-4 associates with the promoter of the ectodermal regulator lin-26 and is required to repress lin-26 expression. We propose that restriction of early blastomeres to the pharyngeal fate depends on both repression of ectodermal genes and activation of pharyngeal genes by PHA-4.