Search

The XMAP215/TOG family of proteins is a closely related set of MAPs (microtubule-associated proteins) found in animals, yeast, and plants [1]. In yeast and animal cells, the XMAP215/TOG proteins are required for both mitosis and meiosis. Although effects of XMAP215/TOG proteins on cytoplasmic microtubules have not previously been shown in animal cells, in plants the Arabidopsis family member MOR1 is required for the organization of cortical microtubule arrays [2]. The Drosophila family member, encoded by the mini spindles ( msps ) gene, is maternally expressed and loaded into the egg, where it is an essential component of meiotic and mitotic spindles [3–5]. Here we show that msps is also required during oogenesis for the structure and function of cytoplasmic microtubules. Localization of bicoid ( bcd ) mRNA in the oocyte is a microtubule-mediated event [6, 7]. We show that bcd RNA localization is defective in msps mutants. We also identify defects in cytoplasmic microtubules in both the germ and follicle cells of mutant ovaries and determine the expression pattern of msps mRNA and protein in developing egg chambers. Our findings reveal a new role for msps in cell patterning and raise the possibility that other family members may perform similar functions.

Subcellular localization of mRNAs within the Drosophila oocyte is an essential step in body patterning. Yps, a Drosophila Y-box protein, is a component of an ovarian ribonucleoprotein complex that also contains Exu, a protein that plays an essential role in mRNA localization. Y-box proteins are known translational regulators, suggesting that this complex might regulate translation as well as mRNA localization. Here we examine the role of the yps gene in these events. We show that yps interacts genetically with orb, a positive regulator of oskar mRNA localization and translation. The nature of the genetic interaction indicates that yps acts antagonistically to orb. We demonstrate that Orb protein is physically associated with both the Yps and Exu proteins, and that this interaction is mediated by RNA. We propose a model wherein Yps and Orb bind competitively to oskar mRNA with opposite effects on translation and RNA localization.

We analyzed the acquisition of fluorescence of a GFP fusion protein expressed from gene constructs designed to regulate either the time of translation of the protein or the time of gene transcription. Both gene constructs expressed a fusion protein with wild-type GFP appended to the N-terminus of the Bicoid (Bcd) transcription factor. One gene construct, P[gfp-bcd] expressed an mRNA that was initially translated at the time of egg deposition. GFP–Bcd fluorescence could first be detected in anterior nuclei in the embryo at syncytial blastoderm stage, within 1.5–2.5 h after translation commenced. The second gene construct, P[hs-gfp-bcd], utilized thehsp70promoter to regulate the induction of gene expression. In larval salivary glands, GFP–Bcd fluorescence could first be detected in nuclei after a 40-min recovery period following a 10-min heat shock induction of gene expression. These estimates of the time required for the acquisition of a fluorescent GFP product inDrosophilatissues are less than suggested by previously published reports.

The Drosophila exuperantia gene (exu) functions in both oogenesis and spermatogenesis. Alternative RNA processing and promoter usage generates sex-specific transcripts which differ in their 5' and 3' untranslated regions, but encode the same predicted protein. We have sequenced the breakpoints of an exu allele which is defective in spermatogenesis but functions normally in oogenesis. This allele deletes most of the sequence specific to the male 3'-UTR, together with some flanking DNA, and causes a reduction in steady-state level of exu mRNA in the testis. In addition, we find that a smaller deletion which removes only sequence within the male 3'-UTR reduces the steady-state level of the mRNA and prevents an exu transgene from rescuing male sterility. Males carrying multiple copies of this transgene are fertile, suggesting that the male-specific 3'-UTR functions to maintain a proper level of exu product in the germline.

Subcellular RNA localization in different cell types leads to asymmetric distribution of proteins in these cells. The localization of bicoid (bcd) messenger RNA to the anterior pole of the developing Drosophila oocyte gives rise in embryogenesis to a steep concentration gradient of the bcd protein, a transcription factor that activates expression of zygotic genes needed for anterior development. The exuperantia (exu) gene is necessary for this localization of bcd mRNA. Here we express a chimaeric gene encoding a fusion between the Acquorea victoria green fluorescent protein (GFP) and the exu protein (Exu) in female germ cells, and find that the fusion protein fluoresces strongly in both live and fixed cells during Drosophila oogenesis. The fusion protein rescues an exu null allele, restoring full fertility to females, and is expressed and localized in a temporal and spatial pattern similar to native Exu. The high sensitivity of the GFP tag provides important new details on the subcellular localization of Exu. The fusion protein is found in particles concentrated at ring canals, where transport occurs between nurse cells and the oocyte. Drugs such as colchicine and taxol that affect microtubule stability alter localization of the particles. We propose that the particles are ribonucleoprotein complexes or vesicles which transport bcd mRNA along microtubules and target it to the anterior oocyte cortex.

Centric regions of eukaryotic genomes are packaged into heterochromatin, which possesses the ability to spread along the chromosome and silence gene expression. The process of spreading has been challenging to study at the molecular level due to repetitious sequences within centric regions. A heterochromatin protein 1 (HP1) tethering system was developed that generates “ectopic heterochromatin” at sites within euchromatic regions of the Drosophila melanogaster genome. Using this system, we show that HP1 dimerization and the PxVxL interaction platform formed by dimerization of the HP1 chromo shadow domain are necessary for spreading to a downstream reporter gene located 3.7 kb away. Surprisingly, either the HP1 chromo domain or the chromo shadow domain alone is sufficient for spreading and silencing at a downstream reporter gene located 1.9 kb away. Spreading is dependent on at least two H3K9 methyltransferases, with SU(VAR)3-9 playing a greater role at the 3.7-kb reporter and dSETDB1 predominately acting at the 1.9 kb reporter. These data support a model whereby HP1 takes part in multiple mechanisms of silencing and spreading.

SET domain proteins are histone lysine methyltransferases (HMTs) that play essential roles in development. Here we show for the first time that histone methylation occurs in both the germ cells and somatic cells of the Drosophila ovary, and demonstrate in vivo that an HMT, the product of the eggless (egg) gene, is required for oogenesis. Egg is a SET domain protein that is similar to the human protein SETDB1 and its mouse ortholog ESET. These proteins are members of a small family of HMTs that contain bifurcated SET domains. Because depletion of SETDB1 in tissue culture cells is cell-lethal, and an ESET mutation causes very early periimplantation embryonic arrest, the role of SETDB1/ESET in development has proven difficult to address. We show that egg is required in the Drosophilaovary for trimethylation of histone H3 at its K9 residue. In females bearing an egg allele that deletes the SET domain, oogenesis arrests at early stages. This arrest is accompanied by reduced proliferation of somatic cells required for egg chamber formation, and by apoptosis in both germ and somatic cell populations. We propose that other closely related SET domain proteins may function similarly in gametogenesis in other species.

RNAs are localized by microtubule-based pathways to both the anterior and posterior poles of the developing Drosophila oocyte. We describe a new gene, wispy, required for localization of mRNAs to both poles of the egg. Embryos from wispy mothers arrest development after abnormal oocyte meiosis and failure of pronuclei to fuse. Our analysis of spindle and chromosome movements during meiosis reveals defects in spindle structures correlated with very high frequencies of chromosome nondisjunction and loss. Spindle defects include abnormally shaped spindles, spindle spurs, and ectopic spindles associated with lost chromosomes, as well as mispositioning of the meiosis II spindles. The polar body nuclei do not associate with their normal monastral arrays of microtubules, the sperm aster is reduced in size, and the centrosomes often dissociate from a mitotic spindle that forms in association with the male pronucleus. We show that wispy is required to recruit or maintain known centrosomal proteins with two types of microtubule organizing centers (MTOCs): (1) the central MTOC that forms between the meiosis II tandem spindles and (2) the centrosomes of the mitotic spindle. We propose that the wispy gene product functions directly in several microtubule-based events in meiosis and early embryogenesis and speculate about its possible mode of action.

Localization of bicoid (bcd) mRNA to the anterior and oskar (osk) mRNA to the posterior of the Drosophila oocyte is critical for embryonic patterning. Previous genetic studies implicated exuperantia (exu) in bcd mRNA localization, but its role in this process is not understood. We have biochemically isolated Exu and show that it is part of a large RNase-sensitive complex that contains at least seven other proteins. One of these proteins was identified as the cold shock domain RNA-binding protein Ypsilon Schachtel (Yps), which we show binds directly to Exu and colocalizes with Exu in both the oocyte and nurse cells of the Drosophila egg chamber. Surprisingly, the Exu–Yps complex contains osk mRNA. This biochemical result led us to reexamine the role of Exu in the localization of osk mRNA. We discovered that exu-null mutants are defective in osk mRNA localization in both nurse cells and the oocyte. Furthermore, both Exu/Yps particles and osk mRNA follow a similar temporal pattern of localization in which they transiently accumulate at the oocyte anterior and subsequently localize to the posterior pole. We propose that Exu is a core component of a large protein complex involved in localizing mRNAs both within nurse cells and the developing oocyte.

The Drosophila exuperantia (exu) gene encodes overlapping sex-specific, germline-dependent mRNAs. In this work, the structural differences between these sex-specific exu mRNAs were determined by sequence analysis of 9 ovary and 10 testis cDNAs. The transformer 2 (tra-2) gene functions in sex determination of female somatic cells through its role in regulating female-specific splicing of doublesex (dsx) RNA. We report here that tra-2 is required in male germ cells for efficient male-specific processing of exu RNA; in the absence of tra-2, X/Y males produce a new mRNA which is processed at its 3' end so that it contains sequences normally specific to the female 3' untranslated region. Although the processing event that requires tra-2 occurs in an untranslated region of the exu transcript, the isolation and characterization of a male-specific exu allele which deletes male 3' untranslated sequence indicate that this processing is biologically significant.

The white gene of Drosophila is required for the pigmentation of the eyes, ocelli, Malpighian tubules and testis sheath. We have examined the regulation of white expression in germ line transformants that carry the mRNA-coding sequences of white flanked by varying amounts of the sequences normally located adjacent to them. A segment with as little as 1.9 kb of 5'- and 2.1 kb of 3'-flanking DNA was expressed to give essentially wild-type pigmentation in all of these tissues and interacted in trans with the zeste-1 allele. In contrast, transformants having segments with 1.1 kb of 5'-flanking DNA had reduced levels of pigment in their eyes, no detectable pigment in their testes and did not interact with zeste-1. The eyes, Malpighian tubules and ocelli of transformants with as little as 0.4 kb of 5'-flanking DNA were pigmented and the eye pigmentation exhibited dosage compensation. We conclude that there are sequence elements present in the region from 1.1 to 1.9 kb upstream from the 5' end of the white transcript which are required for expression in the testes and interaction of white and zeste-1. The same or other elements in this region appear to augment the pigmentation of the eye.

Molecular analysis of the white locus during the past few years has revealed that this gene has a complex array of upstream cis -acting DNA regulatory sequences governing tissue specificity of gene expression. The X-linked white gene is also subject to dosage compensation and transvection effects, as well as to genomic position effects generated by transformation and transposition. The white gene has proved to be a valuable model system for studying these regulatory phenomena.

Nuclear history affects the fates of mRNAs in the cytoplasm of cells. Proteins loaded onto mRNAs in the nucleus mark RNAs for subsequent translational regulation, stability, degradation, and subcellular RNA localization. New results show that the Drosophila heterogeneous nuclear ribonucleoprotein (hnRNP) Hrp48 contributes to coordinated RNA localization and translational control in oocytes.

P-element-mediated DNA transformation was used to generate transformants carrying segments of DNA from the white locus of D. melanogaster. The vast majority of transduced copies of an 11.7 or a 14.3 kb segment of DNA from white successfully rescued the white- eye-color phenotype when inserted in many different chromosomal locations. However, two transformants with abnormal eye pigmentation--apparently a consequence of the genomic positions of the transduced white gene--were also recovered. In all seven cases tested, autosomal insertions of white, which is dosage-compensated in its normal location on the X chromosome, retained the property of dosage compensation. In contrast to the relative insensitivity of eye-color pigmentation and dosage compensation to genomic position, the transduced white DNA segments differed widely in their interactions with the zeste mutation, ranging from greater than normal repression by zeste to insensitivity to the presence of zeste.

The white gene of Drosophila is expressed normally when introduced at many different sites in the genome by P-element-mediated DNA transformation, but is expressed abnormally when inserted at two particular genomic positions. It is now demonstrated that the mutant expression in these two cases is caused by the surrounding chromosomal region into which the white gene has been inserted. The white gene could be moved from these two positions, where it confers a mutant phenotype, to other positions in the genome where it confers a wild-type phenotype. However, flies in which white has been moved to one new location have an unusual mosaic phenotype.