The gypsy retrotransposon of Drosophila melanogaster: mechanisms of mutagenesis and interaction with the suppressor of Hairy-wing locus

Phosphorylated histone variant γH2Av is associated with chromatin insulators in Drosophila

Chromatin insulators are responsible for orchestrating long-range interactions between enhancers and promoters throughout the genome and align with the boundaries of Topologically Associating Domains (TADs). Here, we demonstrate an association between gypsy insulator proteins and the phosphorylated histone variant H2Av (γH2Av), normally a marker of DNA double strand breaks. Gypsy insulator components colocalize with γH2Av throughout the genome, in polytene chromosomes and in diploid cells in which Chromatin IP data shows it is enriched at TAD boundaries. Mutation of insulator components su(Hw) and Cp190 results in a significant reduction in γH2Av levels in chromatin and phosphatase inhibition strengthens the association between insulator components and γH2Av and rescues γH2Av localization in insulator mutants. We also show that γH2Av, but not H2Av, is a component of insulator bodies, which are protein condensates that form during osmotic stress. Phosphatase activity is required for insulator body dissolution after stress recovery. Together, our results implicate the H2A variant with a novel mechanism of insulator function and boundary formation.

Genome-wide studies of the multi-zinc finger Drosophila Suppressor of Hairy-wing protein in the ovary

The Drosophila Suppressor of Hairy-wing [Su(Hw)] protein is a globally expressed, multi-zinc finger (ZnF) DNA-binding protein. Su(Hw) forms a classic insulator when bound to the gypsy retrotransposon and is essential for female germline development. These functions are genetically separable, as exemplified by Su(Hw)(f) that carries a defective ZnF10, causing a loss of insulator but not germline function. Here, we completed the first genome-wide analysis of Su(Hw)-binding sites (SBSs) in the ovary, showing that tissue-specific binding is not responsible for the restricted developmental requirements for Su(Hw). Mapping of ovary Su(Hw)(f) SBSs revealed that female fertility requires binding to only one third of the wild-type sites. We demonstrate that Su(Hw)(f) retention correlates with binding site affinity and partnership with Modifier of (mdg4) 67.2 protein. Finally, we identify clusters of co-regulated ovary genes flanked by Su(Hw)(f) bound sites and show that loss of Su(Hw) has limited effects on transcription of these genes. These data imply that the fertility function of Su(Hw) may not depend upon the demarcation of transcriptional domains. Our studies establish a framework for understanding the germline Su(Hw) function and provide insights into how chromatin occupancy is achieved by multi-ZnF proteins, the most common transcription factor class in metazoans.

A matter of packaging: influence of nucleosome positioning on heterologous gene expression

The organization of DNA into the various levels of chromatin compaction is the main obstacle that restricts the access of transcriptional machinery to genes. Genome-wide chromatin analyses have shown that there are common chromatin organization patterns for most genes but have also revealed important differences in nucleosome positioning throughout the genome. Such chromatin heterogeneity is one of the reasons why recombinant gene expression is highly dependent on integration sites. Different solutions have been tested for this problem, including artificial targeting of chromatin-modifying factors or the addition of DNA elements, which efficiently counteract the influence of the chromatin environment.An influence of the chromatin configuration of the recombinant gene itself on its transcriptional behavior has also been established. This view is especially important for heterologous genes since the general parameters of chromatin organization change from one species to another. The chromatin organization of bacterial DNA proves particularly dramatic when introduced into eukaryotes. The nucleosome positioning of recombinant genes is the result of the interaction between the machinery of the hosting cell and the sequences of both the recombinant genes and the promoter regions. We discuss the key aspects of this phenomenon from the heterologous gene expression perspective.

Effects of chromosomal rearrangements on transvection at the yellow gene of Drosophila melanogaster

Homologous chromosomes are paired in somatic cells of Drosophila melanogaster. This pairing can lead to transvection, which is a process by which the proximity of homologous genes can lead to a change in gene expression. At the yellow gene, transvection is the basis for several examples of intragenic complementation involving the enhancers of one allele acting in trans on the promoter of a paired second allele. Using complementation as our assay, we explored the chromosomal requirements for pairing and transvection at yellow. Following a protocol established by Ed Lewis, we generated and characterized chromosomal rearrangements to define a region in cis to yellow that must remain intact for complementation to occur. Our data indicate that homolog pairing at yellow is efficient, as complementation was disrupted only in the presence of chromosomal rearrangements that break Collapse

Key Words Collapse

MESH Headings

• Animals
• Chromosome Aberrations
• Chromosomes/genetics
• Drosophila Proteins/genetics
• Drosophila melanogaster/genetics
• Female
• Gene Duplication
• Gene Expression Regulation
• Genetic Complementation Test
• Male
• Models, Genetic
• Mutation
• Translocation, Genetic

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Grants

• R01 GM061936 NIGMS NIH HHS
• R01 GM085169 NIGMS NIH HHS
• GM61936 NIGMS NIH HHS
• GM085169 NIGMS NIH HHS

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Affiliation(s)

Sharon A Ou Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
Elaine Chang
Szexian Lee
Katherine So
C-ting Wu
James R Morris

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Characterization of BEAF mutations isolated by homologous recombination in Drosophila

The Drosophila BEAF-32A and BEAF-32B proteins bind to the scs' insulator and to hundreds of other sites on Drosophila chromosomes. These two proteins are encoded by the same gene. We used ends-in homologous recombination to generate the null BEAF(AB-KO) allele and also isolated the BEAF(A-KO) allele that eliminates production of only the BEAF-32A protein. We find that the BEAF proteins together are essential, but BEAF-32B alone is sufficient to obtain viable flies. Our results show that BEAF is important for both oogenesis and development. Maternal or zygotic BEAF is sufficient to obtain adults, although having only maternal BEAF impairs female fertility. In the absence of all BEAF, a few fertile but sickly males are obtained. Using both a chromosomal position-effect assay and an enhancer-blocking assay, we find that BEAF is necessary for scs' insulator function. Lack of BEAF causes a disruption of male X polytene chromosome morphology. However, we did not find evidence that dosage compensation was affected. Position-effect variegation of the w(m4h) allele and different variegating y transgenes was enhanced by the knockout mutation. Combined with the effects on male X polytene chromosomes, we conclude that BEAF function affects chromatin structure or dynamics.

Identification of genomic sites that bind the Drosophila suppressor of Hairy-wing insulator protein

Eukaryotic genomes are divided into independent transcriptional domains by DNA elements known as insulators. The gypsy insulator, a 350-bp element isolated from the Drosophila gypsy retrovirus, contains twelve degenerate binding sites for the Suppressor of Hairy-wing [Su(Hw)] protein. Su(Hw) associates with over 500 non-gypsy genomic sites, the functions of which are largely unknown. Using a bioinformatics approach, we identified 37 putative Su(Hw) insulators (pSIs) that represent regions containing clustered matches to the gypsy insulator Su(Hw) consensus binding sequence. The majority of these pSIs contain fewer than four Su(Hw) binding sites, with only seven showing in vivo Su(Hw) association, as demonstrated by chromatin immunoprecipitation. To understand the properties of the pSIs, these elements were tested for enhancer-blocking capabilities using a transgene assay system. In a complementary set of experiments, effects of the pSIs on transcriptional regulation of genes at the natural genomic location were determined. Our data suggest that pSIs have complex genomic functions and, in some cases, establish insulators. These studies provide the first direct evidence that the Su(Hw) protein contributes to the regulation of gene expression in the Drosophila genome through the establishment of endogenous insulators.

Genotoxicity of retroviral integration in hematopoietic cells

The experience of the past 3 years, since the first case of leukemia was reported in a child cured of X-linked severe combined immunodeficiency (X-SCID) by gene therapy, indicates that the potential genotoxicity of retroviral integration in hematopoietic cells will remain a consideration in evaluating the relative risks versus benefits of gene therapy for specific blood disorders. Although many unique variables may have contributed to an increased risk in X-SCID patients, clonal dominance or frank neoplasia in animal models, clonal dominance in humans with chronic granulomatous disease, and the ability of retroviral integration to immortalize normal bone marrow cells or convert factor-dependent cells to factor independence suggest that transduction of cells with an integrating retrovirus has the potential for altering their subsequent biologic behavior. The selective pressure imposed during in vitro culture or after engraftment may uncover a growth or survival advantage for cells in which an integration event has affected gene expression. Such cells then carry the risk that subsequent mutations may lead to neoplastic evolution of individual clones. Balancing that risk is that the vast majority of integration events seem to be neutral and that optimizing vector design may diminish the probability of altering gene expression by an integrated vector genome. Several cell culture systems and animal models designed to empirically evaluate the safety of vector systems are being developed and should provide useful data for weighing the relative risks and benefits for specific diseases and patient populations. Gene therapy interventions continue to have enormous potential for the treatment of disorders of the hematopoietic system. The future of such efforts seems bright as we continue to evolve and improve various strategies to make such interventions both effective and as safe as possible.

A cis-regulatory sequence within the yellow locus of Drosophila melanogaster required for normal male mating success

Drosophila melanogaster males perform a courtship ritual consisting of a series of dependent fixed-action patterns. The yellow (y) gene is required for normal male courtship behavior and subsequent mating success. To better characterize the requirement for y in the manifestation of innate male sexual behavior, we measured the male mating success (MMS) of 12 hypomorphic y mutants and matched-outbred-background controls using a y+ rescue element on a freely segregating minichromosome. We found that 4 hypomorphs significantly reduced MMS to varying degrees. Reduced MMS was largely independent of adult pigmentation patterns. These mutations defined a 300-bp regulatory region upstream of the transcription start, the mating-success regulatory sequence (MRS), whose function is required for normal MMS. Visualization of gene action via GFP and a Yellow antibody suggests that the MRS directs y transcription in a small number of cells in the third instar CNS, the developmental stage previously implicated in the role of y with regard to male courtship behavior. The presence of Yellow protein in these cells positively correlates with MMS in a subset of mutants. The MRS contains a regulatory sequence controlling larval pigmentation and a 35-bp sequence that is highly conserved within the genus Drosophila and is predicted to bind known transcription factors.

Transcriptional interactions between yeast tRNA genes, flanking genes and Ty elements: a genomic point of view

Retroelement insertion can alter the expression of nearby genes. The Saccharomyces cerevisiae retrotransposons Ty1-Ty4 are transcribed by RNA polymerase II (pol II) and target their integration upstream of genes transcribed by RNA polymerase III (pol III), mainly tRNA genes. Because tRNA genes can repress nearby pol II-transcribed genes, we hypothesized that transcriptional interference may exist between Ty1 insertions and pol III-transcribed genes, the preferred targets for Ty1 integration. Ty1s upstream of two pol III-transcribed genes (SNR6 and SUP2) were recovered and analyzed by RNA blot analysis. Ty1 insertions were found to exert a neutral or modest stimulatory effect on the expression of these genes. Further RNA analysis indicated a modest tRNA position effect on Ty1 transcription. To investigate the possible genomic relevance of these expression effects, we compiled a comprehensive tRNA gene database. This database allowed us to analyze a genome's worth of tRNA genes and Ty elements. It also enabled the prediction and experimental confirmation of tRNA gene position effects at native chromosomal loci. We provide evidence supporting the hypothesis that tRNA genes exert a modest inhibitory effect on adjacent pol II promoters. Direct analysis of PTR3 transcription, promoted by sequences very close to a tRNA gene, shows that this tRNA position effect can operate on a native chromosomal gene.

Differences in insulator properties revealed by enhancer blocking assays on episomes

Insulators are genomic elements that define domains of transcriptional autonomy. Although a large number of insulators have been isolated, it is unclear whether these elements function by shared molecular mechanisms. Novel applications of FLP recombinase technology were used to dissect and compare the function of the Drosophila: gypsy and scs insulators. Inter actions between FLP monomers bound to chromosomally integrated FRT sites were unimpeded by either insulator, demonstrating that these insulators do not establish a chromosomal environment capable of disrupting all types of protein-protein interactions. The gypsy insulator blocked enhancer-activated transcription on FLP-generated extra-chromosomal episomes, whereas the scs insulator displayed silencing effects. These data indicate that these insulators differ in the mechanisms used to prevent enhancer function. That the gypsy insulator blocked enhancer-promoter communication within small episomes suggests that these effects may be accomplished without a global reorganization of chromatin structure. Instead, the gypsy insulator may disrupt enhancer-activated transcription by direct interference with transmission of the enhancer signal to the promoter.

BS a novel LINE-like element in Drosophila melanogaster

Transposable elements with long terminal inverted repeats are rare and only one family of elements of this sort has been identified in the genome of Drosophila melanogaster. An insertion associated with the HSBS mutation of the achaete-scute complex has been reported to be a second element of this type. We have determined the complete sequence of this insertion and have shown that it is in fact two copies of a new LINE-like transposable element, that we have called BS, inserted in opposite orientation 337 bp apart. Like other elements of this type, BS has two open reading frames that appear to encode a gag-like polypeptide and a reverse transcriptase. There are few complete BS elements in the five strains of D.melanogaster that we have tested and they appear to transpose infrequently. The events that may have lead to the double BS insertion are discussed in terms of the supposed mechanism of transposition of LINE-like elements.

Analysis of the antennal phenotype in the Drosophila mutant lozenge

Previous work on the lozenge (lz) gene complex of D. melanogaster has focused on the compound eye. Here we study the effects of 22 lz mutations on the antennal sensilla. The antenna of strong lz alleles is characterized by a lack of basiconic sensilla and by a significantly increased density of coeloconic sensilla. Intermediate alleles have few basiconic sensilla, they exhibit a highly increased density of trichoid sensilla, but a normal coeloconic density. Basiconic sensilla on the maxillary palps are weakly affected even by strong lz alleles. The antennal phenotype for most of the strong and intermediate mutants is partially dominant over wild type. Although this complicates the interpretation of complementation data, 12 selected mutants that were studied in heteroallelic combinations seem to define a single cistron. Temperature shifts of the lztsl allele showed that gene activity is crucial from about 87% of the third larval instar up to 7% of pupal life. Applying restrictive temperature early during this period results in a 'novel' phenotype that is characterized by a dramatic decrease in the density of trichoid sensilla, whereas a late pulse of restrictive temperature leads to a 'normal' intermediate phenotype. Our data suggest that the lz gene controls at least five different functions in the antenna: the size of the third antennal segment, the overall number and density of sensilla, the proportions of the 3 types of sensilla, and the generation of basiconic sensilla.

Two non-gypsy rudimentary mutations and their suppression by mutations of suppressor of Hairy-wing in Drosophila

Two spontaneous mutations of rudimentary, the gene encoding the first steps of de novo pyrimidine biosynthesis in Drosophila, are suppressed by mutant alleles of the suppressor of Hairy-wing locus. This interaction differs from typical su(Hw) suppression in that neither rudimentary allele is associated with an insertion of the gypsy retrotransposon. One allele, rsP1, appears to be a point mutation. Adult rsP1 homozygous females accumulate substantially less 7.3 kb rudimentary transcript than do wild-type females. The other allele, rsP2, is an insertion of an mdg3 retrotransposon in the sixth exon of rudimentary and in the opposite transcriptional orientation. This insertion divides the rudimentary locus into two separate, yet functional, transcription units by truncating transcription from the rudimentary promoter and promoting transcription of downstream rudimentary sequences. Phenotypic suppression of both rsP1 and rsP2 by mutant alleles of the suppressor of Hairy-wing locus correlates with enhanced levels of the rsP1 and rsP2 transcripts.

Broad-complex function during oogenesis in Drosophila melanogaster

The Broad-Complex (BR-C) appears to encode factors that mediate ecdysone effects during the larva-adult transition. The main goal of this study was to gain insight into what roles the BR-C might play during oogenesis. The main findings are as follows. First, as determined by heteroallele studies and clonal analysis, de12 is a somatic line mutation that appears to fall into the broad domain of the BR-C. Second, the de12 mutation is associated with the insertion of the gypsy transposon at position 169.5 (Chao and Guild, Embo J, 1986, 5:143-150) in the BR-C domain. In its new context this gypsy element exhibits ovarian-specific activation. Both this gypsy activation and the de12 phenotype are partially suppressible by su(f) and su(Hw). Third, we have identified a set of transcripts that cross-hybridize with BR-C sequence spanning the gypsy insertion site (166-179). There are significant differences in these cross-hybridizing species, both in size and relative abundance, between de12 and its parent strain. Finally we have determined that in de12 there is a premature arrest of chorion gene amplification in the late stages of oogenesis.

Mutations in the su(s) gene affect RNA processing in Drosophila melanogaster

We have studied the effect of mutations in the suppressor of sable [su(s)] gene on P element-induced yellow alleles. Two independent mutations tested, y76d28 and y1#7, contain a 1.1-kilobase (kb) P element inserted in the 5' transcribed untranslated portion of the yellow gene. Sequences responsible for the y1#7 mutation are inserted in the same transcriptional orientation as yellow and cannot be processed by splicing, and this mutation is not suppressed by su(s) mutations. P element sequences are located in a transcriptional orientation opposite to that of the yellow gene in y76d28; these sequences can be spliced from a composite P element-yellow mRNA, resulting in low accumulation of a functional 1.9-kb yellow transcript. The levels of both the putative precursor P element-yellow RNA and the 1.9-kb yellow transcript increase in y76d28 su(s) flies, suggesting that mutations in su(s) do not affect the efficiency of splicing of the P element sequences. Analysis of y76d28 cDNAs isolated from flies carrying a wild-type or mutant su(s) gene demonstrates that the choice of splice junctions to process P element sequences is unchanged in these different backgrounds, suggesting that mutations in su(s) do not affect the selection of donor and acceptor splice sites. We propose that the su(s) protein functions to control the stability of unprocessed RNA during the splicing reaction.

Somatic reversion/suppression of the mouse mdx phenotype in vivo

The mdx mouse has a myopathy caused by dystrophin deficiency, and is therefore biochemically and genetically homologous to human Duchenne muscular dystrophy. While mdx mouse muscle shows no dystrophin by immunoblotting, a very small percentage of myofibers appear clearly dystrophin-positive by immunofluorescence microscopy. We have characterized these rare positive-staining fibers, and conclude that they are indeed expressing dystrophin despite a nonsense mutation within the dystrophin gene. Thus, the dystrophin-positive fibers probably represent somatic reversion or suppression of the mdx mutation. Cardiac muscle and skeletal muscle from mdx mice showed dramatically different patterns of dystrophin-positive cells. However, this difference is expected given the apparent clonal nature of the reversion/suppression events, the inability of cardiac muscle to regenerate, and other differences in the developmental programs of myofibers and cardiocytes. The prevalence of dystrophin-positive cells in mdx cardiac muscle was determined to be approximately 2 x 10(-5). The observed prevalence of dystrophin-positive cardiocytes in the mdx mouse is a possible estimate of the somatic reversion rate of the mdx mutation in vivo.

DNA bending is a determinant of binding specificity for a Drosophila zinc finger protein

The suppressor of Hairy-wing [su(Hw)] locus encodes a zinc finger-containing protein that binds to specific sequences of the Drosophila gypsy element, mediating the mutagenic effects of this retrotransposon. We carried out a detailed analysis of the su(Hw)-gypsy interaction using various biochemical assays. DNase I footprinting delimits a 37-bp region in the coding strand of gypsy that is protected from digestion by the binding of the su(Hw) protein. Specific DNA contacts involved in the interaction were determined by methylation protection analysis and by missing nucleoside experiments using hydroxyl radical. Results from these experiments indicate that the su(Hw) protein binds to sequences homologous to the octamer motif, which is recognized by homeo box-containing proteins in mammalian organisms. Furthermore, two DNA bends present on both sides of the binding site in the absence of protein favor the strength of this interaction.

A retrotransposon 412 insertion within an exon of the Drosophila melanogaster vermilion gene is spliced from the precursor RNA

Three alleles of the Drosophila melanogaster vermilion (v) gene are suppressed by recessive mutations at the suppressor of sable [su(s)], gene. Previous work has established that these alleles have identical insertions of the 412 retrotransposon in the 5'-untranslated region of the gene. Despite the transposon insertion in an exon, v mutants accumulate trace amounts of apparently wild-type-sized transcripts in a su(s)+ background, and the level of v transcript accumulation is increased by su(s) mutations. Here, we have characterized transcripts from a suppressible v mutant in both su(s)+ and su(s)- backgrounds by S1 nuclease protection experiments and sequence analysis of polymerase chain reaction (PCR) generated cDNA clones. We find that transposon sequences are imprecisely eliminated from v mutant transcripts by splicing at donor and acceptor sites located near the ends of the 412 retrotransposon. Four different 5' donor sites are alternatively spliced to a single 3' acceptor site. The implications of this finding are discussed in relation to possible functions of the su(s)+ gene product.