Tissue-specific and constitutive alpha-tubulin genes of Drosophila melanogaster code for structurally distinct proteins

Tubulin isotypes optimize distinct spindle positioning mechanisms during yeast mitosis

Microtubules are dynamic cytoskeleton filaments that are essential for a wide range of cellular processes. They are polymerized from tubulin, a heterodimer of α- and β-subunits. Most eukaryotic organisms express multiple isotypes of α- and β-tubulin, yet their functional relevance in any organism remains largely obscure. The two α-tubulin isotypes in budding yeast, Tub1 and Tub3, are proposed to be functionally interchangeable, yet their individual functions have not been rigorously interrogated. Here, we develop otherwise isogenic yeast strains expressing single tubulin isotypes at levels comparable to total tubulin in WT cells. Using genome-wide screening, we uncover unique interactions between the isotypes and the two major mitotic spindle positioning mechanisms. We further exploit these cells to demonstrate that Tub1 and Tub3 optimize spindle positioning by differentially recruiting key components of the Dyn1- and Kar9-dependent mechanisms, respectively. Our results provide novel mechanistic insights into how tubulin isotypes allow highly conserved microtubules to function in diverse cellular processes.

α1A and α1C form microtubules to display distinct properties mainly mediated by their C-terminal tails

Microtubules consisting of α/β-tubulin dimers play critical roles in cells. More than seven genes encode α-tubulin in vertebrates. However, the property of microtubules composed of different α-tubulin isotypes is largely unknown. Here, we purified recombinant tubulin heterodimers of mouse α-tubulin isotypes including α1A and α1C with β-tubulin isotype β2A. In vitro microtubule reconstitution assay detected that α1C/β2A microtubules grew faster and underwent catastrophe less frequently than α1A/β2A microtubules. Generation of chimeric tail-swapped and point-mutation tubulins revealed that the carboxyl-terminal (C-terminal) tails of α-tubulin isotypes largely accounted for the differences in polymerization dynamics of α1A/β2A and α1C/β2A microtubules. Kinetics analysis showed that in comparison to α1A/β2A microtubules, α1C/β2A microtubules displayed higher on-rate, lower off-rate, and similar GTP hydrolysis rate at the plus-end, suggesting a contribution of higher plus-end affinity to faster growth and less frequent catastrophe of α1C/β2A microtubules. Furthermore, EB1 had a higher binding ability to α1C/β2A microtubules than to α1A/β2A ones, which could also be attributed to the difference in the C-terminal tails of these two α-tubulin isotypes. Thus, α-tubulin isotypes diversify microtubule properties, which, to a great extent, could be accounted by their C-terminal tails.

An insulator blocks access to enhancers by an illegitimate promoter, preventing repression by transcriptional interference

Several distinct activities and functions have been described for chromatin insulators, which separate genes along chromosomes into functional units. Here, we describe a novel mechanism of functional separation whereby an insulator prevents gene repression. When the homie insulator is deleted from the end of a Drosophila even skipped (eve) locus, a flanking P-element promoter is activated in a partial eve pattern, causing expression driven by enhancers in the 3’ region to be repressed. The mechanism involves transcriptional read-through from the flanking promoter. This conclusion is based on the following. Read-through driven by a heterologous enhancer is sufficient to repress, even when homie is in place. Furthermore, when the flanking promoter is turned around, repression is minimal. Transcriptional read-through that does not produce anti-sense RNA can still repress expression, ruling out RNAi as the mechanism in this case. Thus, transcriptional interference, caused by enhancer capture and read-through when the insulator is removed, represses eve promoter-driven expression. We also show that enhancer-promoter specificity and processivity of transcription can have decisive effects on the consequences of insulator removal. First, a core heat shock 70 promoter that is not activated well by eve enhancers did not cause read-through sufficient to repress the eve promoter. Second, these transcripts are less processive than those initiated at the P-promoter, measured by how far they extend through the eve locus, and so are less disruptive. These results highlight the importance of considering transcriptional read-through when assessing the effects of insulators on gene expression.

Several distinct activities and functions have been described for chromatin insulators, which are regulatory DNA elements that separate genes along chromosomes into functional units. Here, we describe how insulators can prevent repression of one gene by preventing inappropriate transcription of another gene, without blocking read-through of transcription per se. When the insulator homie is deleted from the end of a transgenic eve locus, a flanking transposable element promoter is activated by eve enhancers, causing repression of the eve promoter. The mechanism involves transcriptional read-through from the flanking promoter, which disrupts normal eve enhancer-promoter activities. When the flanking promoter is turned around, repression of eve is minimal. Thus, transcriptional interference, caused by enhancer capture and read-through when the insulator is removed, represses the eve promoter. These results show a novel role for transcriptional read-through in the effects of insulators on gene expression.

Diversity of α-tubulin transcripts in Lolium rigidum

BACKGROUND

Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene families in plants. To better characterize the mechanisms of target-site resistance to dinitroaniline herbicides in the globally important weedy species Lolium rigidum, attempts were made to amplify and sequence α-tubulin transcripts.

RESULTS

Four α-tubulin isoforms (TUA1, TUA2, TUA3 and TUA4) were identified in L. rigidum. Variations in the number and sequence of transcripts encoding these α-tubulin proteins were found in individuals from the two L. rigidum populations examined. Within and among populations, differences in the 5'- and 3'-untranslated regions of cDNA in TUA3 and TUA4 were identified. Furthermore, a novel double mutation, Arg-390-Cys+Asp-442-Glu, in the TUA3 transcript was identified and has the potential to confer dinitroaniline resistance.

CONCLUSION

This research reveals the complexity of the α-tubulin gene family in individuals/populations of the cross-pollinated weedy species L. rigidum, and highlights the need for better understanding of the molecular architecture of tubulin gene families for detecting resistance point mutations. Although TUA4 is a commonly expressed α-tubulin isoform containing most frequently reported resistance mutations, other mutant tubulin isoforms may also have a role in conferring dinitroaniline resistance.

Distinct cis-acting elements mediate targeting and clustering of Drosophila polar granule mRNAs

Specification and development of Drosophila germ cells depend on molecular determinants within the germ plasm, a specialized cytoplasmic domain at the posterior of the embryo. Localization of numerous mRNAs to the germ plasm occurs by their incorporation, as single-transcript ribonucleoprotein (RNP) particles, into complex RNP granules called polar granules. Incorporation of mRNAs into polar granules is followed by recruitment of additional like transcripts to form discrete homotypic clusters. The cis-acting localization signals that target mRNAs to polar granules and promote homotypic clustering remain largely uncharacterized. Here, we show that the polar granule component (pgc) and germ cell-less (gcl) 3' untranslated regions contain complex localization signals comprising multiple, independently weak and partially functionally redundant localization elements (LEs). We demonstrate that targeting of pgc to polar granules and self-assembly into homotypic clusters are functionally separable processes mediated by distinct classes of LEs. We identify a sequence motif shared by other polar granule mRNAs that contributes to homotypic clustering. Our results suggest that mRNA localization signal complexity may be a feature required by the targeting and self-recruitment mechanism that drives germ plasm mRNA localization.

Mutations at the Ser50 residue of translation factor eIF-2alpha dominantly affect developmental rate, body weight, and viability of Drosophila melanogaster

Phosphorylation of the translation initiation factor eIF-2alpha downregulates protein synthesis by sequestering the guanylate exchange factor eIF-2B. The importance of this regulation has been demonstrated in the context of stress and virally induced repression of protein synthesis but has not been investigated relative to the control of protein synthesis during development. Transgenic Drosophila strains bearing aspartic acid or alanine substitutions at the presumed regulatory phosphorylation site (Ser50) of Drosophila eIF-2alpha were established. The expression of the eIF-2alpha mutant transgenes, under the transcriptional control of the hsp70 promoter, was induced at various times during development to assess the developmental and biochemical effects. Flies bearing the aspartic acid eIF-2alpha mutant (HD) transgene displayed a slow growth phenotype and small body size. Repeated induction of the HD transgene resulted in cessation of development. In contrast, flies bearing the alanine eIF-2alpha mutant (HA) displayed a fast growth phenotype and females were significantly larger than nontransgenic control sisters. The HD transgenic flies exhibit a relatively lower level of global protein synthesis than the HA transgenic flies, although the difference is statistically insignificant.

Increased acetylation of microtubules rescues human tau-induced microtubule defects and neuromuscular junction abnormalities in Drosophila

Tau normally associates with and stabilizes microtubules (MTs), but is hyperphosphorylated and aggregated into neurofibrillary tangles in Alzheimer's disease and related neurodegenerative diseases, which are collectively known as tauopathies. MTs are regulated by different forms of post-translational modification, including acetylation; acetylated MTs represent a more stable microtubule population. In our previous study, we showed that inhibition of histone deacetylase 6 (HDAC6), which deacetylates tubulin at lysine 40, rescues defects in MTs and in neuromuscular junction growth caused by tau overexpression. However, HDAC6 also acts on other proteins that are involved in distinct biological processes unrelated to tubulins. In order to examine directly the role of increased tubulin acetylation against tau toxicity, we generated a site-directed α-tubulin^K40Q mutation by CRISPR/Cas9 technology to mimic the acetylated MTs and found that acetylation-mimicking α-tubulin rescued tau-induced MT defects and neuromuscular junction developmental abnormalities. We also showed that late administration of ACY-1215 and tubastatin A, two potent and selective inhibitors of HDAC6, rescued the tau-induced MT defects after the abnormalities had already become apparent. Overall, our results indicate that increasing MT acetylation by either genetic manipulations or drugs might be used as potential strategies for intervention in tauopathies.

Highlighted Article: Increased acetylation of microtubules by genetic and pharmacological approaches rescues human tau overexpression-induced toxicity in Drosophila muscles and neurons.

A Novel Strategy for Live Detection of Viral Infection in Drosophila melanogaster

We have created a transgenic reporter for virus infection, and used it to study Nora virus infection in Drosophila melanogaster. The transgenic construct, Munin, expresses the yeast transcription factor Gal4, tethered to a transmembrane anchor via a linker that can be cleaved by a viral protease. In infected cells, liberated Gal4 will then transcribe any gene that is linked to a promoter with a UAS motif, the target for Gal4 transcription. For instance, infected cells will glow red in the offspring of a cross between the Munin stock and flies with a UAS-RFP(nls) transgene (expressing a red fluorescent protein). In such flies we show that after natural infection, via the faecal-oral route, 5-15% of the midgut cells are infected, but there is little if any infection elsewhere. By contrast, we can detect infection in many other tissues after injection of virus into the body cavity. The same principle could be applied for other viruses and it could also be used to express or suppress any gene of interest in infected cells.

Reproductive workers show queenlike gene expression in an intermediately eusocial insect, the buff-tailed bumble bee Bombus terrestris

Bumble bees represent a taxon with an intermediate level of eusociality within Hymenoptera. The clear division of reproduction between a single founding queen and the largely sterile workers is characteristic for highly eusocial species, whereas the morphological similarity between the bumble bee queen and the workers is typical for more primitively eusocial hymenopterans. Also, unlike other highly eusocial hymenopterans, division of labour among worker subcastes is plastic and not predetermined by morphology or age. We conducted a differential expression analysis based on RNA-seq data from 11 combinations of developmental stage and caste to investigate how a single genome can produce the distinct castes of queens, workers and males in the buff-tailed bumble bee Bombus terrestris. Based on expression patterns, we found males to be the most distinct of all adult castes (2411 transcripts differentially expressed compared to nonreproductive workers). However, only relatively few transcripts were differentially expressed between males and workers during development (larvae: 71 and pupae: 162). This indicates the need for more distinct expression patterns to control behaviour and physiology in adults compared to those required to create different morphologies. Among female castes, reproductive workers and their nonreproductive sisters displayed differential expression in over ten times more transcripts compared to the differential expression found between reproductive workers and their mother queen. This suggests a strong shift towards a more queenlike behaviour and physiology when a worker becomes fertile. This contrasts with eusocial species where reproductive workers are more similar to nonreproductive workers than the queen.

Analysis of protein expression in zebrafish during gonad differentiation by targeted proteomics

The molecular mechanisms governing sex determination and differentiation in the zebrafish (Danio rerio) are not fully understood. To gain more insights into the function of specific genes in these complex processes, the expression of multiple candidates needs to be assessed, preferably on the protein level. Here, we developed a targeted proteomics method based on selected reaction monitoring (SRM) to study the candidate sex-related proteins in zebrafish which were selected based on a global proteomics analysis of adult gonads and representational difference analysis of male and female DNA, as well as on published information on zebrafish and other vertebrates. We employed the developed SRM protocols to acquire time-resolved protein expression profiles during the gonad differentiation period in vas::EGFP transgenic zebrafish. Evidence on protein expression was obtained for the first time for several candidate genes previously studied only on the mRNA level or suggested by bioinformatic predictions. Tuba1b (tubulin alpha 1b), initially included in the study as one of the potential housekeeping proteins, was found to be preferentially expressed in the adult testis with nearly absent expression in the ovary. The revealed changes in protein expression patterns associated with gonad differentiation suggest that several of the examined proteins, especially Ilf2 and Ilf3 (interleukin enhancer-binding factors 2 and 3), Raldh3 (retinaldehyde dehydrogenase type 3), Zgc:195027 (low density lipoprotein-related receptor protein 3) and Sept5a (septin 5a), may play a specific role in the sexual differentiation in zebrafish.

Identification of a tubulin-α gene specifically expressed in testis and adductor muscle during stable reference gene selection in the hermaphrodite gonad of the lion's paw scallop Nodipecten subnodosus

For non-model species, as many used for aquaculture, with minimal or no genomic information, relative quantification of gene expression studies requires preliminary research including the isolation of potential reference genes and the identification of those stably expressed under the biological conditions of interest. Here we report on the isolation of five partial gene sequences from gonad tissue cDNA in the functional hermaphrodite scallop Nodipecten subnodosus to be evaluated as reference genes: 18S-rRNA, riboprotein l8 (rp-l8), actin-β (act-β), elongation factor 1α (ef-1α) and alpha-tubulin-α (tub-α). We found that 18S-rRNA was stably expressed independently of the priming method used to reverse transcribe RNA to cDNA, oligo-dT or random hexamer. Stability analysis for the five putative reference genes with geNorm and NormFinder indicated that 18S together with rp-l8 were the most stable genes for normalization of gene expression during gonad development in both, male and female sexual regions of the hermaphrodite N. subnodosus. The least stable gene was tub-α, showing a biased expression profile between sexual regions of the gonad, therefore this gene was analyzed thereafter as a target gene together with vitellogenin (vit) and a DEAD-box RNA helicase (dbx) gene. Relative expression, estimated by normalization with the combination of 18S and rp-l8 as reference genes, indicated that as gonad development advanced two of the target genes were up-regulated, tub-α in the male region and vit in the female region. Whereas an increased expression was expected during development for vit for its known role in vitellogenesis, the increased expression of tub-α in the male sexual region was unexpected, and pointed toward this gene being a testis-specific α-tubulin isotype. Further analyses of gene expression among tissues indicated that tub-α is specifically and highly expressed in the male gonad, although expression in adductor muscle was also observed at significantly lower levels. The existence of testis specific α- and β-tubulins has been previously reported in other taxa, relating their function to sperm axoneme formation. Tissue-specific tubulin genes, particularly their promoters, have recently found an application as native promoters for transgene tissue-specific expression in research and reproductive control of insect plagues. The third target gene, a putative member of the DEAD-box RNA helicase family (dbx), showed no changes in expression during gonad development or between sexual regions, therefore it was chosen to discuss the different statistical inferences resulting from the arbitrary use of 'randomly chosen' reference genes when normalizing gene expression.

HDAC inhibitors in parasitic diseases

Parasitic diseases cause significant global morbidity and mortality, particularly in underdeveloped regions of the world. Malaria alone causes ~800000 deaths each year, with children and pregnant women being at highest risk. There is no licensed vaccine available for any human parasitic disease and drug resistance is compromising the efficacy of many available anti-parasitic drugs. This is driving drug discovery research on new agents with novel modes of action. Histone deacetylase (HDAC) inhibitors are being investigated as drugs for a range of diseases, including cancers and infectious diseases such as HIV/AIDS, and several parasitic diseases. This review focuses on the current state of knowledge of HDAC inhibitors targeted to the major human parasitic diseases malaria, schistosomiasis, trypanosomiasis, toxoplasmosis and leishmaniasis. Insights are provided into the unique challenges that will need to be considered if HDAC inhibitors are to be progressed towards clinical development as potential new anti-parasitic drugs.

Analysis of pupal head proteome and its alteration in diapausing pupae of Helicoverpa armigera

The proteomic approach has proven to be an useful tool for understanding insect diapause processes. Using 2D gel electrophoresis and matrix assisted laser/desorption ionization (MALDI) time of flight (TOF), we identified 24 proteins in the head of Helicoverpa armigera pupae with diverse functional characteristics, including cytoskeleton proteins, heat-shock proteins, insect development regulation factors, ATPases, proteins regulating signal pathway and enzymes involved in metabolism, etc. A proteomic comparison between nondiapausing and diapausing pupae revealed three proteins that were present only in nondiapausing pupae, and six proteins represented >or=2.0-fold or <or=0.5-fold changes. The differentially expressed proteins, including heat-shock protein 90, chitin deacetylase, alpha-tubulin and transitional endoplasmic reticulum ATPase, etc. were reported for the first time in H. armigera. Identification of these proteins will enable us to further characterize the regulated functions of diapause in this important species.

Decrease in expression of beta-tubulin and microtubule abundance in flight muscles during diapause in adults of Culex pipiens

The cDNA encoding beta-tubulin in the mosquito Culex pipiens has high similarity with the beta-tubulins reported in other insects. In this study, we examine expression of this gene and microtubule abundance in relation to diapause and low temperature. While non-diapausing mosquitoes express beta-tubulin highly in their thoracic muscles, expression is quite low during adult diapause. The abundance of microtubules was also much lower in flight muscles of diapausing adults than in flight muscles from non-diapausing individuals, as confirmed by laser confocal microscopy of tubulins stained using indirect immunofluorescence. Low temperatures decreased microtubule abundance in midguts of non-diapausing mosquitoes, but microtubule abundance in diapausing mosquitoes was already low and remained unchanged by low temperature exposure. Overall, pixel intensity averages were higher in the flight muscles than in the midguts, and again low temperatures decreased microtubule abundance in the flight muscles of non-diapausing females, while levels remained consistently low in diapausing females. These results clearly indicate that a decrease in microtubule abundance is evoked both by the programming of diapause and, in non-diapausing females, by exposure to low temperatures. Quite possibly the reduced microtubule abundance in the flight muscles and reduced expression of beta-tubulin are functionally correlated to the reduction in flight activity that is associated with low temperature and diapause.

Microarray analysis of female- and larval-specific gene expression in the horn fly (Diptera: Muscidae)

The horn fly, Haematobia irritans L., is an obligate blood-feeding parasite of cattle, and control of this pest is a continuing problem because the fly is becoming resistant to pesticides. Dominant conditional lethal gene systems are being studied as population control technologies against agricultural pests. One of the components of these systems is a female-specific gene promoter that drives expression of a lethality-inducing gene. To identify candidate genes to supply this promoter, microarrays were designed from a horn fly expressed sequence tag (EST) database and probed to identify female-specific and larval-specific gene expression. Analysis of dye swap experiments found 432 and 417 transcripts whose expression levels were higher or lower in adult female flies, respectively, compared with adult male flies. Additionally, 419 and 871 transcripts were identified whose expression levels were higher or lower in first-instar larvae compared with adult flies, respectively. Three transcripts were expressed more highly in adult females flies compared with adult males and also higher in the first-instar larval lifestage compared with adult flies. One of these transcripts, a putative nanos ortholog, has a high female-to-male expression ratio, a moderate expression level in first-instar larvae, and has been well characterized in Drosophila. melanogaster (Meigen). In conclusion, we used microarray technology, verified by reverse transcriptase-polymerase chain reaction and massively parallel pyrosequencing, to study life stage- and sex-specific gene expression in the horn fly and identified three gene candidates for detailed evaluation as a gene promoter source for the development of a female-specific conditional lethality system.

The cloning and expression of alpha-tubulin in Monochamus alternatus

The Japanese pine sawyer Monochamus alternatus is one of the major forest pests. It damages pine directly and transfers the nematode Bursaphelenchus xylophilus to pine wood; resulting in serious economic losses around the world every year. Alpha-tubulin is one of most important proteins in most species. We cloned a ubiquitously expressed M. alternatus alpha-tubulin gene and analysed its nucleotides and protein structure; its sequence characters are consistent with what have been reported in other insects. The alignment of proteins showed that there is high homology of alpha-tubulin between M. alternatus and other species. Western blot and immunocytochemistry analyses suggested a common epitope of alpha-tubulin between M. alternatus and Strongylcentrotus purpuratus. We also expressed the protein in Escherichia coli for further functional studies.

Tubulin superfamily genes in Tribolium castaneum and the use of a Tubulin promoter to drive transgene expression

The use of native promoters to drive transgene expression has facilitated overexpression studies in Drosophila and other insects. We identified 12 Tubulin family members from the genome sequence of the red flour beetle, Tribolium castaneum, and used the promoter from one of these to drive constitutive expression of a transgene. The activity of the T. castaneum alpha-Tubulin1 (TcalphaTub1) putative promoter was pre-tested in conjunction with an eye-color gene, T. castaneum vermilion (Tcv), by transient expression in Tcv-deficient embryos. Such embryos showed complete rescue of larval eyespot pigmentation. We also examined the TcalphaTub1 expression pattern in germline transformants using the enhanced green fluorescent protein (EGFP) reporter. Beetles transformed with this piggyBac-based reporter ubiquitously expressed EGFP at all stages.

Axoneme-dependent tubulin modifications in singlet microtubules of the Drosophila sperm tail

Drosophila melanogaster sperm tubulins are posttranslationally glutamylated and glycylated. We show here that axonemes are the substrate for these tubulin C-terminal modifications. Axoneme architecture is required, but full length, motile axonemes are not necessary. Tubulin glutamylation occurs during or shortly after assembly into the axoneme; only glutamylated tubulins are glycylated. Tubulins in other testis microtubules are not modified. Only a small subset of total Drosophila sperm axoneme tubulins have these modifications. Biochemical fractionation of Drosophila sperm showed that central pair and accessory microtubules have the majority of poly-modified tubulins, whereas doublet microtubules have only small amounts of mono- and oligo-modified tubulins. Glutamylation patterns for different beta-tubulins experimentally assembled into axonemes were consistent with utilization of modification sites corresponding to those identified in other organisms, but surrounding sequence context was also important. We compared tubulin modifications in the 9 + 9 + 2 insect sperm tail axonemes of Drosophila with the canonical 9 + 2 axonemes of sperm of the sea urchin Lytichinus pictus and the 9 + 0 motile sperm axonemes of the eel Anguilla japonica. In contrast to Drosophila sperm, L. pictus sperm have equivalent levels of modified tubulins in both doublet and central pair microtubule fractions, whereas the doublets of A. japonica sperm exhibit little glutamylation but extensive glycylation. Tubulin C-terminal modifications are a prevalent feature of motile axonemes, but there is no conserved pattern for placement or amount of these

Drosophila ESC-like can substitute for ESC and becomes required for Polycomb silencing if ESC is absent

The Drosophila esc-like gene (escl) encodes a protein very similar to ESC. Like ESC, ESCL binds directly to the E(Z) histone methyltransferase via its WD region. In contrast to ESC, which is present at highest levels during embryogenesis and low levels thereafter, ESCL is continuously present throughout development and in adults. ESC/E(Z) complexes are present at high levels mainly during embryogenesis but ESCL/E(Z) complexes are found throughout development. While depletion of either ESCL or ESC by RNAi in S2 and Kc cells has little effect on E(Z)-mediated methylation of histone H3 lysine 27 (H3K27), simultaneous depletion of ESCL and ESC results in loss of di- and trimethyl-H3K27, indicating that either ESC or ESCL is necessary and sufficient for di- and trimethylation of H3K27 in vivo. While E(Z) complexes in S2 cells contain predominantly ESC, in ESC-depleted S2 cells, ESCL levels rise dramatically and ESCL replaces ESC in E(Z) complexes. A mutation in escl that produces very little protein is viable and exhibits no phenotypes but strongly enhances esc mutant phenotypes, suggesting they have similar functions. esc escl double homozygotes die at the end of the larval period, indicating that the well-known "maternal rescue" of esc homozygotes requires ESCL. Furthermore, maternal and zygotic over-expression of escl fully rescues the lethality of esc null mutant embryos that contain no ESC protein, indicating that ESCL can substitute fully for ESC in vivo. These data thus indicate that ESC and ESCL play similar if not identical functions in E(Z) complexes in vivo. Despite this, when esc is expressed normally, escl appears to be entirely dispensable, at least for development into morphologically normal fertile adults. Furthermore, the larval lethality of esc escl double mutants, together with the lack of phenotypes in the escl mutant, further suggests that in wild-type (esc(+)) animals it is the post-embryonic expression of esc, not escl, that is important for development of normal adults. Thus escl appears to function in a backup capacity during development that becomes important only when normal esc expression is compromised.

The Kavar D dominant female-sterile mutations of Drosophila reveal a role for the maternally provided α-tubulin4 isoform in cleavage spindle maintenance and elongation

The dominant-negative female-sterile Kavar(D) mutations and their revertant kavar(r) alleles identify the alphaTubulin67C gene of Drosophila melanogaster, which codes for the maternally provided alpha-tubulin(4) isoform. The mutations result in the formation of monopolar, collapsed spindles (each with two nearby centrosomes, a tassel of microtubules and overcondensed chromosomes), thus revealing a novel function for alpha-tubulin(4) in spindle maintenance and elongation. Molecular features of the two Kavar(D) alleles and a kavar(null) allele are described and models for their actions are discussed.

A complementary transposon tool kit for Drosophila melanogaster using P and piggyBac

With the availability of complete genome sequence for Drosophila melanogaster, one of the next strategic goals for fly researchers is a complete gene knockout collection. The P-element transposon, the workhorse of D. melanogaster molecular genetics, has a pronounced nonrandom insertion spectrum. It has been estimated that 87% saturation of the approximately 13,500-gene complement of D. melanogaster might require generating and analyzing up to 150,000 insertions. We describe specific improvements to the lepidopteran transposon piggyBac and the P element that enabled us to tag and disrupt genes in D. melanogaster more efficiently. We generated over 29,000 inserts resulting in 53% gene saturation and a more diverse collection of phenotypically stronger insertional alleles. We found that piggyBac has distinct global and local gene-tagging behavior from that of P elements. Notably, piggyBac excisions from the germ line are nearly always precise, piggyBac does not share chromosomal hotspots associated with P and piggyBac is more effective at gene disruption because it lacks the P bias for insertion in 5' regulatory sequences.

Live imaging of endogenous RNA reveals a diffusion and entrapment mechanism for nanos mRNA localization in Drosophila

BACKGROUND

Localization of nanos mRNA to the posterior pole of the Drosophila embryo directs local synthesis of Nanos protein that is essential for patterning of the anterior-posterior body axis and germ cell function. While nanos RNA is synthesized by the ovarian nurse cells and appears at the posterior pole of the ooctye late in oogenesis, the mechanism by which this RNA is translocated to and anchored at the oocyte posterior is unknown.

RESULTS

By labeling endogenous nanos RNA with GFP, we have been able to follow the dynamic pathway of nanos localization in living oocytes. We demonstrate that nanos localization initiates immediately upon nurse cell dumping, whereby diffusion, enhanced by microtubule-dependent cytoplasmic movements, translocates nanos RNA from the nurse cells to the ooctye posterior. At the posterior, nanos is trapped by association, in particles, with the posteriorly localized germ plasm. Actin-dependent anchoring of nanos RNA complexed to the germ plasm at the posterior maintains localization in the face of rapid cytoplasmic movements.

CONCLUSIONS

These results reveal a diffusion-based, late-acting posterior localization mechanism for long-range transport of nanos mRNA. This mechanism differs from directed transport-based localization mechanisms in its reliance on bulk movement of RNA.

piggyBac-based insertional mutagenesis in the presence of stably integrated P elements in Drosophila

P element-mediated mutagenesis has been used to disrupt an estimated 25% of genes essential for Drosophila adult viability. Mutation of all genes in the fly genome, however, poses a problem, because P elements show significant hotspots of integration. In addition, advanced screening scenarios often require the use of P element-based tools like the generation of germ-line mosaics using FLP recombinase-mediated recombination or gene misexpression using the UAS/Gal4 system. These techniques are P element-based and can therefore not be combined with the use of P elements as mutagenic agents. To circumvent these limitations, we have developed an insertional mutagenesis system using non-P element transposons. An enhanced yellow fluorescent protein-marked piggyBac-based mutator element was mobilized by a piggyBac specific transposase source expressed from a Hermes-based jump-starter transposon marked with enhanced cyan fluorescent protein. In a pilot screen, we have generated 798 piggyBac insertions on FRT bearing third chromosomes of which 9% have sustained a putatively piggyBac-related lethal hit. The FRTs present on the target chromosome remained stably integrated during the screen and could subsequently be used to generate germ-line clones associated with maternal and zygotic phenotypes. PCR-based analysis of insertion loci shows that 57% of the insertions are in genes for which no P element insertions have been reported. Our data demonstrate the potential of this technique to facilitate the quest for saturation mutagenesis of the Drosophila genome. The system is Drosophila nonspecific and potentially applicable in a broad spectrum of nonmodel organisms.

piggyBac-based insertional mutagenesis and enhancer detection as a tool for functional insect genomics

Transposon mutagenesis provides a fundamental tool for functional genomics. Here we present a non-species-specific, combined enhancer detection and binary expression system based on the transposable element piggyBac: For the different components of this insertional mutagenesis system, we used widely applicable transposons and distinguishable broad-range transformation markers, which should enable this system to be operational in nonmodel arthropods. In a pilot screen in Drosophila melanogaster, piggyBac mutator elements on the X chromosome were mobilized in males by a Hermes-based jumpstarter element providing piggyBac transposase activity under control of the alpha1-tubulin promoter. As primary reporters in the piggyBac mutator elements, we employed the heterologous transactivators GAL4delta or tTA. To identify larval and adult enhancer detectors, strains carrying UASp-EYFP or TRE-EYFP as secondary reporter elements were used. Tissue-specific enhancer activities were readily observed in the GAL4delta/UASp-based systems, but only rarely in the tTA/TRE system. Novel autosomal insertions were recovered with an average jumping rate of 80%. Of these novel insertions, 3.8% showed homozygous lethality, which was reversible by piggyBac excision. Insertions were found in both coding and noncoding regions of characterized genes and also in noncharacterized and non-P-targeted CG-number genes. This indicates that piggyBac will greatly facilitate the intended saturation mutagenesis in Drosophila.

Analysis of alpha- and beta-tubulin genes of Bombyx mori using an EST database

Tubulin is one of the most widespread classes of multiprotein families and is well known to construct microtubules with two different subunits, alpha- and beta-tubulin. In the course of genome analysis of Bombyx mori, we have constructed an EST database by large-scale sequencing of clones that were randomly selected from cDNA libraries of various tissues and organs belonging to different developmental stages. Using this EST database, we have identified four types of beta-tubulin gene and three types of alpha-tubulin gene. Based on the analysis of deduced amino acid sequences, we have determined the phylogenetic relationships of tubulins between Bombyx and Drosophila melanogaster as well as two other moth species, suggesting that each tubulin is classified into at least three distinct subfamilies: a ubiquitously expressed one, a developmentally regulated one and a testis specific one.

Cloning and characterization of a divergent alpha-tubulin that is expressed specifically in dividing amebae of Naegleria gruberi

A novel alpha-tubulin gene (alpha6) was cloned from a genomic library of Naegleria gruberi strain NB-1 and characterized. The open reading frame of alpha6 contained 1359 nucleotides encoding a protein of 452 amino acids (aa) with a calculated molecular weight of 50.5 kDa. The nucleotide sequence of the open reading frame of alpha6 showed considerable divergence (68.4% identity) when compared with previously cloned N. gruberi alpha-tubulin genes, which share about 97% identity in DNA sequences. The deduced aa sequence of alpha6-tubulin was 61.9% identical to that of alpha13-tubulin, which was cloned from the same strain, and showed similar identities to those of alpha-tubulins from other species (54 approximately 62%). These data showed that alpha6-tubulin is one of the most divergent alpha-tubulins so far known. Alpha6-tubulin was found to be expressed in actively growing cells and repressed quickly when these cells were induced to differentiate. Immunostaining with an antibody against alpha6-tubulin showed that alpha6-tubulin is present in the nuclei and mitotic spindle-fibers but absent in flagellar axonemes or cytoskeletal microtubules. These data finally established the presence of an alpha-tubulin that is specifically utilized for spindle-fiber microtubules and distinct from the flagellar axonemal alpha-tubulins in N. gruberi, hence confirmed the multi-tubulin hypothesis in this organism.

The odorant-binding proteins of Drosophila melanogaster: annotation and characterization of a divergent gene family

Insect odorant-binding proteins (OBPs) are thought to facilitate the delivery of hydrophobic odorants, such as sex pheromones or food odors, to receptors on sensory neurons. Increasingly, OBP family members are also being found in non-sensory tissues where they might carry other types of small hydrophobic molecules. They are identifiable by four or six conserved Cys residues and contain six alpha-helices which enclose a hydrophobic ligand-binding pocket. Through exhaustive BLAST searches we have increased the total number of OBPs identified in Drosophila melanogaster to 38, and have amplified the DNA complementary to RNA corresponding to 21 of these by reverse transcriptase polymerase chain reaction. Isoforms frequently share less than 30% amino acid identity and appear to have radically changed since the separation of the major insect orders. However, their sequences are consistent with known OBP structures. Most are located in clusters of between four and 14 genes and several were unusual in that they contained additions, deletions, or fusions. These hexa-helical insect OBPs are structurally unrelated to the functionally analogous lipocalin-like beta-barrel OBPs of vertebrates. As only two lipocalin-like proteins have been found in D. melanogaster, these helical proteins appear to be the dominant carrier of small hydrophobic molecules in insects.

Short Stop provides an essential link between F-actin and microtubules during axon extension

Coordination of F-actin and microtubule dynamics is important for cellular motility and morphogenesis, but little is known about underlying mechanisms. short stop (shot) encodes an evolutionarily conserved, neuronally expressed family of rod-like proteins required for sensory and motor axon extension in Drosophila melanogaster. We identify Shot isoforms that contain N-terminal F-actin and C-terminal microtubule-binding domains, and that crosslink F-actin and microtubules in cultured cells. The F-actin- and microtubule-binding domains of Shot are required in the same molecule for axon extension, though the length of the connecting rod domain can be dramatically reduced without affecting activity. Shot therefore functions as a cytoskeletal crosslinker in axon extension, rather than mediating independent interactions with F-actin and microtubules. A Ca2+-binding motif located adjacent to the microtubule-binding domain is also required for axon extension, suggesting that intracellular Ca2+ release may regulate Shot activity. These results suggest that Shot coordinates regulated interactions between F-actin and microtubules that are crucial for neuronal morphogenesis.

Headless flies produced by mutations in the paralogous Pax6 genes eyeless and twin of eyeless

The two Pax6 gene homologs eyeless and twin of eyeless play decisive early roles in Drosophila eye development. Strong mutants of twin of eyeless or of eyeless are headless, which suggests that they are required for the development of all structures derived from eye-antennal discs. The activity of these genes is crucial at the very beginning of eye-antennal development in the primordia of eye-antennal discs when eyeless is first activated by the twin of eyeless gene product. This activation does not strictly depend on the Twin of eyeless protein, but is temperature-dependent in its absence. Twin of eyeless acts also in parallel to the eyeless gene and exerts functions that are partially redundant with those of Eyeless, while Eyeless is mainly required to prevent early cell death and promote eye development in eye-antennal discs.

A Drosophila model of HIV-Tat-related pathogenicity

To analyze the mechanism of Tat-mediated HIV pathogenicity, we produced a Drosophila melanogaster strain transgenic for HIV-tat gene and induced the expression of the protein during Drosophila development. By in vitro and in vivo experiments, we demonstrated that Tat specifically binds to tubulin via the MAP-binding domain of tubulin, and that this interaction delays the polymerization of tubulin and induces a premature stop to microtubule-dependent cytoplasmic streaming. The delay in the polymerization of microtubules, the tracks for the transport of the axes determinants, alters the positioning of the dorso-ventral axis as shown by the mislocalization of Gurken and Kinesin in oocyte of Drosophila after Tat induction. These results validate the use of Drosophila as a tool to study the molecular mechanism of viral gene products and suggest that Tat-tubulin interaction is responsible for neurodegenerative diseases associated with AIDS.

Promoter analysis for daily expression of Drosophila timeless gene

Drosophila circadian clock gene timeless (tim) displays circadian oscillation in its mRNA level, and such oscillation is transcriptionally regulated. The promoter region up to -756 of tim is suggested to promote the circadian mRNA expression, however, the role of the sequence upstream of tim promoter region in the transcriptional regulation is still unrevealed. We novelly isolated and determined tim 5'-flanking sequence -2764 to -757, and found a putative cAMP-response element, six regions of the half site for PAR-basic leucine zipper transcription factors and six nonpalindromic E-boxes. Our in vivo reporter assay showed that 966 bp of tim promoter region, including a palindromic CACGTG E-box and a half site of PAR-basic leucine zipper transcription factors, was minimally required for its daily mRNA expression. While, the deletion of the 5'-flanking region -1970 to -967 caused a slight decrease in the reporter mRNA levels. These results indicate that the 5'-flanking sequence upstream of the promoter region have a role in the daily regulation of tim mRNA expression in Drosophila.

A role for cyclin J in the rapid nuclear division cycles of early Drosophila embryogenesis

The nuclear division cycles of early Drosophila embryogenesis have a number of unique features that distinguish them from later cell cycles. These features include the lack of some checkpoints that operate in later cell cycles, the absence of gap phases, and very rapid DNA synthesis phases. The molecular mechanisms that control these rapid nuclear division cycles are poorly understood. Here we describe analysis of cyclin J, a previously uncharacterized cyclin which has an RNA expression pattern that suggests a possible role in early embryogenesis. We show that the cyclin J protein is present in early embryos where it forms active kinase complexes with cyclin-dependent kinase (Cdk) 2. To determine whether cyclin J plays a role in controlling the early nuclear cycles we isolated peptide aptamers that specifically bind to cyclin J and inhibit its ability to activate Cdks. We injected the inhibitory aptamers into syncytial Drosophila embryos and demonstrated that they caused defects in chromosome segregation and progression through mitosis. We obtained similar results by injecting cyclin J antibodies into embryos. Our results suggest that a cyclin J-associated kinase activity is required for the early embryonic division cycles.

Drosophila Lis1 is required for neuroblast proliferation, dendritic elaboration and axonal transport

Haplo-insufficiency of human Lis1 causes lissencephaly. Reduced Lis1 activity in both humans and mice results in a neuronal migration defect. Here we show that Drosophila Lis1 is highly expressed in the nervous system. Lis1 is essential for neuroblast proliferation and axonal transport, as shown by a mosaic analysis using a Lis1 null mutation. Moreover, it is cell-autonomously required for dendritic growth, branching and maturation. Analogous mosaic analysis shows that neurons containing a mutated cytoplasmic-dynein heavy chain (Dhc64C) exhibit phenotypes similar to Lis1 mutants. These results implicate Lis1 as a regulator of the microtubule cytoskeleton and show that it is important for diverse physiological functions in the nervous system.

The fusome organizes the microtubule network during oocyte differentiation in Drosophila

Differentiation of the Drosophila oocyte takes place in a cyst of 16 interconnected germ cells and is dependent on a network of microtubules that becomes polarized as differentiation progresses (polarization). We have investigated how the microtubule network polarizes using a GFP-tubulin construct that allows germ-cell microtubules to be visualized with greater sensitivity than in previous studies. Unexpectedly, microtubules are seen to associate with the fusome, an asymmetric germline-specific organelle, which elaborates as cysts form and undergoes complex changes during cyst polarization. This fusome-microtubule association occurs periodically during late interphases of cyst divisions and then continuously in 16-cell cysts that have entered meiotic prophase. As meiotic cysts move through the germarium, microtubule minus ends progressively focus towards the center of the fusome, as visualized using a NOD-lacZ marker. During this same period, discrete foci rich in gamma tubulin that very probably correspond to migrating cystocyte centrosomes also associate with the fusome, first on the fusome arms and then in its center, subsequently moving into the differentiating oocyte. The fusome is required for this complex process, because microtubule network organization and polarization are disrupted in hts(1) mutant cysts, which lack fusomes. Our results suggest that the fusome, a specialized membrane-skeletal structure, which arises in early germ cells, plays a crucial role in polarizing 16-cell cysts, at least in part by interacting with microtubules and centrosomes.

Quantitative analysis of gene function in the Drosophila embryo

The specific functions of gene products frequently depend on the developmental context in which they are expressed. Thus, studies on gene function will benefit from systems that allow for manipulation of gene expression within model systems where the developmental context is well defined. Here we describe a system that allows for genetically controlled overexpression of any gene of interest under normal physiological conditions in the early Drosophila embryo. This regulated expression is achieved through the use of Drosophila lines that express a maternal mRNA for the yeast transcription factor GAL4. Embryos derived from females that express GAL4 maternally activate GAL4-dependent UAS transgenes at uniform levels throughout the embryo during the blastoderm stage of embryogenesis. The expression levels can be quantitatively manipulated through the use of lines that have different levels of maternal GAL4 activity. Specific phenotypes are produced by expression of a number of different developmental regulators with this system, including genes that normally do not function during Drosophila embryogenesis. Analysis of the response to overexpression of runt provides evidence that this pair-rule segmentation gene has a direct role in repressing transcription of the segment-polarity gene engrailed. The maternal GAL4 system will have applications both for the measurement of gene activity in reverse genetic experiments as well as for the identification of genetic factors that have quantitative effects on gene function in vivo.

Mutations in the alpha-tubulin 67C gene specifically impair achiasmate segregation in Drosophila melanogaster

Drosophila melanogaster oocytes heterozygous for mutations in the alpha-tubulin 67C gene (alphatub67C) display defects in centromere positioning during prometaphase of meiosis I. The centromeres do not migrate to the poleward edges of the chromatin mass, and the chromatin fails to stretch during spindle lengthening. These results suggest that the poleward forces acting at the kinetochore are compromised in the alphatub67C mutants. Genetic studies demonstrate that these mutations also strongly and specifically decrease the fidelity of achiasmate chromosome segregation. Proper centromere orientation, chromatin elongation, and faithful segregation can all be restored by a decrease in the amount of the Nod chromokinesin. These results suggest that the accurate segregation of achiasmate chromosomes requires the proper balancing of forces acting on the chromosomes during prometaphase.

Different 3′ untranslated regions target alternatively processed hu-li tai shao (hts) transcripts to distinct cytoplasmic locations during Drosophila oogenesis

Cytoplasmic mRNA localization is one method by which protein production is restricted to a particular intracellular site. We report here a novel mechanism for localization of transcripts encoding distinct protein isoforms to different destinations. Alternative processing of transcripts produced in the Drosophila ovary by the hu-li tai shao (hts) locus introduces distinct 3′ untranslated regions (3′UTRs) that differentially localize the mRNAs. Three classes of hts mRNA (R2, N32 and N4) are synthesized in the germ line nurse cells and encode proteins with adducin-homologous amino-terminal regions but divergent carboxy-terminal domains. The R2 and N32 classes of mRNA remain in the nurse cells and are not transported into the oocyte. In contrast, the N4 class of transcripts is transported from the nurse cells into the oocyte starting at stage 1, is subsequently localized to the oocyte cortex at stage 8 and then to the anterior pole from stage 9 on. All aspects of N4 transcript transport and localization are directed by the 345-nucleotide(nt)-long 3′ untranslated region (3′UTR). The organization of localization elements in the N4 3′UTR is modular: a 150 nt core is sufficient to direct transport and localization throughout oogenesis. Additional 3′UTR elements function additively together with this core region at later stages of oogenesis to maintain or enhance anterior transcript anchoring. The swallow locus is required to maintain hts transcripts at the anterior pole of the oocyte and functions through the N4 3′UTR. In addition to the three classes of germ line-expressed hts transcripts, a fourth class (R1) is expressed in the somatic follicle cells that surround the germ line cells. This transcript class encodes the Drosophila orthologue of mammalian adducin.

Drosophila S6 kinase: a regulator of cell size

Cell proliferation requires cell growth; that is, cells only divide after they reach a critical size. However, the mechanisms by which cells grow and maintain their appropriate size have remained elusive. Drosophila deficient in the S6 kinase gene (dS6K) exhibited an extreme delay in development and a severe reduction in body size. These flies had smaller cells rather than fewer cells. The effect was cell-autonomous, displayed throughout larval development, and distinct from that of ribosomal protein mutants (Minutes). Thus, the dS6K gene product regulates cell size in a cell-autonomous manner without impinging on cell number.

Computational prediction of the three-dimensional structures for the Caenorhabditis elegans tubulin family

In this article we characterize, from a structural point of view, all 16 members of the tubulin gene family of Caenorhabditis elegans (9 alpha-tubulins, 6 beta-tubulins, and 1 gamma-tubulin). We obtained their tertiary structures by computationally modifying the X-ray crystal structure of the pig brain alpha/beta-tubulin dimer published by Nogales et al. [Nature (London) 1998;391:199-203]. Our computational protocol involves changing the amino acids (with MIDAS; Jarvis et al., UCSF MIDAS. University of California, San Francisco, 1986) in the 3D structure of pig brain alpha/beta-tubulin dimer followed by geometry optimization with the AMBER force field (Perlman et al., AMBER 4. University of California, San Francisco, 1990). We subsequently analyze and compare the resulting structures in terms of the differences in their secondary and tertiary structures. In addition, we compare the pattern of hydrogen bonds and hydrophobic contacts in the guanosine triphosphate (GTP)-binding site for all members of the tubulin family. Our computational results show that, except for gamma-tubulin, all members of the C. elegans tubulin family have similar secondary and 3D structures and that the change in the pattern of hydrogen bonds in the GTP-binding site may be used to assess the relative stability of different alpha/beta-tubulin dimers formed by monomers of the tubulin family.

Identification of a Drosophila homologue to vertebrate Crk by interaction with MBC

The vertebrate adapter protein termed Crk was initially identified from the chicken CT10 retrovirus on the basis of its transforming activity (Mayer et al., 1988. Nature 332, 272-275). We have identified a Drosophila protein with homology to vertebrate Crk, termed dCRK, by interaction with the protein encoded by the Drosophila myoblast city (mbc) gene. The dCRK protein has extensive homology to the both the Crk-II form of vertebrate Crk and the Crk-related protein CRKL, and includes one SH2 domain followed by two SH3 domains. A single protein of approx. 37kDa is detected in extracts from embryos, and Northern analysis revealed a single transcript of 1.3kb. The dCrk mRNA is abundant throughout embryogenesis, declines during the larval stages, and reappears during pupation. In addition, maternally-provided transcripts have been detected. During embryogenesis, the spatial distribution of this transcript is relatively broad and appears to include all germ layers. Finally, dCrk is located on the fourth chromosome, approximately at cytological position 101F-102A.

Cloning, characterisation and expression of the alpha-tubulin genes of the leech, Hirudo medicinalis

We have isolated two alpha-tubulin cDNAs from the leech, Hirudo medicinalis. Both encode putative proteins of 451 amino-acids which differ from each other at only two positions. Southern blotting suggests that there are only two alpha-tubulin genes in the leech. The genes contain two introns and, because of the extremely high homology of the nucleotide sequence from the second intron to the end of the genes, we have inferred that a gene conversion event about 9.5 million years ago has homogenised the Hirudo alpha-tubulin sequences. Using in situ hybridisation to tissue sections, we have shown that the two genes are probably expressed in all neurons of the leech ganglia and that their spatial distribution remains unchanged during neuronal regeneration. The deduced amino-acid sequences of the leech alpha-tubulins show that they have greatest similarity to those from a platyhelminth, echiuran and mollusc with rather less to arthropod alpha-tubulins. The protein sequences of the leech alpha-tubulins have been compared with representatives of those from across all phyla to determine if any specific feature labels certain isotypes of tubulin for neuronal expression.

Alterations of per RNA in noncoding regions affect periodicity of circadian behavioral rhythms

Circadian rhythms in Drosophila depend on a molecular feedback loop that includes products of the period (per) and timeless (tim) genes. RNA and protein products of both genes cycle with a circadian period and the proteins feedback to inhibit expression of their own mRNAs. While cyclic expression of PER protein appears to be necessary for rhythmic behavior, the function of per RNA cycling is somewhat controversial. Rhythmic transcription accounts, in part, for cycling of per RNA, but it is clear now that posttranscriptional mechanisms also contribute to the cyclic expression of both per RNA and protein. As posttranscriptional mechanisms, such as mRNA stability and translation, are frequently mediated by 3' untranslated regions (UTR) of genes, the authors examined the role of this region of per in the regulation of circadian rhythms. Removal of most of per's 3' UTR had a small effect on the function of a per transgene. However, replacement of per's 3'UTR with corresponding sequences of the tubulin gene led to the rescue of behavioral rhythms in per01 flies with periods that were 3 h shorter than those generated by a wild-type per transgene. The hybrid RNA cycles, but the protein produced by it accumulates earlier in a day-night cycle than the PER protein produced by a control per transgene carrying its own 3'UTR, perhaps because the tubulin sequences counteract the effect of destabilizing elements in the per RNA at earlier points in the circadian cycle. These data indicate that the appropriate regulation of per RNA expression, effected by transcriptional as well as posttranscriptional mechanisms, is critical for the determination of circadian period.

The organization of the alpha-tubulin gene family in the Drosophila montium subgroup of the melanogaster species group

The alpha 1-, alpha 2-, alpha 3-, and alpha 4-tubulin genes have been mapped by in situ hybridization to the polytene chromosomes of five species representative of the Drosophila montium subgroup geographical distribution. A lambda phage clone containing alpha 1-tubulin specific sequences was isolated from a genomic DNA library of Drosophila auraria and its restriction endonuclease pattern is presented. Both well-characterized heterologous and homologous probes were used to assess orthogonality of gene members between species groups. The in situ hybridization pattern observed in all species studied is consistent with that of Drosophila melanogaster, since alpha 1-, alpha 2-, and alpha 3-tubulin genes are located on the same polytene arm, and the alpha 4-tubulin gene is found on a different arm. Cross-hybridization was observed among alpha 1-, alpha 2-, and alpha 3-tubulin specific sequences in all species studied, using either heterologous or homologous probes. However, unlike D. melanogaster, in all montium species studied, both alpha 1- and alpha 3-tubulin specific probes hybridize to the same polytene band, indicating a clustered organization of the above genes. The chromosomal organization of this gene family would suggest that taxa within the montium subgroup are closer to their common ancestors than are the taxa in the melanogaster species group. A mode of evolution for this gene family in Drosophila is proposed.

The Tomaj mutant alleles of alpha Tubulin67C reveal a requirement for the encoded maternal specific tubulin isoform in the sperm aster, the cleavage spindle apparatus and neurogenesis during embryonic development in Drosophila

The three dominant TomajD and their eleven revertant (TomajR) alleles have been localized to the alpha Tubulin67C gene of Drosophila melanogaster. Although the meiotic divisions are normally completed in eggs laid by TomajD/+, TomajD/-, TomajR/- females, embryogenesis arrests prior to the gonomeric division. The arrest is caused by: (1) the failure of prominent sperm aster formation; and (2) a consequent lack of female pronuclear migration towards the male pronucleus. Concomitant with the sperm aster defect, the four female meiotic products fuse (tetra-fusion), similar to what is seen in eggs of wild-type virgin females. In eggs of females heterozygous for weaker TomajR alleles, embryogenesis comes to a cessation before or shortly after cortical migration of cleavage nuclei. The apparent source of embryonic defect is the cleavage spindle apparatus. One of the three TomajD alleles is cold-sensitive and its cold-sensitive period coincides with the completion of female meiosis and pronuclear migration. Disorganized central and peripheral nervous systems are also characteristic of embryos derived from the temperature-sensitive TomajD/+ females. The Tomaj mutant phenotypes indicate an involvement of the normal alpha Tubulin67C gene product in: (1) the formation of the sperm aster; (2) cleavage spindle apparatus formation/function; and (3) the differentiation of the embryonic nervous system. The TomajD alleles encode a normal-sized alpha Tubulin67C isotype. Sequence analyses of the TomajD alleles revealed the replacement in different positions of a single negatively charged or neutral amino acid with a positively charged one. These residues presumably identify important functional sites.

Identification and developmental expression of a Bombyx mori alpha-tubulin gene

A cDNA clone isolated from the wing discs at the metamorphosis of Bombyx mori during the period of morphogenesis has been characterized. The amino acid sequence predicted for the putative protein is highly homologous to the Drosophila alpha1-tubulin. This is the first alpha-tubulin gene isolated in Bombyx mori and other isotype sequences are present in the Bombyx genome. The transcript is detected in the wing discs at every postembryonic stage examined, and is also expressed in other tissues, but at different levels. Although the mRNA level is maximum when the 20-hydroxyecdysone titre is high, its accumulation is independent of the hormone level both in vivo and in vitro. Significance of the accumulation of the mRNA of an ubiquitous alpha-tubulin in developing wing discs is discussed by comparison with our knowledge of the alpha-tubulin family in Drosophila and in other organisms.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Different levels of Ras activity can specify distinct transcriptional and morphological consequences in early Drosophila embryos

The terminal portions of the Drosophila body pattern are specified by the localized activity of the receptor tyrosine kinase Torso (Tor) at each pole of the early embryo. Tor activity elicits the transcription of two ‘gap’ genes, tailless (tll) and huckebein (hkb), in overlapping but distinct domains by stimulating the Ras signal transduction pathway. Here, we show that quantitative variations in the level of Ras activity can specify qualitatively distinct transcriptional and morphological responses. Low levels of Ras activity at the posterior pole direct tll but not hkb transcription; higher levels drive transcription of both genes. Correspondingly, low levels of Ras activity specify a limited subset of posterior terminal structures, whereas higher levels specify a larger subset. However, we also show that the response to Ras activity is not uniform along the body. Instead, levels of Ras activity which suffice to drive tll and hkb transcription at the posterior pole fail to drive their expression in more central portions of the body, apparently due to repression by other gap gene products. We conclude that tll and hkb transcription, as well as the terminal structures, are specified by two inputs: a gradient of Ras activity which emanates from the pole, and the opposing influence of more centrally deployed gap genes which repress the response to Ras.

Drosophila myoblast city encodes a conserved protein that is essential for myoblast fusion, dorsal closure, and cytoskeletal organization

The Drosophila myoblast city (mbc) locus was previously identified on the basis of a defect in myoblast fusion (Rushton et al., 1995. Development [Camb.]. 121:1979-1988). We describe herein the isolation and characterization of the mbc gene. The mbc transcript and its encoded protein are expressed in a broad range of tissues, including somatic myoblasts, cardial cells, and visceral mesoderm. It is also expressed in the pole cells and in ectodermally derived tissues, including the epidermis. Consistent with this latter expression, mbc mutant embryos exhibit defects in dorsal closure and cytoskeletal organization in the migrating epidermis. Both the mesodermal and ectodermal defects are reminiscent of those induced by altered forms of Drac1 and suggest that mbc may function in the same pathway. MBC bears striking homology to human DOCK180, which interacts with the SH2-SH3 adapter protein Crk and may play a role in signal transduction from focal adhesions. Taken together, these results suggest the possibility that MBC is an intermediate in a signal transduction pathway from the rho/rac family of GTPases to events in the cytoskeleton and that this pathway may be used during myoblast fusion and dorsal closure.

The mago nashi gene is required for the polarisation of the oocyte and the formation of perpendicular axes in Drosophila

BACKGROUND

Drosophila axis formation requires a series of inductive interactions between the oocyte and the somatic follicle cells. Early in oogenesis, Gurken protein, a member of the transforming growth factor alpha family, is produced by the oocyte to induce the adiacent follicle cells to adopt a posterior cell fate. These cells subsequently send an unidentified signal back to the oocyte to induce the formation of a polarised microtubule array that defines the anterior-posterior axis. The polarised microtubules also direct the movement of the nucleus and gurken mRNA from the posterior to the anterior of the oocyte, where Gurken signals a second time to induce the dorsal follicle cells, thereby polarising the dorsal-ventral axis.

RESULTS

In addition to its previously described role in the localisation of oskar mRNA, the mago nashi gene is required in the germ line for the transduction of the polarising signal from the posterior follicle cells. Using a new in vivo marker for microtubules, we show that mago nashi mutant oocytes develop a symmetric microtubule cytoskeleton that leads to the transient localisation of bicoid mRNA to both poles. Furthermore, the oocyte nucleus often fails to migrate to the anterior, causing the second Gurken signal to be sent in the same direction as the first. This results in a novel phenotype in which the anterior of the egg is ventralised and the posterior dorsalised, demonstrating that the migration of the oocyte nucleus determines the relative orientation of the two principal axes of Drosophila. The mago nashi gene is highly conserved from plants to animals, and encodes a protein that is predominantly localised to nuclei.

CONCLUSIONS

The mago nashi gene plays two essential roles in Drosophila axis formation: it is required downstream of the signal from the posterior follicle cells for the polarisation of the oocyte microtubule cytoskeleton, and has a second, independent role in the localisation of oskar mRNA to the posterior of the oocyte.