Interrelationship of RNA interference and transcriptional gene silencing in Drosophila.
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Animals↗
Biomedical subjects
U Bhadra
Publications and source records attributed to U Bhadra.
Dosage-dependent gene regulation in multicellular eukaryotes: implications for dosage compensation, aneuploid syndromes, and quantitative traits.
Evidence from a variety of data suggests that regulatory mechanisms in multicellular eukaryotes have evolved in such a manner that the stoichiometric relationship of the components of regulatory complexes affects target gene expression. This type of mechanism sets the level of gene expression and, as a consequence, the phenotypic characteristics. Because many types of regulatory processes exhibit dosage-dependent behavior, they would impact quantitative traits and contribute to their multigenic control in a semidominant fashion. Many dosage-dependent effects would also account for the extensive modulation of gene expression throughout the genome that occurs when chromosomes are added to or subtracted from the karyotype (aneuploidy). Moreover, because the majority of dosage-dependent regulators act negatively, this property can account for the up-regulation of genes in monosomics and hemizygous sex chromosomes to achieve dosage compensation.
Aneuploidy↗
Making noise about silence: repression of repeated genes in animals.
Repeated copies of genes, whether in tandem or dispersed, are often recognized by the cell and silenced. Tandem repeat silencing is associated with a heterochromatin-like complex. Dispersed gene silencing can be mediated by the repressive Polycomb Group complex or involve post-transcriptional silencing presumably involving double-stranded RNA. The I retrotransposable element in Drosophila appears to be susceptible to dispersed gene silencing, potentially by both post-transcriptional and transcriptional processes. Some mutations that eliminate RNA interference in Caenorhabditis elegans result in the mobilization of many transposons and two of these mutations desilence tandem repeats in the germline. One challenge for the future is to determine the nature of any relationship between post-transcriptionally and transcriptionally based mechanisms. The silencing mechanisms potentially act as a protection against high expression of transposons and viruses.
Animals↗
Histone acetylation and gene expression analysis of sex lethal mutants in Drosophila.
The evolution of sex determination mechanisms is often accompanied by reduction in dosage of genes on a whole chromosome. Under these circumstances, negatively acting regulatory genes would tend to double the expression of the genome, which produces compensation of the single-sex chromosome and increases autosomal gene expression. Previous work has suggested that to reduce the autosomal expression to the female level, these dosage effects are modified by a chromatin complex specific to males, which sequesters a histone acetylase to the X. The reduced autosomal histone 4 lysine 16 (H4Lys16) acetylation results in lowered autosomal expression, while the higher acetylation on the X is mitigated by the male-specific lethal complex, preventing overexpression. In this report, we examine how mutations in the principal sex determination gene, Sex lethal (Sxl), impact the H4 acetylation and gene expression on both the X and autosomes. When Sxl expression is missing in females, we find that the sequestration occurs concordantly with reductions in autosomal H4Lys16 acetylation and gene expression on the whole. When Sxl is ectopically expressed in Sxl(M) mutant males, the sequestration is disrupted, leading to an increase in autosomal H4Lys16 acetylation and overall gene expression. In both cases we find relatively little effect upon X chromosomal gene expression.
Acetylation↗
Cosuppression of nonhomologous transgenes in Drosophila involves mutually related endogenous sequences.
Cosuppression refers to the phenomenon in which silencing among dispersed homologous genes occurs. Here we demonstrate that two nonhomologous reciprocal fusion genes, white-Alcohol dehydrogenase (w-Adh) and Adh-w, exhibit cosuppression using the endogenous Adh sequence as an intermediary. Deletion of the endogenous Adh gene eliminates the interaction, while reintroduction of an 8.6 kb Adh fragment restores the silencing. Using truncated Adh constructs, a nontranscribed segment in the Adh regulatory region was found to be one of the sequences required for homology recognition. The silencing interaction is initiated during early development. The silenced transgenes are associated with the Polycomb group complex of chromatin proteins.
Alcohol Dehydrogenase↗
Less from more: cosuppression of transposable elements.
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Animals↗
Role of the male specific lethal (msl) genes in modifying the effects of sex chromosomal dosage in Drosophila.
Immunostaining of chromosomes shows that the male-specific lethal (MSL) proteins are associated with all female chromosomes at a low level but are sequestered to the X chromosome in males. Histone-4 Lys-16 acetylation follows a similar pattern in normal males and females, being higher on the X and lower on the autosomes in males than in females. However, the staining pattern of acetylation and the mof gene product, a putative histone acetylase, in msl mutant males returns to a uniform genome-wide distribution as found in females. Gene expression on the autosomes correlates with the level of histone-4 acetylation. With minor exceptions, the expression levels of X-linked genes are maintained with either an increase or decrease of acetylation, suggesting that the MSL complex renders gene activity unresponsive to H4Lys16 acetylation. Evidence was also found for the presence of nucleation sites for association of the MSL proteins with the X chromosome rather than individual gene binding sequences. We suggest that sequestration of the MSL proteins occurs in males to nullify on the autosomes and maintain on the X, an inverse effect produced by negatively acting dosage-dependent regulatory genes as a consequence of the evolution of the X/Y sex chromosomal system.
Animals↗
Role of multiple trans-acting regulators in modifying the effect of the retrotransposon copia on host gene expression in Drosophila.
We have examined the combinatorial effect of five trans-acting modifiers of white apricot, a copia retrotransposon-induced allele of the white eye-color gene. Quantitation of steady-state levels of copia transcripts for various modifier combinations at different developmental stages using northern analysis reveals nonadditivity and epistasis. Interestingly, extensive overexpression of copia transcripts caused by the modifier Ufo is altered up to twofold in level when Ufo is combined with either of the modifiers Mow or Wow. In general, double combinations do not elevate the level of copia transcripts more than twofold. The role of suppressor mutations in modifying the effect of transposable elements on host gene expression is discussed.
Animals↗
Interactions among dosage-dependent trans-acting modifiers of gene expression and position-effect variegation in Drosophila.
We have investigated the effect of dosage-dependent trans-acting regulators of the white eye color gene in combinations to understand their interaction properties. The consequences of the interactions will aid in an understanding of aneuploid syndromes, position-effect variegation (PEV), quantitative traits, and dosage compensation, all of which are affected by dosage-dependent modifiers. Various combinations modulate two functionally related transcripts, white and scarlet, differently. The overall trend is that multiple modifiers are noncumulative or epistatic to each other. In some combinations, developmental transitions from larvae to pupae to adults act as a switch for whether the effect is positive or negative. With position-effect variegation, similar responses were found as with gene expression. The highly multigenic nature of dosage-sensitive modulation of both gene expression and PEV suggests that dosage effects can be progressively transduced through a series of steps in a hierarchical manner.
ATP-Binding Cassette Transporters↗
Cosuppression in Drosophila: gene silencing of Alcohol dehydrogenase by white-Adh transgenes is Polycomb dependent.
When two to six copies of a white promoter-Alcohol dehydrogenase (Adh) reporter fusion gene are introduced into the genome, the expression is progressively reduced both in larvae and adults rather than the expected gene dosage effect. In addition, multiple transgenes reduce endogenous Adh transcripts, a result that is strongly analogous to "cosuppression" phenomena described in many plant species but which has not been previously observed in animals. Silencing of the Adh gene is not influenced by zeste-dependent transvection but strongly affected by the Polycomb and Polycomblike mutations. Polycomb and polyhomeotic proteins are bound to the chromatin at the sites of the repressed w-Adh transgenes.
Alcohol Dehydrogenase↗
A trans-acting modifier causing extensive overexpression of genes in Drosophila melanogaster.
A modified P element-induced mutation at cytological position 47A11-12 on chromosome 2 of Drosophila melanogaster inversely alters the expression of several genes examined. In addition, the effect is nearly exponential in some developmental stages for five of the eleven genes tested (white, brown, scarlet, rudimentary and vermilion). Interestingly, the exponential overexpression of the transcripts leads to a sexually dimorphic effect, with female levels being greater. The white allele specificity analysis and the lack of response of an Alcohol dehydrogenase promoter-white reporter fusion construct suggest that the 5' regulatory sequences of white are required for the response. Because the cases of extreme overexpression are more prominent in females than males, the locus is termed Ultra female overexpression (Ufo). Ufo revertants have no discernible effect on the transcript levels, indicating that the mutation is responsible for the modulation of the tested genes. Regulatory genes such as Ufo may contribute to the wide fluctuations in gene expression between tissues and stages of development.
ATP-Binding Cassette Transporters↗
A sex-influenced modifier in Drosophila that affects a broad spectrum of target loci including the histone repeats.
A second chromosomal trans-acting modifier, Lightener of white (Low), modulates the phenotypic expression of various alleles of the white eye color gene. This modifier has an unusually broad spectrum of affected genes including white, brown, scarlet and the eye developmental genes, Bar and Lobe. In addition, Low weakly suppresses position effect variegation. Northern blot hybridization with different X and autosomal probes reveals that Low modulates genes of independent expression patterns. Interestingly, many of the modulations of gene expression are developmentally restricted and differ in intensity between the sexes. Low also elevates the expression of the histone tandem repeats in three distinct developmental stages. A deficiency encompassing the histone cluster reduces their transcript levels and significantly alters the expression of some of the tested genes. Thus, Low is a modifier that plays a role in modulating the expression of genes governing various processes including pigment deposition, eye development, chromosomal proteins and position effect variegation.
Alleles↗
Characterization of a sex-influenced modifier of gene expression and suppressor of position-effect variegation in Drosophila.
Modifier of white (Mow), a dominant transacting gene, has been identified through a mutagenic screen for second-site loci that alter the level of expression of the white eye color locus. Mow reduces the expression of white in most developmental stages, but enhances its expression in the pupal stage, the time at which the major contribution to the adult phenotype is made. Tests with an Alcohol dehydrogenase promoter-white reporter and a series of white truncation constructs have shown that Mow fails to affect the reporter; cis-regulatory mutations of white also show no response, suggesting a requirement for white regulatory domains for interaction with Mow. A quantitative analysis of steady-state transcript levels reveals that the white mRNA level decreases in the presence of one dose of Mow in larvae and adults, but the reduction is greater in females than males. Two other functionally related genes, brown and scarlet, also exhibit a similar sexually dimorphic alteration in expression, mediated by Mow. In the mid-pupal stage, by contrast, the level of white and brown mRNA is increased by Mow. In addition, Mow acts as a weak suppressor of position effect variegation (PEV). These observations suggest a connection between dosage modulation of gene expression and suppression of position-effect variegation.
Alcohol Dehydrogenase↗
Weakener of white (Wow), a gene that modifies the expression of the white eye color locus and that suppresses position effect variegation in Drosophila melanogaster.
A locus is described in Drosophila melanogaster that modifies the expression of the white eye color gene. This trans-acting modifier reduces the expression of the white gene in the eye, but elevates the expression in other adult tissues. Because of the eye phenotype in which the expression of white is lessened but not eliminated, the newly described locus is called the Weakener of white (Wow). Northern analysis reveals that Wow can exert an inverse or direct modifying effect depending upon the developmental stage. Two related genes, brown and scarlet, that are coordinately expressed with white, are also affected by Wow. In addition, Wow modulates the steady state RNA level of the retrotransposon, copia. When tested with a white promoter-Alcohol dehydrogenase reporter. Wow confers the modifying effect to the reporter, suggesting a requirement of the white regulatory sequences for mediating the response. In addition to being a dosage sensitive regulator of white, brown, scarlet and copia, Wow acts as a suppressor of position effect variegation. There are many dosage sensitive suppressors of position effect variegation and many dosage-sensitive modifiers of gene expression. The Wow mutations provide evidence for an overlap between the two types of modifiers.
ATP-Binding Cassette Transporters↗
Drug inventory management in Medical College Hospital, Kolkata.
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Drug Storage↗