GLHs are homologous to Drosophila VASA, a polar granule component necessary for oogenesis and embryonic pattern formation, the GLHs are distinguished by containing multiple CCHC zinc fingers. RNA-mediated interference (RNAi)

Germ granules are cytoplasmic, non-membranous RNA/protein complexes aggregated in the primordial germ cells of many higher eucaryotes (Eddy, 1975). In the nematode Caenorhabditis elegans, germ granules are called P granules (Strome and Wood, 1982, 1983; Wolf et al., 1983). P granule components are candidate cytoplasmic determinants, specifying germ cell identity in the embryo and gametogenesis in the next generation. P granules are dispersed throughout the cytoplasm in the mature oocyte, segregate to the posterior cortex after fertilization and continue to segregate into the P germline lineage in the early embryo until they surround the nucleus of the P4 germ cell progenitor at the 16-24 cell stage. P granules remain in the germline throughout the life of the worm, surrounding each germ cell nucleus during mitotic proliferation and gametogenesis. In Drosophila melanogaster germ granules are called polar granules. Transplantation of cytoplasm containing polar granules can induce germ cells to ectopically form at the anterior of the fly embryo; genetically-marked anterior pole cells can become primordial germ cells when transplanted into foster mothers (Illmensee and Mahowald, 1975). Mutations that affect pole cells include ones in the ‘posterior group’ genes: vasa, tudor, staufen, valois, mago-nashi, oskar and nanos (reviewed by St Johnston and Nüsslein-Volhard, 1992). Because of maternal effects, mutations in many of these genes have a grandchildless phenotype. Ectopic osk expressed at the anterior pole is sufficient to recruit VASA and TUDOR; these three polar granule components must be functional to form pole cells (Ephrussi and Lehmann, 1992). Thus, one or more of these components may be the ‘germline determinants’ in the fly. VASA is a germline-specific, ATP-dependent DEAD-box RNA helicase (Hay et al., 1988; Lasko and Ashburner, 1988). VASA regulates the translation of at least two mRNAs, gurken and nanos (Styhler et al., 1998; Gavis et al., 1996). A null vasa mutation causes a range of defects in oogenesis (Styhler et al., 1998). Germline RNA helicases are conserved components of germ granules, with VASA homologues reported in several species, including Xenopus, mouse, and Caenorhabditis (Komiya et al., 1994; Fujiwara et al., 1994; Roussell and Bennett, 1993; Gruidl et al., 1996). The germline helicase (glh) genes, vasa homologues, encode P granule components in C. elegans (Roussell and Bennett, 1993; Gruidl et al., 1996; this report). GLH-1 and GLH-2 contain the eight conserved motifs characteristic of DEAD-box RNA helicases and, like VASA, they contain N-terminal glycine-rich repeats. The glycine repeats in the GLHs differ from VASA; they contain no charged amino acids. GLH-1 and GLH-2 also differ from VASA and most other RNA helicases 2907 Development 127, 2907-2916 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV3205