Blastoderm-differential and blastoderm-specific genes of Drosophila melanogaster

Loss of function of the Drosophila Ninein-related centrosomal protein Bsg25D causes mitotic defects and impairs embryonic development

The centrosome-associated proteins Ninein (Nin) and Ninein-like protein (Nlp) play significant roles in microtubule stability, nucleation and anchoring at the centrosome in mammalian cells. Here, we investigate Blastoderm specific gene 25D (Bsg25D), which encodes the only Drosophila protein that is closely related to Nin and Nlp. In early embryos, we find that Bsg25D mRNA and Bsg25D protein are closely associated with centrosomes and astral microtubules. We show that sequences within the coding region and 3′UTR of Bsg25D mRNAs are important for proper localization of this transcript in oogenesis and embryogenesis. Ectopic expression of eGFP-Bsg25D from an unlocalized mRNA disrupts microtubule polarity in mid-oogenesis and compromises the distribution of the axis polarity determinant Gurken. Using total internal reflection fluorescence microscopy, we show that an N-terminal fragment of Bsg25D can bind microtubules in vitro and can move along them, predominantly toward minus-ends. While flies homozygous for a Bsg25D null mutation are viable and fertile, 70% of embryos lacking maternal and zygotic Bsg25D do not hatch and exhibit chromosome segregation defects, as well as detachment of centrosomes from mitotic spindles. We conclude that Bsg25D is a centrosomal protein that, while dispensable for viability, nevertheless helps ensure the integrity of mitotic divisions in Drosophila.

Summary: In humans, mutations in Ninein or Ninein-like protein result in microcephaly and other severe diseases. We show that while flies lacking the Ninein orthologue can survive, many die as embryos with defects in mitosis.

Stochastic model for gene transcription on Drosophila melanogaster embryos

We examine immunostaining experimental data for the formation of stripe 2 of even-skipped (eve) transcripts on D. melanogaster embryos. An estimate of the factor converting immunofluorescence intensity units into molecular numbers is given. The analysis of the eve dynamics at the region of stripe 2 suggests that the promoter site of the gene has two distinct regimes: an earlier phase when it is predominantly activated until a critical time when it becomes mainly repressed. That suggests proposing a stochastic binary model for gene transcription on D. melanogaster embryos. Our model has two random variables: the transcripts number and the state of the source of mRNAs given as active or repressed. We are able to reproduce available experimental data for the average number of transcripts. An analysis of the random fluctuations on the number of eves and their consequences on the spatial precision of stripe 2 is presented. We show that the position of the anterior or posterior borders fluctuate around their average position by ∼1% of the embryo length, which is similar to what is found experimentally. The fitting of data by such a simple model suggests that it can be useful to understand the functions of randomness during developmental processes.

Expression and sequence analysis of the Drosophila blastoderm-specific gene bsg25A

By differential screening of a genomic library, we have cloned a gene expressed specifically during the blastoderm stage of Drosophila embryogenesis. Northern blot analysis and in situ hybridization to embryos reveal that the transcript is maximally expressed during the late syncytial blastoderm stage, disappears rapidly during the cellular blastoderm stage and is not detected at any other point in the Drosophila life cycle. On the basis of its temporally restricted expression and its polytene chromosomal map position at 25A1,2, we have designated this gene blastoderm-specific gene 25A (bsg25A). bsg25A encodes a novel protein of 23 kDa.

Molecular characterization of the Drosophila Mo25 gene, which is conserved among Drosophila, mouse, and yeast

To study the general physiological role of the Mo25 gene, which has been cloned from mouse cleavage-stage embryos, we isolated a Drosophila equivalent, dMo25, cDNA from an embryo cDNA library. The 2,222 nucleotides contained a single open reading frame encoding a polypeptide of 339 amino acid residues with a calculated molecular mass of 39,278 daltons. The deduced amino acid sequence of the dMo25 cDNA had 69.3% identity with mouse Mo25. A homology search revealed that these were similar to a protein encoded in an open reading frame near the calcineurin B subunit gene on chromosome XI in Saccharomyces cerevisiae. In particular, the carboxy-terminal region was highly conserved in Drosophila, mouse, and yeast. The dMo25 gene was mapped to the left arm of the third chromosome at 73AB, and 2.3- and 1.8-kb mRNA bands were detected during development and in adult Drosophila. Conservation of the gene structure and the wide expression profile indicated that the function of the gene is likely to be fundamental in many cell types as well as during development.

cis-regulatory elements of the Drosophila blastoderm-specific serendipity alpha gene: ectopic activation in the embryonic PNS promoted by the deletion of an upstream region

The Drosophila serendipity alpha gene (sry alpha) is specifically expressed at the blastoderm stage in all somatic cells. By deletion analysis of sry alpha-lacZ fusion genes, the sry alpha cis-acting regulatory elements have been restricted to the [-311, +130] 5'-region of the gene and separated in two domains. The [-118, +130] domain is sufficient for transcriptional activation at the blastoderm stage. The [-311, -118] domain is required for a full level of expression. Deletion of this upstream domain leads to a secondary pattern of lacZ expression in precursor cells of the peripheral nervous system (PNS). The sry alpha gene is not itself secondarily expressed in the PNS, as shown by in situ hybridization. The patterns of expression of the different sry alpha-lacZ fusion genes suggest a combinatorial mode of regulation of sry alpha expression at blastoderm.

Drosophila genes encoding maternal-specific and maternal-differential RNAs

The unique cellular and genetic events which occur during the first few hours of Drosophila embryogenesis suggest that there are genes whose function is entirely or largely limited to this stage; this is supported by both genetic and molecular evidence. To identify some of these genes and characterize the relative contribution of specifically maternal and specifically zygotic transcription to early embryogenesis, we used competition and differential screening of a Drosophila genomic DNA library to obtain blastoderm- and maternal-differential sequences [Roark et al.: Dev Biol 109:476-488, 1985]. We describe here the Eco RI restriction fragments, chromosomal location, and size and developmental pattern of expression of the RNAs transcribed from 19 maternal-differential sequences. Five sequences encode maternal-specific transcripts (50-150-fold more abundant in maternal RNA than at any other stage). The maternal-specific and maternal-differential sequences are located at single sites on all major chromosome arms. Comparison of these sites with the sites of presently mapped maternal-effect genes shows several possible correlations, including one region containing three maternal-effect lethal mutations and two maternal-specific sequences.

Transcripts of the Drosophila blastoderm-specific locus, terminus, are concentrated posteriorly and encode a potential DNA-binding finger

The commitment of cells to specific fates, as well as the transitions in the cell cycle and transcription that occur at the cellular blastoderm stage of Drosophila embryogenesis, suggest that there are genes with unique functions expressed specifically at this stage. In an attempt to identify such genes, we used molecular screening to isolate several loci which encode blastoderm-specific transcripts (Roark et al., (1985). Dev. Biol. 109, 476-488). We report here the complete nucleotide sequence of one of these genes, terminus (ter), which maps to 75C1,2. The predicted ter protein possesses a transcription factor IIIA (TFIIIA)-like putative Zn-binding, DNA-binding finger. The ter RNA, detected by in situ hybridization, is distributed uniformly in the embryo during the syncytial blastoderm stage, and then becomes more concentrated in the posterior during the late cellular blastoderm stage. During gastrulation, the RNA is most concentrated in the amnioproctodeal invagination; it is also found at a lower concentration in the ventral furrow and in the anterodorsal neurogenic region. By the end of germ band extension, ter RNA is no longer detected.

Isolation and characterization of a region of the Drosophila genome which contains a cluster of differentially expressed maternal genes (yema gene region)

We have characterized a genomic clone of Drosophila melanogaster which codes for four transcripts that are synthesized during oogenesis, remain abundant in the preblastoderm embryo, and then vanish during gastrulation. One of the transcripts varies in concentration along the anterior-posterior axis of the oocyte. This cluster of maternally acting genes (yema) maps to 98 F3-10 on chromosome arm 3 R.

Molecular characterization of bsg25D: a blastoderm-specific locus of Drosophila melanogaster

The blastoderm stage of Drosophila embryogenesis is a time of crucial transitions in RNA transcription, the cell cycle and segment determination. We have previously identified three loci encoding RNAs specific to this stage (Roark et al., Dev. Biol. 109, 476-488, 1985). We present here the complete nucleotide sequence of one of these loci, bsg25D, which encodes a 2.7 kb blastoderm-specific RNA. The primary structure of this RNA, and that of an overlapping 4.5 kb RNA, has been determined. The amino acid sequence of the predicted bsg25D protein has been compared to the NBRF protein database. Structural similarities between domains in the bsg25D, fos, and tropomyosin proteins, and their possible significance for early embryogenesis are discussed.

The spatial distribution of a blastoderm stage-specific mRNA from the serendipity locus of Drosophila melanogaster

The spatial distribution of a blastoderm stage-specific mRNA was examined by hybridization in situ to tissue sections of early embryos of Drosophila melanogaster. The mRNA examined, the C mRNA, is coded for by the molecularly-complex locus, serendipity (sry). The C mRNA was found to be expressed in embryos in all nuclei and cells except pole cells, during the syncytial and cellular blastoderm stages and its transcription correlates well with the formation of the cellular blastoderm.

Characterization of the myosin light-chain-2 gene of Drosophila melanogaster

Recombinant DNA clones encoding the Drosophila melanogaster homolog of the vertebrate myosin light-chain-2 (MLC-2) gene have been isolated. This single-copy gene maps to the chromosomal locus 99E. The nucleotide sequence was determined for a 3.4-kilobase genomic fragment containing the gene and for two MLC-2 cDNA clones generated from late pupal mRNA. Comparison of these sequences shows that the gene contains two introns, the positions of which are conserved in the corresponding rat sequence. Extension of a primer homologous to the mRNA reveals two start sites for transcription 12 nucleotides apart. The sequence TATA is not present ahead of the mRNA cap site. There are two major sites of poly(A) addition separated by 356 nucleotides. The protein sequence derived from translation of the cDNA sequence shows a high degree of homology with that for the DTNB myosin light chain (MLC-2) of chicken. A lower degree of sequence homology was seen in comparisons with other evolutionarily related calcium-binding proteins. RNA blots show high levels of expression of several transcripts during the developmental time stages when muscle is being produced. In vitro translation of hybrid-selected RNA produces two polypeptides which comigrate on two-dimensional gels with proteins from Drosophila actomyosin, although the cDNA sequence reveals only one 26-kilodalton primary translation product.

Sequence and structure of the serendipity locus of Drosophila melanogaster. A densely transcribed region including a blastoderm-specific gene

The transcriptional organization of the Drosophila melanogaster serendipity (sry) locus (previously designated EH8) has been investigated by DNA sequencing, S1 nuclease mapping, and primer extension analysis. The data indicate that the five different (and partially overlapping) sry messenger RNAs detectable in early embryos are initiated at three separate sites, each directly upstream from one of the three protein-coding regions, designated (in 5' to 3' order) beta, alpha and delta. All of the sry mRNAs are transcribed in the same direction. The two blastoderm stage-specific sry mRNAs both include the alpha-coding region and have the same 5' terminus, only 183 nucleotides downstream from the 3' terminus of a beta region transcript (which is transcribed in ovaries). Similarly, only 331 nucleotides separate the 5' end of a delta region transcript from the major 3' end of the alpha region transcripts. One of the five sry embryonic mRNAs includes both the beta and alpha protein-coding segments and the spacer region in between, while another mRNA includes both the alpha and delta protein-coding regions as well as the spacer in between. The two read-through transcripts probably result from the failure to undergo a 3' cleavage and polyadenylation event rather than from differential splicing. As the three other sry embryo mRNAs are each transcribed from a single protein-coding region, it would appear that the alpha, beta and delta open reading frames correspond to three separate genes. Codon bias analysis reinforces the notion that these three genes code for bona fide proteins with translation starting at the first in-frame AUG codon. The predicted beta and delta polypeptides show partial amino acid sequence homology, suggesting a common evolutionary origin.

Characterization of the myosin light-chain-2 gene of Drosophila melanogaster

Recombinant DNA clones encoding the Drosophila melanogaster homolog of the vertebrate myosin light-chain-2 (MLC-2) gene have been isolated. This single-copy gene maps to the chromosomal locus 99E. The nucleotide sequence was determined for a 3.4-kilobase genomic fragment containing the gene and for two MLC-2 cDNA clones generated from late pupal mRNA. Comparison of these sequences shows that the gene contains two introns, the positions of which are conserved in the corresponding rat sequence. Extension of a primer homologous to the mRNA reveals two start sites for transcription 12 nucleotides apart. The sequence TATA is not present ahead of the mRNA cap site. There are two major sites of poly(A) addition separated by 356 nucleotides. The protein sequence derived from translation of the cDNA sequence shows a high degree of homology with that for the DTNB myosin light chain (MLC-2) of chicken. A lower degree of sequence homology was seen in comparisons with other evolutionarily related calcium-binding proteins. RNA blots show high levels of expression of several transcripts during the developmental time stages when muscle is being produced. In vitro translation of hybrid-selected RNA produces two polypeptides which comigrate on two-dimensional gels with proteins from Drosophila actomyosin, although the cDNA sequence reveals only one 26-kilodalton primary translation product.

A Drosophila Minute gene encodes a ribosomal protein

Minute genes have long constituted a special problem in Drosophila genetics. For at least 50-60 different genes scattered throughout the genome, dominant mutations and/or deficiencies have been recognized which result in a common phenotype consisting of short thin bristles, slow development, reduced viability, rough eyes, small body size and etched tergites. Schultz proposed that the Minute loci encode similar but separate functions involved in growth and division common to all cells. Atwood and Ritossa suggested that Minute loci encode components of the protein synthetic machinery, specifically the transfer RNA genes; this now seems unlikely on grounds of both mapping and mutability studies. More recently, we and others suggested that the Minute loci are ribosomal protein genes. We report here that transformation with a cloned 3.3-kilobase (kb) region containing the gene encoding the large subunit ribosomal protein 49 (rp49) suppresses the dominant phenotypes of Minute (3)99D, a previously undescribed Minute associated with a chromosomal deficiency of the 99D interval. This activity is specific to the 99D Minute as it does not suppress other Minute loci elsewhere in the genome. This result provides direct evidence that the Minute locus at the 99D interval encodes the ribosomal protein 49.