PUM1

Abstract: Germline stem cells play a pivotal role in gametogenesis; yet little is known about how they are formed, how they divide to self-renew, and how these processes are genetically controlled. Here we describe the self-renewing asymmetric division of germline stem cells in the Drosophila ovarian germline, as marked by the spectrosome, a cytoplasmic structure rich in membrane skeletal proteins. The ontogeny of the spectrosome marks the lineage of germline stem cells. We identified two new groups of mutations in which the divisional asymmetry is disrupted. The first, which we refer to as ovarette (ovt) mutations, was shown to correspond to a novel class of mutations in the pumilio locus. Since pumilio is known to posttranscriptionally repress the expression of target genes at earlier stages of germ cell development, our results suggest that a similar activity is needed to maintain germ line stem cells. We have also identified a second and novel gene, piwi, whose mutations abolish germline stem cell division.

Abstract: ......... The nematode Caenorhabditis elegans has two sexes, males and hermaphrodites. Hermaphrodites initially produce sperm but switch to producing oocytes. This switch appears to be controlled by the 39 untranslated region of fem-3 messenger RNA. We have now identified a binding factor (FBF) which is a cytoplasmic protein that binds specifically to the regulatory region of fem-3 39UTR and mediates the sperm/oocyte switch. The RNA-binding domain of FBF consists of a stretch of eight tandem repeats and two short flanking regions. This structural element is conserved in several proteins including Drosophila Pumilio, a regulatory protein that controls pattern formation in the fly by binding to a 39UTR. We propose that FBF and Pumilio are members of a widespread family of sequence-specific RNA-binding proteins.

Abstract: The zinc-finger protein Nanos and the RNA-binding protein Pumilio act together to repress the translation of maternal hunchback RNA in the posterior of the Drosophila embryo, thereby allowing abdomen formation. nanos RNA is localized to the posterior pole during oogenesis and the posteriorly synthesized Nanos protein is sequestered into the germ cells as they form in the embryo. This maternally provided Nanos protein is present in germ cells throughout embryogenesis. Here we show that maternally deposited Nanos protein is essential for germ cell migration. Lack of zygotic activity of nanos and pumilio has a dramatic effect on germline development of homozygous females. Given the coordinate function of nanos and pumilio in embryonic patterning, we analyzed the role of these genes in oogenesis. We find that both genes act in the germline. Although the nanos and pumilio ovarian phenotypes have similarities and both genes ultimately affect germline stem cell development, the focus of these phenotypes appears to be different. While pumilio mutant ovaries fail to maintain stem cells and all germline cells differentiate into egg chambers, the focus of nanos function seems to lie in the differentiation of the stem cell progeny, the cystoblast. Consistent with the model that nanos and pumilio have different phenotypic foci during oogenesis, we detect high levels of Pumilio protein in the germline stem cells and high levels of Nanos in the dividing cystoblasts. We therefore suggest that, in contrast to embryonic patterning, Nanos and Pumilio may interact with different partners in the germline.