Organization and expression of alpha-tubulin genes in Drosophila melanogaster. One member of the alpha-tubulin multigene family is transcribed in both oogenesis and later embryonic development

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.

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.

Hyperactive spruce budworm antifreeze protein expression in transgenic Drosophila does not confer cold shock tolerance

Drosophila melanogaster, a freeze intolerant and cold shock sensitive insect, was transformed with the hyperactive insect antifreeze protein gene (AFP) from the spruce budworm, Choristoneura fumiferana. Transformation P-element constructs (pCasper) were made with CfAFP 337 isoform DNA using a strong constitutive promoter, Actin 5c. This is the first report of insect AFP used to transform another insect. Properly folded active insect AFP was only detected when signal sequences were used to target proteins to the endoplasmic reticulum for secretion into the hemolymph. The 18 residue Drosophila binding protein signal sequence (BiP) constructs resulted in transformed fly lines with significantly higher AFP expression in hemolymph than when the native C. fumiferana AFP signal sequence was used. The resultant transgene fly lines have the highest levels of thermal hysteresis, 0.8 degrees C, seen for any engineered Drosophila. Despite the high level of expression, even higher than some overwintering fish with natural levels of endogenous AFP, the transformants did not display any cold shock resistance compared to controls or low AFP expressing lines. These results indicate that insect AFP alone cannot protect Drosophila from cold shock and may not be useful for Drosophila cryopreservation.

Cloning and characterization of a member of the hsp70 gene family from Locusta migratoria, a highly thermotolerant insect

A complementary deoxyribonucleic acid (cDNA) and the corresponding gene segment encoding a member of the 70-kDa heat shock protein (Hsp70) family have been cloned and sequenced from Locusta migratoria, the African migratory locust. These animals are noted for their thermotolerance, which can exceed temperatures of 50 degrees C. Conceptually translated, the sequence shows a 654-residue protein with theoretical molecular weight of 71.4 kDa, which more closely resembles the mammalian Hsp70 (84-85% similarity) than Hsp70 from other insects, with approximately 75% similarity to the sequence from the fruit fly. Comparisons of cDNA and genomic sequences show that the gene contains 2 introns, a 245-bp intron located in the 5' untranslated region and a 91-bp intron in the coding region. Transcript abundance, as estimated by Northern blot analysis and reverse transcription-polymerase chain reaction, shows that heat shock treatment (45 degrees C for 3 hours) does not elevate hsp70 messenger ribonucleic acid levels in fat bodies or in neural tissues. Immunological assays of Hsp70 show that the protein is constitutively expressed, with a modest, approximately 2-fold induction after a 3-hour heat shock in fat body preparations. Although this sequence could be an hsc70 rather than an hsp70, it was the only cDNA isolated from heat-shocked tissue. Whatever the formal designation, such modest induction and constitutive expression may be ideally suited as an adaptation to the locust's chronic exposure to heat shock temperatures and the consequent demand for chaperone proteins.

Developmental and environmental regulation of antifreeze proteins in the mealworm beetle Tenebrio molitor

The yellow mealworm beetle, Tenebrio molitor, contains a family of small Cys-rich and Thr-rich thermal hysteresis proteins that depress the hemolymph freezing point below the melting point by as much as 5. 5 degrees C (DeltaT = thermal hysteresis). Thermal hysteresis protein expression was evaluated throughout development and after exposure to altered environmental conditions. Under favorable growth conditions, small larvae (11-13 mg) had only low levels of thermal hysteresis proteins or thermal hysteresis protein message, but these levels increased 10-fold and 18-fold, respectively, by the final larval instar (> 190 mg), resulting in thermal hysteresis > 3 degrees C. Exposure of small larvae (11-13 mg) to 4 weeks of cold (4 degrees C) caused an approximately 20-fold increase in thermal hysteresis protein concentration, well in excess of the less than threefold developmental increase seen after 4 weeks at 22 degrees C. Exposure of large larvae (100-120 mg) to cold caused 12-fold and sixfold increases in thermal hysteresis protein message and protein levels, respectively, approximately double the maximum levels they would have attained in the final larval instar at 22 degrees C. Thus, thermal hysteresis increased to similar levels (> 4 degrees C) in the cold, irrespective of the size of the larvae (the overwintering stage). At pupation, thermal hysteresis protein message levels decreased > 20-fold and remained low thereafter, but thermal hysteresis activity decreased much more slowly. Exposure to cold did not reverse this decline. Desiccation or starvation of larvae had comparable effects to cold exposure, but surprisingly, short daylength photoperiod or total darkness had no effect on either thermal hysteresis or message levels. As all environmental conditions that caused increased thermal hysteresis also inhibited growth, we postulate that developmental arrest is a primary factor in the regulation of T. molitor thermal hysteresis proteins.

Molecular characterization of a cDNA from the true armyworm Pseudaletia unipuncta encoding Manduca sexta allatotropin peptide(1)

Allatotropin (AT) is an insect neuropeptide isolated from the tobacco hornworm, Manduca sexta, stimulates juvenile hormone (JH) biosynthesis by the corpora allata. A cDNA isolated from the true armyworm, Pseudaletia unipuncta, encodes a 135 amino acid AT precursor peptide which contains the AT peptide, with processing sites necessary for its endoproteolytic cleavage and amidation, plus two additional peptides of unknown function. The encoded AT peptide is identical to that isolated from M. sexta and Agrius convolvuli. Southern blot analysis indicated that AT is a single copy gene per haploid genome and is present in two allelic forms. A single transcript of approximately 1.5 kilobases was detected by northern blot analysis. The expression of the AT gene was analyzed during development from sixth instar larvae to five day-old moths. Initial expression was observed in late pupae and this expression was maintained throughout the adult stages in both sexes. In one day-old moths, expression was at its lowest level of the stages that express AT mRNA but levels increased in day 3 and day 5 adults. This pattern of AT expression in adult P. unipuncta moths mirrors that of JH biosynthesis and supports the notion that AT may act in the adult stages. Immunohistochemistry and in situ hybridization revealed that AT expression was localized to numerous structures of the nervous system, suggesting that AT may have functions distinct from regulation of JH biosynthesis.

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.

Molecular characterization of a cDNA from Pseudaletia unipuncta encoding the Manduca sexta allatostatin peptide (Mas-AST)

A 15-residue neuropeptide, Manduca sexta allatostatin (Mas-AST), strongly inhibits juvenile hormone (JH) biosynthesis in vitro by corpora allata (CA) from Manduca fifth-stadium larvae and adult females as well as Helicoverpa zea adult females (Kramer et al., 1991 Proc. Natl. Acad. Sci (USA) 88, 9458-9462). In contrast, this study found that 1.0 microM Mas-AST has no JH biosynthesis inhibitory activity in Pseudaletia unipuncta sixth instar larvae or newly-emerged (day 0) adults but inhibited CA of 5-day-old adult females by 60%. From a P. unipuncta brain cDNA library, was isolated a cDNA that encodes a 125 amino acid polypeptide containing the Mas-AST sequence. Within the precursor, Mas-AST is situated at the carboxy terminus and is flanked by different dibasic proteolytic cleavage signals. The Pseudaletia gene specifying the Mas-AST peptide is present as a single copy per haploid genome. Expression of this gene was low in Pseudaletia sixth instar larvae, prepupae and early pupae but was relatively high in late pupae, and day 1 and 3 adults of both sexes. In day 5 adults, the relative transcript level appears to be maintained in females but declines in males. This pattern of Mas-AST expression does not correlate well with the profile of JH biosynthesis in Pseudaletia, which increases during the first 5 days of adult life, suggesting additional or alternative functions for this peptide.

Cloning and baculovirus expression of a desiccation stress gene from the beetle, Tenebrio molitor

The cDNA sequence encoding a novel desiccation stress protein (dsp28) found in the hemolymph of the common yellow mealworm beetle, Tenebrio molitor, has been determined. The sequence encodes a 225 amino acid protein containing a 20 amino acid signal peptide. Dsp28 shows no significant similarity to any known nucleic acid or protein sequence. Levels of dsp28 mRNA were found to increase approx 5-fold following desiccation. Dsp28 cDNA has been cloned into a baculovirus expression vector and the expressed protein was compared to native dsp28. Both dsp28 expressed by recombinant baculovirus and native dsp28 are glycosylated and N-terminally processed. Although dsp28 is induced by cold in addition to desiccation stress, it does not contribute to the freezing point depression (thermal hysteresis) observed in Tenebrio hemolymph.

Juvenile hormone regulation and developmental expression of a Tenebrio desiccation stress protein gene

Levels of a 28 kDa hemolymph protein, desiccation stress protein (dsp28), in the mealworm beetle, Tenebrio molitor, increase in response to desiccation and cold stress and are also developmentally regulated under nonstress conditions. Dsp28 mRNA is produced in the fat body, and its abundance changes dramatically throughout development. Transcript abundance increases throughout larval development, drops at pupation, and increases again in adults, the highest levels being found in females. The juvenile hormone (JH) analogue, methoprene, increased dsp28 transcript accumulation in pupae, suggesting that changing JH titres have a role in developmental expression and also perhaps contribute to dsp28 regulation during conditions of environmental stress. Genomic DNA, containing the entire dsp28 coding region plus 1.3 kb of upstream sequence, was isolated and potential regulatory sequences, including putative JH response elements, were identified.

Selective expression of the tba-1 alpha tubulin gene in a set of mechanosensory and motor neurons during the development of Caenorhabditis elegans

In the nematode Caenorhabditis elegans, a monoclonal antibody 3A5 raised against Drosophila alpha tubulins selectively stains the nervous system immuno-cytochemically. Direct screening of a C. elegans cDNA expression library with 3A5 has allowed cloning of the tba-1 (tubulin alpha-1) gene from C. elegans. The corresponding genomic DNA encodes a protein of 449 amino acid residues that has a high homology with the vertebrate alpha tubulins but a lower homology with yeast alpha tubulins. Interestingly, the carboxyl-terminus sequence EEEGEEY (Glu-Glu-Glu-Gly-Glu-Glu-Tyr) of the nematode tba-1 encoded isotype is identical to these residues in human, mouse, rat, pig and chicken alpha-1 tubulin isotypes that are expressed in the brain. Temporal and spatial expression studies of the tba-1 gene using Northern blot analysis and tba-1::lacZ fusion gene expression analysis during embryonic and the postembryonic development of C. elegans reveal that the tba-1 tubulin is preferentially expressed in the nematode nervous system, especially in a set of mechanosensory neurons and a set of ventral cord motor neurons (DA, DB, VA, and VB) during embryonic and postembryonic development. Our results indicate an inter-species conservation of the alpha tubulin carboxyl-terminal domain in functionally related brain specific isotypes from metazoans as divergent as mammals and nematodes. These results also suggest specificity of the individual alpha tubulin isotypes during neural development.

Flightin, a novel myofibrillar protein of Drosophila stretch-activated muscles

The indirect flight muscles of Drosophila are adapted for rapid oscillatory movements which depend on properties of the contractile apparatus itself. Flight muscles are stretch activated and the frequency of contraction in these muscles is independent of the rate of nerve impulses. Little is known about the molecular basis of these adaptations. We now report a novel protein that is found only in flight muscles and has, therefore, been named flightin. Although we detect only one gene (in polytene region 76D) for flightin, this protein has several isoforms (relative gel mobilities, 27-30 kD; pIs, 4.6-6.0). These isoforms appear to be created by posttranslational modifications. A subset of these isoforms is absent in newly emerged adults but appears when the adult develops the ability to fly. In intact muscles flightin is associated with the A band of the sarcomere, where evidence suggests it interacts with the myosin filaments. Computer database searches do not reveal extensive similarity to any known protein. However, the NH2-terminal 12 residues show similarity to the NH2-terminal sequence of actin, a region that interacts with myosin. These features suggest a role for flightin in the regulation of contraction, possibly by modulating actin-myosin interaction.

A functionally specialized alpha-tubulin is required for oocyte meiosis and cleavage mitoses in Drosophila

Three alpha-tubulin proteins contribute to microtubules during oogenesis and early embryogenesis in Drosophila melanogaster: alpha TUB84B, alpha TUB84D, and alpha TUB67C. alpha TUB67C is unique in two respects. It is a structurally divergent alpha-tubulin, sharing only 67% amino acid identity with the generic isotypes alpha TUB84B and alpha TUB84D, and its expression is exclusively maternal. The genetic analysis of the alpha Tub67C gene described here demonstrates that alpha TUB67C is required for nuclear division in the oocyte and early embryo. Both meiosis and cleavage-stage mitoses are severely affected by mutations that result in a substantial decrease in the ratio of alpha TUB67C/alpha TUB84B+alpha TUB84D. A large increase in this ratio, achieved by increasing the gene dosage of alpha Tub67C, has little or no effect on meiosis, but severely disrupts mitotic spindle function. Thus, both classes of alpha-tubulin isotype present in the mature oocyte, alpha TUB67C and alpha TUB84B/84D, are essential for normal spindle function in early Drosophila development. These alpha-tubulins provide the first example of tubulin isotypes known to be coexpressed in wild-type animals whose encoded variation is required for the normal function of a microtubule array.

Behavior of structurally divergent alpha-tubulin isotypes during Drosophila embryogenesis: evidence for post-translational regulation of isotype abundance

Two major alpha-tubulin isotypes are present during Drosophila embryogenesis: an evolutionarily divergent maternal isotype that is synthesized only in the ovary and deposited in the oocyte and a highly conserved constitutive isotype that is both maternally supplied and zygotically synthesized. A maternal isotype-specific antibody and a monoclonal antibody that recognizes both the maternal and constitutive isotypes were characterized and used to determine the distribution and abundance of alpha-tubulins during embryogenesis. Both isotypes are abundant and assemble into all classes of microtubules from the syncytial blastoderm stage until completion of germ band retraction. During subsequent development, however, the maternal isotype is retained only in the developing CNS, and later in a subset of connective fibers within the CNS. In contrast, total alpha-tubulin levels remain high in essentially all tissues throughout embryogenesis, indicating that most tissues selectively accumulate the constitutive isotype. To determine if selective accumulation of the constitutive isotype requires zygotic synthesis of this protein, mutant embryos that do not contain functional constitutive alpha-tubulin genes were examined. In these embryos, as in wild type, the maternal isotype decreases to background levels in tissues that retain high levels of the constitutive isotype. The constitutive isotype therefore appears to be more stable than the maternal isotype in most tissues. Differences in isotype stability may play an important role in determining the developmental pattern of isotype accumulation in Drosophila embryos.

Expression of heat-shock locus hsr-omega in nonstressed cells during development in Drosophila melanogaster

The hsr-omega locus forms one of the largest Drosophila heat-shock puffs and produces three major transcripts. These three transcripts are also produced constitutively, at lower levels, in almost all tissues and developmental stages. The amounts of the transcripts in nonstressed cells are modulated during development. The hormone ecdysone leads to increased levels of hsr-omega transcripts in cultured cells, suggesting that changing ecdysone titers may play a role in the developmental changes of hsr-omega transcript levels. By in situ hybridization to RNA in tissue sections, we detect only two cell types that lack hsr-omega transcripts--the preblastoderm embryo and the primary spermatocyte. There are no maternal transcripts of hsr-omega in the embryo. Transcripts appear abruptly at the time that the zygotic genome becomes transcriptionally active, shortly before the formation of the cellular blastoderm. No constitutive hsr-omega transcripts are found in primary spermatocytes. The spermatocytes cannot respond to heat shock by transcribing either hsr-omega or hsp70 RNA. Constitutive hsr-omega transcription is resumed later in spermatogenesis and hsr-omega RNA is detected in differentiating spermatids. These spermatids are also capable of mounting a heat-shock response, as measured by increases in hsr-omega and hsp70 RNA.

Functional implications of the unusual spatial distribution of a minor alpha-tubulin isotype in Drosophila: a common thread among chordotonal ligaments, developing muscle, and testis cyst cells

Three of the four alpha-tubulin genes in Drosophila melanogaster are temporally regulated. mRNA from one of these genes, alpha 85E-tubulin, first appears in 6- to 8-hr embryos and persists, with marked fluctuations, through the end of pupal development. In adults, alpha 85E mRNA has been unequivocally identified only in testes. In the present study, isotype-specific antibodies have been used to localize alpha 85E tubulin protein in whole tissues. The results demonstrate a spatially restricted expression pattern of the alpha 85E gene that includes tissues of both ectodermal and mesodermal origins. Specifically, embryonic accumulation of alpha 85E tubulin is limited to support cells of chordotonal organs and the developing musculature of the viscera and body wall. In late third instar larvae, chordotonal organs and a subset of larval nerves, but not muscle, stain with anti-alpha 85E. The timing of protein accumulation during pupal development suggests that alpha 85E tubulin is involved in the construction of the adult as well as the larval musculature. In testis, only the somatically derived cyst cells that surround developing spermatid bundles accumulate alpha 85E-tubulin. The cell types that express alpha 85E share a requirement for extensive cell shape changes during development, suggesting that this minor alpha-tubulin may have distinct functional properties.

Interacting proteins identified by genetic interactions: a missense mutation in alpha-tubulin fails to complement alleles of the testis-specific beta-tubulin gene of Drosophila melanogaster

In this paper we demonstrate that failure to complement between mutations at separate loci can be used to identify genes that encode interacting structural proteins. A mutation (nc33) identified because it failed to complement mutant alleles of the gene encoding the testis-specific beta 2-tubulin of Drosophila melanogaster (B2t) did not map to the B2t locus. We show that this second-site noncomplementing mutation is a missense mutation in alpha-tubulin that results in substitution of methionine in place of valine at amino acid 177. Because alpha- and beta-tubulin form a heterodimer, our results suggest that the genetic interaction, failure to complement, is based on the structural interaction between the protein products of the two genes. Although the nc33 mutation failed to complement a null allele of B2t (B2tn), a deletion of the alpha-tubulin gene to which nc33 mapped complemented B2tn. Thus, the failure to complement appears to require the presence of the altered alpha-tubulin encoded by the nc33 allele, which may act as a structural poison when incorporated into either the tubulin heterodimer or microtubules.

Interacting proteins identified by genetic interactions: a missense mutation in alpha-tubulin fails to complement alleles of the testis-specific beta-tubulin gene of Drosophila melanogaster

In this paper we demonstrate that failure to complement between mutations at separate loci can be used to identify genes that encode interacting structural proteins. A mutation (nc33) identified because it failed to complement mutant alleles of the gene encoding the testis-specific beta 2-tubulin of Drosophila melanogaster (B2t) did not map to the B2t locus. We show that this second-site noncomplementing mutation is a missense mutation in alpha-tubulin that results in substitution of methionine in place of valine at amino acid 177. Because alpha- and beta-tubulin form a heterodimer, our results suggest that the genetic interaction, failure to complement, is based on the structural interaction between the protein products of the two genes. Although the nc33 mutation failed to complement a null allele of B2t (B2tn), a deletion of the alpha-tubulin gene to which nc33 mapped complemented B2tn. Thus, the failure to complement appears to require the presence of the altered alpha-tubulin encoded by the nc33 allele, which may act as a structural poison when incorporated into either the tubulin heterodimer or microtubules.

Developmental distribution of RNA and protein products of the Drosophila alpha-tubulin gene family

The developmental pattern of gene expression of the Drosophila melanogaster alpha-tubulin family has been examined in detail at both the mRNA and protein levels. Northern data from 16 stages of development have been quantified to produce estimates of pool sizes of each of the alpha-tubulin transcripts through development. The in situ pattern of alpha 67C RNA localization in developing oocytes and early embryos has also been determined. At the protein level, two of the three previously unidentified products of alpha-tubulin genes (alpha 67C and alpha 85E) have been identified. Evidence that protein from the fourth gene comigrates with the ubiquitously expressed alpha 84B is presented. In addition to the primary translational products of the alpha-tubulin genes, an elaborate series of post-translationally modified alpha-tubulins has been resolved. The developmental profiles of both synthesis and accumulation of these alpha-tubulin proteins are described.

Characterization of the gene for mp20: a Drosophila muscle protein that is not found in asynchronous oscillatory flight muscle

A Drosophila melanogaster gene encoding a muscle specific protein was isolated by differential screening with RNA from primary cultures of myotubes. The gene encodes a 20-kD protein, muscle protein 20 (mp20), that is not detected in the asynchronous oscillatory flight muscles, but is found in most, if not all, other muscles (the synchronous muscles). The sequence of the protein, deduced from the DNA, contains two regions of 12 amino acids with significant similarity to high-affinity calcium-binding sites of other proteins. This protein is easily extracted from the contractile apparatus and thus does not seem to be a tightly bound structural component. The gene (located in polytene region 49F 9-13) is unique in the D. melanogaster genome and yields two transcripts, 1.0 and 0.9 kb long. The levels of the two transcripts are regulated differently during development, yet the coding regions of the two transcripts are identical.

Studies of He-T DNA sequences in the pericentric regions of Drosophila chromosomes

He-T DNA is a complex set of repeated DNA sequences with sharply defined locations in the polytene chromosomes of Drosophila melanogaster. He-T sequences are found only in the chromocenter and in the terminal (telomere) band on each chromosome arm. Both of these regions appear to be heterochromatic and He-T sequences are never detected in the euchromatic arms of the chromosomes (Young et al. 1983). In the study reported here, in situ hybridization to metaphase chromosomes was used to study the association of He-T DNA with heterochromatic regions that are under-replicated in polytene chromosomes. Although the metaphase Y chromosome appears to be uniformly heterochromatic, He-T DNA hybridization is concentrated in the pericentric region of both normal and deleted Y chromosomes. He-T DNA hybridization is also concentrated in the pericentric regions of the autosomes. Much lower levels of He-T sequences were found in pericentric regions of normal X chromosomes; however compound X chromosomes, constructed by exchanges involving Y chromosomes, had large amounts of He-T DNA, presumably residual Y sequences. The apparent co-localization of He-T sequences with satellite DNAs in pericentric heterochromatin of metaphase chromosomes contrasts with the segregation of satellite DNA to alpha heterochromatin while He-T sequences hybridize to beta heterochromatin in polytene nuclei. This comparison suggests that satellite sequences do not exist as a single block within each chromosome but have interspersed regions of other sequences, including He-T DNA. If this is so, we assume that the satellite DNA blocks must associate during polytenization, leaving the interspersed sequences looped out to form beta heterochromatin. DNA from D. melanogaster has many restriction fragments with homology to He-T sequences. Some of these fragments are found only on the Y. Two of the repeated He-T family restriction fragments are found entirely on the short arm of the Y, predominantly in the pericentric region. Under conditions of moderate stringency, a subset of He-T DNA sequences cross-hybridizes with DNA from D. simulans and D. miranda. In each species, a large fraction of the cross-hybridizing sequences is on the Y chromosome.

Mapping protein-DNA interactions in vivo with formaldehyde: evidence that histone H4 is retained on a highly transcribed gene

We have used formaldehyde-mediated protein-DNA crosslinking within intact cells to examine the in vivo chromatin structure of the D. melanogaster heat shock protein 70 (hsp70) genes. In agreement with previous in vitro studies, we find that the heat shock-mediated transcriptional induction of the hsp70 genes perturbs their chromatin structure, resulting in fewer protein-DNA contacts crosslinkable in vivo by formaldehyde. However, contrary to earlier in vitro evidence that histones may be absent from actively transcribed genes, we show directly, by immunoprecipitation of in vivo-crosslinked chromatin fragments, that at least histone H4 remains bound to hsp70 DNA in vivo, irrespective of its rate of transcription. The formaldehyde-based in vivo mapping techniques described in this work are generally applicable, and can be used both to probe protein-DNA interactions within specific genes and to determine the genomic location of specific chromosomal proteins.

An investigation of microtubule organization and functions in living Drosophila embryos by injection of a fluorescently labeled antibody against tyrosinated alpha-tubulin

Rhodamine-labeled monoclonal antibodies, which react with tyrosinated alpha-tubulin (clone YL 1/2; Kilmartin, J. V., B. Wright, and C. Milstein, 1982, J. Cell Biol., 93:576-582) and label microtubules in vivo (Wehland, J., M. C. Willingham, and I. Sandoval, 1983, J. Cell Biol., 97:1467-1475) were microinjected into syncytial stage Drosophila embryos. At 1 mg/ml antibody concentration, the microtubule arrays of the surface caps became labeled by YL 1/2 but normal development was found to continue. The results are compared with the data from fixed material particularly with regard to interphase microtubules, centrosome separation, and spindle and midbody formation. At 5 mg/ml antibody concentration the microtubules took up larger quantities of antibodies and clumped around the nuclei. Nuclei with clumped microtubules lost their position in the surface layer and moved into the interior. As a result, the F-actin cap meshwork associated with such nuclei either failed to form or subsided. It is concluded that microtubule activity is required to maintain the nuclei in the surface layer and organize the F-actin meshwork of the caps.

Isolation and characterization of the gene for myosin light chain two of Drosophila melanogaster

A recombinant lambda-phage DNA clone containing Drosophila melanogaster sequences encoding the gene for myosin light chain (MLC) two has been isolated from a library of randomly sheared DNA. The Drosophila MLC2 gene is located in region 99E1-3 on the right arm of chromosome 3, several bands removed from the site reported for the other myosin light chain gene at 98B. The MLC2 sequence at 99E1-3 appears to encode all of the isoforms of Drosophila MLC2. The polypeptide encoded at 99E was identified as MLC2 by the following criteria: the in vitro translation product is identical in size to MLC2 isolated from Drosophila muscle, and on two-dimensional gels the in vitro translation product can be separated into two or more peptides that co-migrate with isoforms of larval and thoracic MLC2. RNA encoding the polypeptide was detected in embryos only after the onset of muscle differentiation and was also abundant in adult thoracic muscle. The nucleotide sequence of cDNA generated from late embryonic RNA would be translated to yield a protein sequence with multiple regions of homology to vertebrate MLC2. (There are shorter regions of homology to vertebrate MLC1). Like a number of vertebrate muscle proteins, Drosophila MLC2 has an acetylated amino-terminus.

Developmental consequences of mutations in the 84B alpha-tubulin gene of Drosophila melanogaster

Developmental effects of six mutations in the gene encoding the majority of alpha-tubulin in all tissues at all stages of Drosophila melanogaster development have been examined. All six alleles produce at least partially stable alpha 84B protein. In genetic assays, two of these alleles approximate the null condition. The other four alleles appear to form a graded series of hypomorphs. The two most severe alleles produce a semidominant maternal-effect polyphasic lethality, plus a predominantly larval recessive zygotic lethality. Clonal analysis of one of these alleles suggests it is a cell lethal. Worsening of the lethal phenotype (negative complementation) occurs in most interallelic heterozygotes involving these two mutations. As hemizygotes, the other four alleles are predominantly larval/pupal lethals. Partial complementation is achieved by most interallelic heterozygotes involving these four alleles. Phenotypic defects associated with the six tubulin mutation include disrupted embryos, pseudopupae, pharate adults with defects in various cuticular pattern elements, pharate adults with retarded head development, adults with leg tremors and extremely short life spans, and viable but sterile adults with bristle defects.

Differential expressions of essential and nonessential alpha-tubulin genes in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has two alpha-tubulin genes and one beta-tubulin gene. Gene disruption experiments showed that the alpha 1-tubulin gene (NDA2) is essential whereas the alpha 2 gene is dispensable. The alpha 2-disrupted cells missing alpha 2 transcript and alpha 2 polypeptide could grow and sporulate normally. The alpha 2 gene, however, was expressed in the wild type and the alpha 1 mutant. Alpha 2-Tubulin was distinguished as an electrophoretic band and was assembled into microtubules. The alpha 2-disrupted cells had an increased sensitivity to an antimicrotubule drug thiabendazole, and the alpha 1(cold-sensitive [cs]) alpha 2 (disrupted) cells became not only cs but also temperature sensitive. Northern blot experiments indicated that alpha 2 transcription was minor and constitutive whereas alpha 1 transcription was major and modulated, depending on the gene copy number of the alpha 2 gene. The amounts of alpha 1 and alpha 2 polypeptides estimated by beta-galactosidase activities of the lacZ-fused genes integrated on the chromosome and by intensities of the electrophoretic bands in crude tubulin fractions, however, were comparable, indicating that alpha 2 tubulin is not a minor subtype. We assume that the cells of Schizosaccharomyces pombe have no excess tubulin pool. alpha 1 mutants would then be blocked in the cell cycle because only half the amount of functional alpha-tubulin required for growth can be produced by the alpha 2 gene. On the other hand, the alpha 2-disrupted cells became viable because the synthesis of alpha 1 tubulin was increased by transcriptional or translational modulation or both. The real cause for essential alpha 1 and dispensable alpha 2 genes seems to be in their regulatory sequences instead of the coding sequences.

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

We have determined the nucleotide sequences of all four Drosophila alpha-tubulin genes (alpha 1, alpha 2, alpha 3, and alpha 4). Two of the genes, alpha 1 and alpha 3, are constitutively expressed and code for proteins that are very similar to previously sequenced alpha-tubulins. They differ from each other by only two amino acid substitutions. These two genes also have blocks of homology between the noncoding leader regions of their transcription units. In contrast to these constitutive genes, the tissue-specific alpha 2 and alpha 4 genes code for tubulins with different structures. The alpha 2 mRNA is male-specific in adults and codes for a tubulin that differs from alpha 1 at 21 of the 450 residues. Six nonconservative substitutions are clustered within the 14 carboxyl-terminal amino acids, a region implicated in the regulation of microtubule assembly. The alpha 4 mRNA is maternal and is found only in ovarian nurse cells, eggs, and early embryos. It codes for the most highly divergent alpha-tubulin yet reported and differs from alpha 1 at 149 positions.

The expression and genomic organization of randomly selected cloned Drosophila melanogaster genes

A lambda recombinant DNA library containing Drosophila melanogaster nuclear DNA inserts was screened with cDNA made from oocyte and gastrula poly(A)+ RNA. 124 clones were isolated which represented sequences complementary to a distribution of abundancies of their RNAs. The clone set was then used as probes to identify those whose RNA abundancies changed during embryonic development. The vast majority of clones showed little difference during development. Four different clones were identified whose poly(A)+ RNAs were quantitatively regulated; two were oocyte-specific, and two were embryonic-specific. 44 clones were chosen for in situ hybridization to salivary gland polytene chromosomes. The location and distribution of their sites are described. A class of clones, identified by in situ hybridization to the nucleolus, is further described. These clones contain a scrambled array of ribosomal intervening sequences.

Differential expressions of essential and nonessential alpha-tubulin genes in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has two alpha-tubulin genes and one beta-tubulin gene. Gene disruption experiments showed that the alpha 1-tubulin gene (NDA2) is essential whereas the alpha 2 gene is dispensable. The alpha 2-disrupted cells missing alpha 2 transcript and alpha 2 polypeptide could grow and sporulate normally. The alpha 2 gene, however, was expressed in the wild type and the alpha 1 mutant. Alpha 2-Tubulin was distinguished as an electrophoretic band and was assembled into microtubules. The alpha 2-disrupted cells had an increased sensitivity to an antimicrotubule drug thiabendazole, and the alpha 1(cold-sensitive [cs]) alpha 2 (disrupted) cells became not only cs but also temperature sensitive. Northern blot experiments indicated that alpha 2 transcription was minor and constitutive whereas alpha 1 transcription was major and modulated, depending on the gene copy number of the alpha 2 gene. The amounts of alpha 1 and alpha 2 polypeptides estimated by beta-galactosidase activities of the lacZ-fused genes integrated on the chromosome and by intensities of the electrophoretic bands in crude tubulin fractions, however, were comparable, indicating that alpha 2 tubulin is not a minor subtype. We assume that the cells of Schizosaccharomyces pombe have no excess tubulin pool. alpha 1 mutants would then be blocked in the cell cycle because only half the amount of functional alpha-tubulin required for growth can be produced by the alpha 2 gene. On the other hand, the alpha 2-disrupted cells became viable because the synthesis of alpha 1 tubulin was increased by transcriptional or translational modulation or both. The real cause for essential alpha 1 and dispensable alpha 2 genes seems to be in their regulatory sequences instead of the coding sequences.

Expression of ras cellular oncogenes during development of Drosophila melanogaster

The transcription patterns of three v-Ha-ras-related cellular oncogenes in Drosophila melanogaster were studied. Each gene coded for at least two distinct transcripts. The larger transcript of each gene was expressed at a similar abundance during the entire life cycle of fruit flies, whereas the shorter transcripts were much more abundant in embryonic stages than at later stages.

Developmental changes in expression of the Drosophila melanogaster epidermal growth factor receptor gene

A Drosophila gene homologous to the human EGF receptor gene has recently been isolated and sequenced. Two transcripts, 7.6- and 7.1-kb long, encoded by this gene were identified in Drosophila melanogaster. The transcripts are present at low abundance in the maternal RNA stored in unfertilized eggs and in 2-hr-old embryos. The abundance of both transcripts increases sharply between 2 and 5 hr after egg deposition, it remains high throughout embryogenesis, and decreases again in the larval and pupal stages. In adult flies the two transcripts are expressed differentially. The 7.1-kb transcript is present during adulthood at the same level detected previously in pupal stage, but the abundance of the 7.6-kb transcript decreases substantially and it remains low during adulthood.

Expression of ras cellular oncogenes during development of Drosophila melanogaster

The transcription patterns of three v-Ha-ras-related cellular oncogenes in Drosophila melanogaster were studied. Each gene coded for at least two distinct transcripts. The larger transcript of each gene was expressed at a similar abundance during the entire life cycle of fruit flies, whereas the shorter transcripts were much more abundant in embryonic stages than at later stages.

Quantitative determination of tubulin and characterization of tubulin forms during development in Drosophila melanogaster

The proportion of tubulin and its isoform pattern have been analyzed at different stages in the development of Drosophila melanogaster. Tubulin proportion varied during development, the highest proportion being found at embryogenesis where two alpha- and beta- (one of them transitory) tubulin subunits were found. During the larval stage, the proportion of total tubulin decreased but new alpha-isotubulins were identified. These alpha-isotubulins were also present at the adult stage and all of them could be incorporated into microtubules assembled in vitro.

Post-transcriptional regulation of the abundance of mRNAs encoding alpha-tubulin and a 94,000-dalton protein in teratocarcinoma-derived stem cells versus differentiated cells

Changes in the expression of the genes encoding alpha-tubulin and a 94,000-dalton protein (p94) specified by a cDNA clone, p4-30, were examined in a differentiated teratocarcinoma-derived parietal endoderm cell line, PYS-2, and an undifferentiated teratocarcinoma stem cell line, F9. Relative to other proteins or mRNA species, the synthesis rate of the alpha-tubulins and of p94, as well as the levels of their corresponding cytoplasmic mRNAs, were lower in PYS-2 than in F9 cells. The decrease was greater for the relative abundance of cytoplasmic alpha-tubulin mRNA than for p94 mRNA. Similarly, induction of differentiation of F9 cells by simultaneous exposure to retinoic acid (RA) and dibutyryl cyclic AMP resulted in reduced relative levels of the cytoplasmic mRNAs for these proteins. The reduction in abundance of the two RNA species was not due to a decrease in growth rate since the differentiated cells, PYS-2, RA-treated F9, and RA plus dibutyryl cyclic AMP-treated F9 cells, grew at a rate similar to that of undifferentiated F9 cells. However, induction of differentiation of F9 cells by treatment with RA alone did not cause down-regulation of the two RNA species. The relative levels of total cellular RNA encoding alpha-tubulin and p94 in PYS-2 cells were also lower than those in F9 cells to an extent comparable to the decrease in the cytoplasmic RNAs. Since the apparent relative rates of RNA transcription were similar in both cell types, we conclude that the reduction in relative levels of the alpha-tubulin and p94 RNAs in the cell depends largely on the relative stability of the two RNAs and not on the relative rates of transcription. The faster disappearance of the two RNA species relative to other cellular RNAs from actinomycin D-treated PYS-2 compared with F9 cells is consistent with this interpretation.

Structure and arrangement of the beta-tubulin genes of Leishmania tropica

We studied the organization and arrangement of the genes encoding beta-tubulin in the protozoan parasite Leishmania tropica and examined the structure and orientation of the beta-tubulin mRNA relative to the gene. There were found to be eight to nine beta-tubulin genes arranged in an array of direct tandem repeat units with a length of 3.8 kilobase pairs, and they were extremely homologous, if not identical, in sequence. These repeat units did not contain the alpha-tubulin genes. The transcribed sequences within the beta-tubulin genes were localized, and the orientation of the major alpha-tubulin mRNA was mapped on the gene by S1 nuclease analysis.

Differential expression of two neural cell-specific beta-tubulin mRNAs during rat brain development

We recently demonstrated that beta-tubulin mRNA expression is regulated during rat brain development. This is manifested by a dramatic decrease in both 1.8- and 2.9-kilobase (kb) mRNAs when extensive neurite elongation is occurring. Coincident with these decreases is the increased production of a 2.5-kb mRNA. (J.F. Bond and S.R. Farmer, Mol. Cell. Biol. 3:1333-1342, 1983). In the present study, we have isolated and characterized three different cDNAs corresponding to beta-tubulin mRNAs (R beta T.1, R beta T.2, and R beta T.3). Hybridization of 3' untranslated region subclones of R beta T.1 and R beta T.2 cDNAs to RNA from a variety of rat tissues and cells revealed that these two cDNAs are neural cell specific. R beta T.1 corresponds to an abundant 1.8-kb mRNA expressed only at early stages of rat brain development. R beta T.2 corresponds to the 2.5-kb mRNA expressed at later stages. These data strongly suggest that there is differential expression of the beta-tubulin multigene family during rat brain development.

Tubulin genes of the African trypanosome Trypanosoma brucei rhodesiense:nucleotide sequence of a 3.7-kb fragment containing genes for alpha and beta tubulins

Most tubulin genes of the African trypanosome Trypanosoma rhodesiense are contained in 3.7-kb tandemly repeating units. One member of the 3.7-kb repeat family has been isolated from a T. rhodesiense genomic library, cloned, and sequenced. The 3646-bp fragment contains a complete alpha-tubulin gene and portions of two beta-tubulin genes. No introns are present. The genes are separated by 634- and 333-bp intergenic regions, which lack typical eukaryotic promoter and poly(A) signal sequences. However, both intergenic regions exhibit some structural similarity with sequences proposed to be involved in transcription termination and poly(A) addition in yeast. The 634-bp intergenic region shows homology to the "mini-exon" sequence associated with variable surface glycoprotein (VSG) and other trypanosome mRNAs. A comparable sequence is not found in the 333-bp intergenic region. T. rhodesiense alpha and beta-tubulins exhibit about 84-85% amino acid (aa) sequence homology with tubulins of mammals; the genes show about 74-75% nucleotide sequence homology. The alpha-tubulin contains 451 aa and the beta tubulin 442 aa; both have tyrosine as the C-terminal aa.

Post-transcriptional regulation of the abundance of mRNAs encoding alpha-tubulin and a 94,000-dalton protein in teratocarcinoma-derived stem cells versus differentiated cells

Changes in the expression of the genes encoding alpha-tubulin and a 94,000-dalton protein (p94) specified by a cDNA clone, p4-30, were examined in a differentiated teratocarcinoma-derived parietal endoderm cell line, PYS-2, and an undifferentiated teratocarcinoma stem cell line, F9. Relative to other proteins or mRNA species, the synthesis rate of the alpha-tubulins and of p94, as well as the levels of their corresponding cytoplasmic mRNAs, were lower in PYS-2 than in F9 cells. The decrease was greater for the relative abundance of cytoplasmic alpha-tubulin mRNA than for p94 mRNA. Similarly, induction of differentiation of F9 cells by simultaneous exposure to retinoic acid (RA) and dibutyryl cyclic AMP resulted in reduced relative levels of the cytoplasmic mRNAs for these proteins. The reduction in abundance of the two RNA species was not due to a decrease in growth rate since the differentiated cells, PYS-2, RA-treated F9, and RA plus dibutyryl cyclic AMP-treated F9 cells, grew at a rate similar to that of undifferentiated F9 cells. However, induction of differentiation of F9 cells by treatment with RA alone did not cause down-regulation of the two RNA species. The relative levels of total cellular RNA encoding alpha-tubulin and p94 in PYS-2 cells were also lower than those in F9 cells to an extent comparable to the decrease in the cytoplasmic RNAs. Since the apparent relative rates of RNA transcription were similar in both cell types, we conclude that the reduction in relative levels of the alpha-tubulin and p94 RNAs in the cell depends largely on the relative stability of the two RNAs and not on the relative rates of transcription. The faster disappearance of the two RNA species relative to other cellular RNAs from actinomycin D-treated PYS-2 compared with F9 cells is consistent with this interpretation.

Structure and arrangement of the beta-tubulin genes of Leishmania tropica

We studied the organization and arrangement of the genes encoding beta-tubulin in the protozoan parasite Leishmania tropica and examined the structure and orientation of the beta-tubulin mRNA relative to the gene. There were found to be eight to nine beta-tubulin genes arranged in an array of direct tandem repeat units with a length of 3.8 kilobase pairs, and they were extremely homologous, if not identical, in sequence. These repeat units did not contain the alpha-tubulin genes. The transcribed sequences within the beta-tubulin genes were localized, and the orientation of the major alpha-tubulin mRNA was mapped on the gene by S1 nuclease analysis.

Genetics of microtubule systems

In most eucaryotes the tubulin genes comprise small multigene families with approximately equal numbers of genes for alpha- and beta-tubulin, the structural proteins of microtubules. The recent isolation of tubulin mutations in several species is proving to be a powerful tool for examining the structure and function of specific sets of microtubules. In Drosophila melanogaster, genetic analysis of a testis-specific beta-tubulin gene has shown that a single tubulin gene product may fulfill a number of different microtubule functions. In addition to tubulin mutations, mutations in other genes whose products are involved in the regulation or structure of specific microtubule arrays have also been isolated. The combination of analysis of both classes of mutations is beginning to allow a molecular description of the construction and function of three-dimensional cellular structures. In addition, such studies may also shed light on the evolutionary pressures that gave rise to and serve to maintain small families of genes encoding very similar proteins.

Differential expression of two neural cell-specific beta-tubulin mRNAs during rat brain development

We recently demonstrated that beta-tubulin mRNA expression is regulated during rat brain development. This is manifested by a dramatic decrease in both 1.8- and 2.9-kilobase (kb) mRNAs when extensive neurite elongation is occurring. Coincident with these decreases is the increased production of a 2.5-kb mRNA. (J.F. Bond and S.R. Farmer, Mol. Cell. Biol. 3:1333-1342, 1983). In the present study, we have isolated and characterized three different cDNAs corresponding to beta-tubulin mRNAs (R beta T.1, R beta T.2, and R beta T.3). Hybridization of 3' untranslated region subclones of R beta T.1 and R beta T.2 cDNAs to RNA from a variety of rat tissues and cells revealed that these two cDNAs are neural cell specific. R beta T.1 corresponds to an abundant 1.8-kb mRNA expressed only at early stages of rat brain development. R beta T.2 corresponds to the 2.5-kb mRNA expressed at later stages. These data strongly suggest that there is differential expression of the beta-tubulin multigene family during rat brain development.

Heterogeneity of vertebrate brain tubulins

We have examined the extent of brain tubulin heterogeneity in six vertebrate species commonly used in tubulin research (rat, calf, pig, chicken, human, and lamb) using isoelectric focusing, two-dimensional electrophoresis, and peptide mapping procedures that provide higher resolution than previously available. The extent of heterogeneity is extremely similar in all of these organisms, as judged by number, range of isoelectric points, and distribution of the isotubulins. A minimum of 6 alpha and 12 beta tubulins was resolved from all sources. Even the pattern of spots on two-dimensional peptide maps is remarkably similar. These similarities suggest that the populations of tubulin in all of these brains should have similar overall physical properties. It is particularly interesting that chicken, which has only four or five beta-tubulin genes, contains approximately 12 beta tubulins. Thus, post-translational modification must generate at least some of the tubulin heterogeneity. Mammalian species, which contain 15-20 tubulin DNA sequences, do not show any more tubulin protein heterogeneity than does chicken. This suggests that expression of only a small number of the mammalian genes may be required to generate the observed tubulin heterogeneity.

Regulation of Drosophila alpha- and beta-tubulin genes during development

Both the alpha and the beta subunit of tubulin in Drosophila melanogaster are encoded by small multigene families. Using heterologous hybridization probes representing chicken alpha- and beta-tubulin genes, four complementary alpha- and four beta-tubulin gene sequences from Drosophila have been isolated. Each gene has been individually mapped cytogenically by in situ hybridization of nucleic acid probes to polytene chromosomes. It is clear that the genes in each family are dispersed rather than arranged in tandem arrays or clusters. Furthermore, alpha- and beta-tubulin genes are not physically linked as alpha-beta pairs. Transcripts from individual genes are differentially accumulated at particular stages of Drosophila development. This differential gene activity may provide expression of functionally specialized tubulin subunits. Alternatively, differences in expression may reflect tissue specific patterns of gene utilization.

Developmental and comparative aspects of brine shrimp tubulin

Tubulin from embryos of the brine shrimp Artemia has been purified to apparent homogeneity by chromatography on phosphocellulose P11 and DEAE-cellulose, (NH4)2SO4 fractionation and assembly-disassembly of microtubules. Peptide mapping indicated that Artemia and bovine brain tubulin were very similar in spite of differences in the electrophoretic behaviour of tubulin from these two organisms. Isoelectric focusing and two-dimensional gel electrophoresis were used to resolve and identify several Artemia isotubulins . The isotubulin composition and the quantity of tubulin did not change during pre-emergence development of Artemia embryos. Formation of microtubules with tubulin purified from embryos at different stages of development did not require glycerol or microtubule-associated proteins and formation of structurally normal microtubules was actually hindered by glycerol and Mg2+. The characteristics of Artemia tubulin, in concert with the unusual life history of Artemia, suggest that this organism will be very useful for the study of tubulin gene expression and tubulin utilization during embryo development.