Monitoring of switches in heterochromatin-induced silencing shows incomplete establishment and developmental instabilities

Position effect variegation (PEV) in Drosophila results from new juxtapositions of euchromatic and heterochromatic chromosomal regions, and manifests as striking bimodal patterns of gene expression. The semirandom patterns of PEV, reflecting clonal relationships between cells, have been interpreted as gene-expression states that are set in development and thereafter maintained without change through subsequent cell divisions. The rate of instability of PEV is almost entirely unexplored beyond the final expression of the modified gene; thus the origin of the expressivity and patterns of PEV remain unexplained. Many properties of PEV are not predicted from currently accepted biochemical and theoretical models. In this work we investigate the time at which expressivity of silencing is set, and find that it is determined before heterochromatin exists. We employ a mathematical simulation and a corroborating experimental approach to monitor switching (i.e., gains and losses of silencing) through development. In contrast to current views, we find that gene silencing is incompletely set early in embryogenesis, but nevertheless is repeatedly lost and gained in individual cells throughout development. Our data support an alternative to locus-specific “epigenetic” silencing at variegating gene promoters that more fully accounts for the final patterns of PEV.

Download Full-text

Interactions Among Dosage-Dependent Trans-Acting Modifiers of Gene Expression and Position-Effect Variegation in Drosophila

Genetics ◽

10.1093/genetics/150.1.251 ◽

1998 ◽

Vol 150 (1) ◽

pp. 251-263 ◽

Cited By ~ 3

Author(s):

Utpal Bhadra ◽

Manika Pal Bhadra ◽

James A Birchler

Keyword(s):

Gene Expression ◽

Quantitative Traits ◽

Position Effect ◽

Position Effect Variegation ◽

Developmental Transitions ◽

Eye Color ◽

Dosage Effects ◽

Effect Variegation ◽

Hierarchical Manner ◽

Color Gene

Abstract 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.

Download Full-text

Mutation in P0, a Dual Function Ribosomal Protein/Apurinic/Apyrimidinic Endonuclease, Modifies Gene Expression and Position Effect Variegation in Drosophila

Genetics ◽

10.1093/genetics/150.4.1487 ◽

1998 ◽

Vol 150 (4) ◽

pp. 1487-1495

Author(s):

Maxim V Frolov ◽

James A Birchler

Keyword(s):

Gene Expression ◽

Ribosomal Protein ◽

Position Effect ◽

P Element ◽

Dual Function ◽

Insertion Mutant ◽

Position Effect Variegation ◽

Element Insertion ◽

Effect Variegation ◽

Ribosomal Protein P0

Abstract In a search for modifiers of gene expression with the white eye color gene as a target, a third chromosomal P-element insertion mutant l(3)01544 has been identified that exhibits a strong pigment increase in a white-apricot background. Molecular analysis shows that the P-element insertion is found in the first intron of the gene surrounding the insertion site. Sequencing both the cDNA and genomic fragments revealed that the identified gene is identical to one encoding ribosomal protein P0/apurinic/apyrimidinic endonuclease. The P-element-induced mutation, l(3)01544, affects the steady-state level of white transcripts and transcripts of some other genes. In addition, l(3)01544 suppresses the variegated phenotypes of In(1)wm4h and In(1)y3P, suggesting a potential involvement of the P0 protein in modifying position effect variegation. The revertant generated by the precise excision of the P element has lost all mutant phenotypes. Recent work revealed that Drosophila ribosomal protein P0 contains an apurinic/apyrimidinic endonuclease activity. Our results suggest that this multifunctional protein is also involved in regulation of gene expression in Drosophila.

Download Full-text

Evidence for intrinsic differences in the formation of chromatin domains in Drosophila melanogaster.

Genetics ◽

10.1093/genetics/132.4.1063 ◽

1992 ◽

Vol 132 (4) ◽

pp. 1063-1069 ◽

Cited By ~ 1

Author(s):

C P Bishop

Keyword(s):

Gene Expression ◽

Drosophila Melanogaster ◽

Genetic Background ◽

Position Effect ◽

The Other ◽

Position Effect Variegation ◽

Compaction Process ◽

Chromatin Domains ◽

Effect Variegation ◽

Reporter Systems

Abstract The results of an investigation into intrinsic differences in the formation of two different heterochromatic domains are presented. The study utilized two different position effect variegation mutants in Drosophila melanogaster for investigating the process of compacting different stretches of DNA into heterochromatin. Each stretch of DNA encodes for a gene that affects different aspects of bristle morphology. The expression of each gene is prevented when it is compacted into heterochromatin thus the genes serve as effective reporter systems to monitor the spread of heterochromatin. Both variegating mutants are scored in the same cell such that environmental and genetic background differences are unambiguously eliminated. Any differences observed in the repression of the two genes must therefore be the result of intrinsic differences in the heterochromatic compaction process for the two stretches of DNA. Studies of the effects different enhancers of variegation have upon the compaction of the two genes indicate each compaction event occurs independently of the other, and that different components are involved in the two processes. These results are discussed with regard to spreading heterochromatin and the role this process may play in regulating gene expression.

Download Full-text

Genetic factors controlling white gene expression of the transposon AR4-24 at a telomere in Drosophila melanogaster

Genome ◽

10.1139/g02-074 ◽

2002 ◽

Vol 45 (6) ◽

pp. 1025-1034 ◽

Cited By ~ 2

Author(s):

M L Balasov

Keyword(s):

Gene Expression ◽

Drosophila Melanogaster ◽

Y Chromosome ◽

Genetic Factors ◽

Position Effect ◽

Position Effect Variegation ◽

Heterochromatin Protein 1 ◽

Heterochromatin Protein ◽

Effect Variegation ◽

Restricted Pattern

The position effect of the AR 4-24 P[white, rosy] transposon was studied at cytological position 60F. Three copies of the transposon (within ~50-kb region) resulted in a spatially restricted pattern of white variegation. This pattern was modified by temperature and by removal of the Y chromosome, suggesting that it was due to classical heterochromatin-induced position effect variegation (PEV). In contrast with classical PEV, extra dose of the heterochromatin protein 1 (HP1) suppressed white variegation and one dose enhanced it. The effect of Pc-G, trx-G, and other PEV suppressors was also tested. It was found that E(Pc)1, TrlR85, and mutations of Su(z)2C relieve AR 4-24- silencing and z1 enhances it. To explain the results obtained with these modifiers, it is proposed that PEV and telomeric position effect can counteract each other at this particular cytological site.Key words: position effect variegation, heterochromatin protein 1, Drosophila melanogaster.

Download Full-text

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.

Genetics ◽

10.1093/genetics/137.4.1057 ◽

1994 ◽

Vol 137 (4) ◽

pp. 1057-1070 ◽

Cited By ~ 1

Author(s):

J A Birchler ◽

U Bhadra ◽

L Rabinow ◽

R Linsk ◽

A T Nguyen-Huynh

Keyword(s):

Gene Expression ◽

Drosophila Melanogaster ◽

Position Effect ◽

Northern Analysis ◽

Regulatory Sequences ◽

Position Effect Variegation ◽

Eye Color ◽

Effect Variegation ◽

Modifying Effect ◽

Color Gene

Abstract 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.

Download Full-text

Dynamics and stability: epigenetic conversions in position effect variegation

Biochemistry and Cell Biology ◽

10.1139/bcb-2012-0048 ◽

2013 ◽

Vol 91 (1) ◽

pp. 6-13 ◽

Cited By ~ 10

Author(s):

Krassimir Yankulov

Keyword(s):

Gene Expression ◽

Saccharomyces Cerevisiae ◽

Position Effect ◽

Specific Gene ◽

Position Effect Variegation ◽

Human Pathogens ◽

Dynamic Nature ◽

Host Environment ◽

Effect Variegation ◽

Transcription And Replication

Position effect variegation (PEV) refers to quasi-stable patterns of gene expression that are observed at specific loci throughout the genomes of eukaryotes. The genes subjected to PEV can be completely silenced or fully active. Stochastic conversions between these 2 states are responsible for the variegated phenotypes. Positional variegation is used by human pathogens (Trypanosoma, Plasmodium, and Candida) to evade the immune system or adapt to the host environment. In the yeasts Saccharomyces cerevisiae and S accharomyces pombe, telomeric PEV aids the adaptation to a changing environment. In metazoans, similar epigenetic conversions are likely to accompany cell differentiation and the setting of tissue-specific gene expression programs. Surprisingly, we know very little about the mechanisms of epigenetic conversions. In this article, earlier models on the nature of PEV are revisited and recent advances on the dynamic nature of chromatin are reviewed. The normal dynamic histone turnover during transcription and DNA replication and its perturbation at transcription and replication pause sites are discussed. It is proposed that such perturbations play key roles in epigenetic conversions and in PEV.

Download Full-text

Regena (Rga), a Drosophila Homolog of the Global Negative Transcriptional Regulator CDC36 (NOT2) from Yeast, Modifies Gene Expression and Suppresses Position Effect Variegation

Genetics ◽

10.1093/genetics/148.1.317 ◽

1998 ◽

Vol 148 (1) ◽

pp. 317-329

Author(s):

Maxim V Frolov ◽

Elizaveta V Benevolenskaya ◽

James A Birchler

Keyword(s):

Gene Expression ◽

Position Effect ◽

Transcriptional Regulator ◽

Binding Motif ◽

Position Effect Variegation ◽

Nucleic Acid Binding ◽

Eye Color ◽

Effect Variegation ◽

Chromatin Proteins ◽

Transcriptional Complex

Abstract A mutation in Regena (Rga) was isolated in screens for modifiers of white eye color gene expression. The reduction in the level of the Rga product results in a complex modulation of white mRNA both positively and negatively, depending on the developmental stage. In addition to white, Rga also affects the expression of several other tested genes, with one of them, Vinculin, being regulated in a strong sex-specific manner. Rga was cloned by transposon tagging. Its predicted product lacks any recognized nucleic acid–binding motif but is homologous to a global negative transcriptional regulator, CDC36 (NOT2), from yeast. Rga also acts as a suppressor of position effect variegation, suggesting that a possible function of Rga could be mediation of an interaction between chromatin proteins and the transcriptional complex.

Download Full-text