Chromosome-wide gene-specific targeting of the Drosophila dosage compensation complex

YY1-mediated enhancer-promoter communication in the immunoglobulin μ locus is regulated by MSL/MOF recruitment

The rearrangement and expression of the immunoglobulin μ heavy chain (Igh) gene require communication of the intragenic Eμ and 3' regulatory region (RR) enhancers with the variable (VH) gene promoter. Eμ binding of the transcription factor YY1 has been implicated in enhancer-promoter communication, but the YY1 protein network remains obscure. By analyzing the comprehensive proteome of the 1-kb Eμ wild-type enhancer and that of Eμ lacking the YY1 binding site, we identified the male-specific lethal (MSL)/MOF complex as a component of the YY1 protein network. We found that MSL2 recruitment depends on YY1 and that gene knockout of Msl2 in primary pre-B cells reduces μ gene expression and chromatin looping of Eμ to the 3' RR enhancer and VH promoter. Moreover, Mof heterozygosity in mice impaired μ expression and early B cell differentiation. Together, these data suggest that the MSL/MOF complex regulates Igh gene expression by augmenting YY1-mediated enhancer-promoter communication.

Identification of X chromatin is modulated by complementary pathways in Drosophila melanogaster

Drosophila melanogaster males have one X chromosome while females have two. This creates an imbalance in X:A gene dosage between the sexes. This imbalance is corrected by increasing transcription from male X-linked genes approximately 2-fold. This process involves the Male-Specific Lethal (MSL) complex, which is recruited to Chromatin Entry Sites (CES) and transcribed X-linked genes, where it modifies chromatin to increase expression. Repetitive sequences strikingly enriched in X euchromatin, the 1.688X satellite repeats, also promote recruitment of the MSL complex to nearby genes. Unlike CES, the 1.688X repeats do not recruit the MSL complex directly. The genetic architecture of recruitment by these DNA elements remains speculative. To facilitate dissection of the mechanism of recruitment, we developed a luciferase reporter system for recruitment of compensation to an autosome. The system was validated by knock down of genes known to participate in compensation. Knock down of factors genetically linked to X recognition reveals that 1.688X repeats recruit through a different mechanism than the CES. Our findings suggest that 1.688X repeats play a larger role during embryogenesis, whereas the contribution of 1.688X repeats and CES is equivalent later in development. Our studies also reveal unexpected complexity and potential interdependence of recruiting elements.

Dosage Compensation in Drosophila: Its Canonical and Non-Canonical Mechanisms

Dosage compensation equalizes gene expression in a single male X chromosome with that in the pairs of autosomes and female X chromosomes. In the fruit fly Drosophila, canonical dosage compensation is implemented by the male-specific lethal (MSL) complex functioning in all male somatic cells. This complex contains acetyl transferase males absent on the first (MOF), which performs H4K16 hyperacetylation specifically in the male X chromosome, thus facilitating transcription of the X-linked genes. However, accumulating evidence points to an existence of additional, non-canonical dosage compensation mechanisms operating in somatic and germline cells. In this review, we discuss current advances in the understanding of both canonical and non-canonical mechanisms of dosage compensation in Drosophila.

Structural insights reveal the specific recognition of roX RNA by the dsRNA-binding domains of the RNA helicase MLE and its indispensable role in dosage compensation in Drosophila

In Drosophila, dosage compensation globally upregulates the expression of genes located on male single X-chromosome. Maleless (MLE) helicase plays an essential role to incorporate the roX lncRNA into the dosage compensation complex (MSL-DCC), and such function is essentially dependent on its dsRNA-binding domains (dsRBDs). Here, we report a 2.90Å crystal structure of tandem dsRBDs of MLE in complex with a 55mer stem-loop of roX2 (R2H1). MLE dsRBDs bind to R2H1 cooperatively and interact with two successive minor grooves and a major groove of R2H1, respectively. The recognition of R2H1 by MLE dsRBDs involves both shape- and sequence-specificity. Moreover, dsRBD₂ displays a stronger RNA affinity than dsRBD₁, and mutations of key residues in either MLE dsRBD remarkably reduce their affinities for roX2 both in vitro and in vivo. In Drosophila, the structure-based mle mutations generated using the CRISPR/Cas9 system, are partially male-lethal and indicate the inter-regulation among the components of the MSL-DCC at multiple levels. Hence, our research provides structural insights into the interactions between MLE dsRBDs and R2H1 and facilitates a deeper understanding of the mechanism by which MLE tandem dsRBDs play an indispensable role in specific recognition of roX and the assembly of the MSL-DCC in Drosophila dosage compensation.

Factor cooperation for chromosome discrimination in Drosophila

Transcription regulators select their genomic binding sites from a large pool of similar, non-functional sequences. Although general principles that allow such discrimination are known, the complexity of DNA elements often precludes a prediction of functional sites. The process of dosage compensation in Drosophila allows exploring the rules underlying binding site selectivity. The male-specific-lethal (MSL) Dosage Compensation Complex (DCC) selectively binds to some 300 X chromosomal ‘High Affinity Sites’ (HAS) containing GA-rich ‘MSL recognition elements’ (MREs), but disregards thousands of other MRE sequences in the genome. The DNA-binding subunit MSL2 alone identifies a subset of MREs, but fails to recognize most MREs within HAS. The ‘Chromatin-linked adaptor for MSL proteins’ (CLAMP) also interacts with many MREs genome-wide and promotes DCC binding to HAS. Using genome-wide DNA-immunoprecipitation we describe extensive cooperativity between both factors, depending on the nature of the binding sites. These are explained by physical interaction between MSL2 and CLAMP. In vivo, both factors cooperate to compete with nucleosome formation at HAS. The male-specific MSL2 thus synergises with a ubiquitous GA-repeat binding protein for refined X/autosome discrimination.

Progressive dosage compensation during Drosophila embryogenesis is reflected by gene arrangement

In Drosophila melanogaster males, X-chromosome monosomy is compensated by chromosome-wide transcription activation. We found that complete dosage compensation during embryogenesis takes surprisingly long and is incomplete even after 10 h of development. Although the activating dosage compensation complex (DCC) associates with the X-chromosome and MOF acetylates histone H4 early, many genes are not compensated. Acetylation levels on gene bodies continue to increase for several hours after gastrulation in parallel with progressive compensation. Constitutive genes are compensated earlier than developmental genes. Remarkably, later compensation correlates with longer distances to DCC binding sites. This time-space relationship suggests that DCC action on target genes requires maturation of the active chromosome compartment.

The X-linked 1.688 Satellite in Drosophila melanogaster Promotes Specific Targeting by Painting of Fourth

Repetitive DNA, represented by transposons and satellite DNA, constitutes a large portion of eukaryotic genomes, being the major component of constitutive heterochromatin. There is a growing body of evidence that it regulates several nuclear functions including chromatin state and the proper functioning of centromeres and telomeres. The 1.688 satellite is one of the most abundant repetitive sequences in Drosophila melanogaster, with the longest array being located in the pericentromeric region of the X-chromosome. Short arrays of 1.688 repeats are widespread within the euchromatic part of the X-chromosome, and these arrays were recently suggested to assist in recognition of the X-chromosome by the dosage compensation male-specific lethal complex. We discovered that a short array of 1.688 satellite repeats is essential for recruitment of the protein POF to a previously described site on the X-chromosome (PoX2) and to various transgenic constructs. On an isolated target, i.e., an autosomic transgene consisting of a gene upstream of 1.688 satellite repeats, POF is recruited to the transgene in both males and females. The sequence of the satellite, as well as its length and position within the recruitment element, are the major determinants of targeting. Moreover, the 1.688 array promotes POF targeting to the roX1-proximal PoX1 site in trans Finally, binding of POF to the 1.688-related satellite-enriched sequences is conserved in evolution. We hypothesize that the 1.688 satellite functioned in an ancient dosage compensation system involving POF targeting to the X-chromosome.

Chromosome topology guides the Drosophila Dosage Compensation Complex for target gene activation

X chromosome dosage compensation in Drosophila requires chromosome-wide coordination of gene activation. The male-specific lethal dosage compensation complex (DCC) identifies and binds to X-chromosomal high-affinity sites (HAS) from which it boosts transcription. A sub-class of HAS, PionX sites, represent first contacts on the X. Here, we explored the chromosomal interactions of representative PionX sites by high-resolution 4C and determined the global chromosome conformation by Hi-C in sex-sorted embryos. Male and female X chromosomes display similar nuclear architecture, concordant with clustered, constitutively active genes. PionX sites, like HAS, are evenly distributed in the active compartment and engage in short- and long-range interactions beyond compartment boundaries. Long-range, inter-domain interactions between DCC binding sites are stronger in males, suggesting that the complex refines chromatin organization. By de novo induction of DCC in female cells, we monitored the extent of activation surrounding PionX sites. This revealed a remarkable range of DCC action not only in linear proximity, but also at megabase distance if close in space, suggesting that DCC profits from pre-existing chromosome folding to activate genes.

Dosage Compensation in Drosophila-a Model for the Coordinate Regulation of Transcription

The sex chromosomes have special significance in the history of genetics. The chromosomal basis of inheritance was firmly established when Calvin Bridges demonstrated that exceptions to Mendel's laws of segregation were accompanied at the cytological level by exceptional sex chromosome segregation. The morphological differences between X and Y exploited in Bridges' experiments arose as a consequence of the evolution of the sex chromosomes. Originally a homologous chromosome pair, the degeneration of the Y chromosome has been accompanied by a requirement for increased expression of the single X chromosome in males. Drosophila has been a model for the study of this dosage compensation and has brought key strengths, including classical genetics, the exceptional cytology of polytene chromosomes, and more recently, comprehensive genomics. The impact of these studies goes beyond sex chromosome regulation, providing valuable insights into mechanisms for the establishment and maintenance of chromatin domains, and for the coordinate regulation of transcription.

Ubiquitylation of the acetyltransferase MOF in Drosophila melanogaster

The nuclear acetyltransferase MOF (KAT8 in mammals) is a subunit of at least two multi-component complexes involved in transcription regulation. In the context of complexes of the ‘Non-Specific-Lethal’ (NSL) type it controls transcription initiation of many nuclear housekeeping genes and of mitochondrial genes. While this function is conserved in metazoans, MOF has an additional, specific function in Drosophila in the context of dosage compensation. As a subunit of the male-specific-lethal dosage compensation complex (MSL-DCC) it contributes to the doubling of transcription output from the single male X chromosome by acetylating histone H4. Proper dosage compensation requires finely tuned levels of MSL-DCC and an appropriate distribution of MOF between the regulatory complexes. The amounts of DCC formed depends directly on the levels of the male-specific MSL2, which orchestrates the assembly of the DCC, including MOF recruitment. We found earlier that MSL2 is an E3 ligase that ubiquitylates most MSL proteins, including MOF, suggesting that ubiquitylation may contribute to a quality control of MOF’s overall levels and folding state as well as its partitioning between the complex entities. We now used mass spectrometry to map the lysines in MOF that are ubiquitylated by MSL2 in vitro and identified in vivo ubiquitylation sites of MOF in male and female cells. MSL2-specific ubiquitylation in vivo could not be traced due to the dominance of other, sex-independent ubiquitylation events and conceivably may be rare or transient. Expressing appropriately mutated MOF derivatives we assessed the importance of the ubiquitylated lysines for dosage compensation by monitoring DCC formation and X chromosome targeting in cultured cells, and by genetic complementation of the male-specific-lethal mof² allele in flies. Our study provides a comprehensive analysis of MOF ubiquitylation as a reference for future studies.

An H4K16 histone acetyltransferase mediates decondensation of the X chromosome in C. elegans males

Background

In C. elegans, in order to equalize gene expression between the sexes and balance X and autosomal expression, two steps are believed to be required. First, an unknown mechanism is hypothesized to upregulate the X chromosome in both sexes. This mechanism balances the X to autosomal expression in males, but creates X overexpression in hermaphrodites. Therefore, to restore the balance, hermaphrodites downregulate gene expression twofold on both X chromosomes. While many studies have focused on X chromosome downregulation, the mechanism of X upregulation is not known.

Results

To gain more insight into X upregulation, we studied the effects of chromatin condensation and histone acetylation on gene expression levels in male C. elegans. We have found that the H4K16 histone acetyltransferase MYS-1/Tip60 mediates dramatic decondensation of the male X chromosome as measured by FISH. However, RNA-seq analysis revealed that MYS-1 contributes only slightly to upregulation of gene expression on the X chromosome. These results suggest that the level of chromosome decondensation does not necessarily correlate with the degree of gene expression change in vivo. Furthermore, the X chromosome is more sensitive to MYS-1-mediated decondensation than the autosomes, despite similar levels of H4K16ac on all chromosomes, as measured by ChIP-seq. H4K16ac levels weakly correlate with gene expression levels on both the X and the autosomes, but highly expressed genes on the X chromosome do not contain exceptionally high levels of H4K16ac.

Conclusion

These results indicate that H4K16ac and chromosome decondensation influence regulation of the male X chromosome; however, they do not fully account for the high levels of gene expression observed on the X chromosomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-016-0097-x) contains supplementary material, which is available to authorized users.

Expansion of GA Dinucleotide Repeats Increases the Density of CLAMP Binding Sites on the X-Chromosome to Promote Drosophila Dosage Compensation

Dosage compensation is an essential process that equalizes transcript levels of X-linked genes between sexes by forming a domain of coordinated gene expression. Throughout the evolution of Diptera, many different X-chromosomes acquired the ability to be dosage compensated. Once each newly evolved X-chromosome is targeted for dosage compensation in XY males, its active genes are upregulated two-fold to equalize gene expression with XX females. In Drosophila melanogaster, the CLAMP zinc finger protein links the dosage compensation complex to the X-chromosome. However, the mechanism for X-chromosome identification has remained unknown. Here, we combine biochemical, genomic and evolutionary approaches to reveal that expansion of GA-dinucleotide repeats likely accumulated on the X-chromosome over evolutionary time to increase the density of CLAMP binding sites, thereby driving the evolution of dosage compensation. Overall, we present new insight into how subtle changes in genomic architecture, such as expansions of a simple sequence repeat, promote the evolution of coordinated gene expression.

Male-Killing Spiroplasma Alters Behavior of the Dosage Compensation Complex during Drosophila melanogaster Embryogenesis

Numerous arthropods harbor maternally transmitted bacteria that induce the preferential death of males [1-7]. This sex-specific lethality benefits the bacteria because males are "dead ends" regarding bacterial transmission, and their absence may result in additional resources for their viable female siblings who can thereby more successfully transmit the bacteria [5]. Although these symbionts disrupt a range of developmental processes [8-10], the underlying cellular mechanisms are largely unknown. It was previously shown that mutations in genes of the dosage compensation pathway of Drosophila melanogaster suppressed male killing caused by the bacterium, Spiroplasma [10]. This result suggested that dosage compensation is a target of Spiroplasma. However, it remains unclear how this pathway is affected, and whether the underlying interactions require the male-specific cellular environment. Here, we investigated the cellular basis of male embryonic lethality in D. melanogaster induced by Spiroplasma. We found that the dosage compensation complex (DCC), which acetylates X chromatin in males [11], becomes mis-localized to ectopic regions of the nucleus immediately prior to the killing phase. This effect was accompanied by inappropriate histone acetylation and genome-wide mis-regulation of gene expression. Artificially induced formation of the DCC in infected females, through transgenic expression of the DCC-specific gene msl-2, resulted in mis-localization of this complex to non-X regions and early Spiroplasma-induced death, mirroring the killing effects in males. These findings strongly suggest that Spiroplasma initiates male killing by targeting the dosage compensation machinery directly and independently of other cellular features characteristic of the male sex.

High-Affinity Sites Form an Interaction Network to Facilitate Spreading of the MSL Complex across the X Chromosome in Drosophila

Dosage compensation mechanisms provide a paradigm to study the contribution of chromosomal conformation toward targeting and spreading of epigenetic regulators over a specific chromosome. By using Hi-C and 4C analyses, we show that high-affinity sites (HAS), landing platforms of the male-specific lethal (MSL) complex, are enriched around topologically associating domain (TAD) boundaries on the X chromosome and harbor more long-range contacts in a sex-independent manner. Ectopically expressed roX1 and roX2 RNAs target HAS on the X chromosome in trans and, via spatial proximity, induce spreading of the MSL complex in cis, leading to increased expression of neighboring autosomal genes. We show that the MSL complex regulates nucleosome positioning at HAS, therefore acting locally rather than influencing the overall chromosomal architecture. We propose that the sex-independent, three-dimensional conformation of the X chromosome poises it for exploitation by the MSL complex, thereby facilitating spreading in males.

Active chromatin and transcription play a key role in chromosome partitioning into topologically associating domains

Recent advances enabled by the Hi-C technique have unraveled many principles of chromosomal folding that were subsequently linked to disease and gene regulation. In particular, Hi-C revealed that chromosomes of animals are organized into topologically associating domains (TADs), evolutionary conserved compact chromatin domains that influence gene expression. Mechanisms that underlie partitioning of the genome into TADs remain poorly understood. To explore principles of TAD folding in Drosophila melanogaster, we performed Hi-C and poly(A)⁺ RNA-seq in four cell lines of various origins (S2, Kc167, DmBG3-c2, and OSC). Contrary to previous studies, we find that regions between TADs (i.e., the inter-TADs and TAD boundaries) in Drosophila are only weakly enriched with the insulator protein dCTCF, while another insulator protein Su(Hw) is preferentially present within TADs. However, Drosophila inter-TADs harbor active chromatin and constitutively transcribed (housekeeping) genes. Accordingly, we find that binding of insulator proteins dCTCF and Su(Hw) predicts TAD boundaries much worse than active chromatin marks do. Interestingly, inter-TADs correspond to decompacted inter-bands of polytene chromosomes, whereas TADs mostly correspond to densely packed bands. Collectively, our results suggest that TADs are condensed chromatin domains depleted in active chromatin marks, separated by regions of active chromatin. We propose the mechanism of TAD self-assembly based on the ability of nucleosomes from inactive chromatin to aggregate, and lack of this ability in acetylated nucleosomal arrays. Finally, we test this hypothesis by polymer simulations and find that TAD partitioning may be explained by different modes of inter-nucleosomal interactions for active and inactive chromatin.

Active promoters give rise to false positive 'Phantom Peaks' in ChIP-seq experiments

Chromatin immunoprecipitation (ChIP) is widely used to identify chromosomal binding sites. Chromatin proteins are cross-linked to their target sequences in living cells. The purified chromatin is sheared and the relevant protein is enriched by immunoprecipitation with specific antibodies. The co-purifying genomic DNA is then determined by massive parallel sequencing (ChIP-seq).

We applied ChIP-seq to map the chromosomal binding sites for two ISWI-containing nucleosome remodeling factors, ACF and RSF, in Drosophila embryos. Employing several polyclonal and monoclonal antibodies directed against their signature subunits, ACF1 and RSF-1, robust profiles were obtained indicating that both remodelers co-occupied a large set of active promoters.

Further validation included controls using chromatin of mutant embryos that do not express ACF1 or RSF-1. Surprisingly, the ChIP-seq profiles were unchanged, suggesting that they were not due to specific immunoprecipitation. Conservative analysis lists about 3000 chromosomal loci, mostly active promoters that are prone to non-specific enrichment in ChIP and appear as ‘Phantom Peaks’. These peaks are not obtained with pre-immune serum and are not prominent in input chromatin.

Mining the modENCODE ChIP-seq profiles identifies potential Phantom Peaks in many profiles of epigenetic regulators. These profiles and other ChIP-seq data featuring prominent Phantom Peaks must be validated with chromatin from cells in which the protein of interest has been depleted.

Dosage compensation in Drosophila

Dosage compensation in Drosophila increases the transcription of genes on the single X chromosome in males to equal that of both X chromosomes in females. Site-specific histone acetylation by the male-specific lethal (MSL) complex is thought to play a fundamental role in the increased transcriptional output of the male X. Nucleation and sequence-independent spreading of the complex to active genes serves as a model for understanding the targeting and function of epigenetic chromatin-modifying complexes. Interestingly, two noncoding RNAs are key for MSL assembly and spreading to active genes along the length of the X chromosome.

Increased expression of X-linked genes in mammals is associated with a higher stability of transcripts and an increased ribosome density

Mammalian sex chromosomes evolved from the degeneration of one homolog of a pair of ancestral autosomes, the proto-Y. This resulted in a gene dose imbalance that is believed to be restored (partially or fully) through upregulation of gene expression from the single active X-chromosome in both sexes by a dosage compensatory mechanism. We analyzed multiple genome-wide RNA stability data sets and found significantly longer average half-lives for X-chromosome transcripts than for autosomal transcripts in various human cell lines, both male and female, and in mice. Analysis of ribosome profiling data shows that ribosome density is higher on X-chromosome transcripts than on autosomal transcripts in both humans and mice, suggesting that the higher stability is causally linked to a higher translation rate. Our results and observations are in accordance with a dosage compensatory upregulation of expressed X-linked genes. We therefore propose that differential mRNA stability and translation rates of the autosomes and sex chromosomes contribute to an evolutionarily conserved dosage compensation mechanism in mammals.

Proximity ligation assays of protein and RNA interactions in the male-specific lethal complex on Drosophila melanogaster polytene chromosomes

In Drosophila, the male-specific lethal (MSL) complex specifically targets the male X chromosome and participates in a twofold increase in expression output leading to functional dosage compensation. The complex includes five proteins and two non-coding RNAs (ncRNAs). A number of additional associated factors have also been identified. However, the components’ roles and interactions have not been fully elucidated. The in situ proximity ligation assay (PLA) provides a sensitive means to determine whether proteins and other factors have bound to chromosomes in close proximity to each other, and thus may interact. Thus, we modified, tested, and applied the assay to probe interactions of MSL complex components on polytene chromosomes. We show that in situ PLA can detect and map both protein-protein and protein-ncRNA interactions on polytene chromosomes at high resolution. We further show that all five protein components of the MSL complex are in close proximity to each other, and the ncRNAs roX1 and roX2 bind the complex in close proximity to MLE. Our results also indicate that JIL1, a histone H3 Ser10 kinase enriched on the male X chromosome, interacts with MSL1 and MSL2, but not MSL3 of the MSL complex. In addition, we corroborate proposed interactions of the MSL complex with both CLAMP and TopoII.

Structural basis of X chromosome DNA recognition by the MSL2 CXC domain during Drosophila dosage compensation

The male-specific lethal dosage compensation complex (MSL-DCC) selectively assembles on the X chromosome in Drosophila and activates gene transcription by twofold through histone acetylation. An MSL recognition element (MRE) sequence motif nucleates the initial MSL association. Here, Zheng et al. identified the CXC domain of MSL2 specifically recognizing the MRE motif and determined its crystal structure bound to specific and nonspecific DNAs. Specific DNA-binding mutants of MSL2 are impaired in MRE binding and X chromosome localization in vivo.

The male-specific lethal dosage compensation complex (MSL-DCC) selectively assembles on the X chromosome in Drosophila males and activates gene transcription by twofold through histone acetylation. An MSL recognition element (MRE) sequence motif nucleates the initial MSL association, but how it is recognized remains unknown. Here, we identified the CXC domain of MSL2 specifically recognizing the MRE motif and determined its crystal structure bound to specific and nonspecific DNAs. The CXC domain primarily contacts one strand of DNA duplex and employs a single arginine to directly read out dinucleotide sequences from the minor groove. The arginine is flexible when bound to nonspecific sequences. The core region of the MRE motif harbors two binding sites on opposite strands that can cooperatively recruit a CXC dimer. Specific DNA-binding mutants of MSL2 are impaired in MRE binding and X chromosome localization in vivo. Our results reveal multiple dynamic DNA-binding modes of the CXC domain that target the MSL-DCC to X chromosomes.

Mechanisms of x chromosome dosage compensation

In many animals, males have one X and females have two X chromosomes. The difference in X chromosome dosage between the two sexes is compensated by mechanisms that regulate X chromosome transcription. Recent advances in genomic techniques have provided new insights into the molecular mechanisms of X chromosome dosage compensation. In this review, I summarize our current understanding of dosage imbalance in general, and then review the molecular mechanisms of X chromosome dosage compensation with an emphasis on the parallels and differences between the three well-studied model systems, M. musculus, D. melanogaster and C. elegans.

Non-coding roX RNAs prevent the binding of the MSL-complex to heterochromatic regions

Long non-coding RNAs contribute to dosage compensation in both mammals and Drosophila by inducing changes in the chromatin structure of the X-chromosome. In Drosophila melanogaster, roX1 and roX2 are long non-coding RNAs that together with proteins form the male-specific lethal (MSL) complex, which coats the entire male X-chromosome and mediates dosage compensation by increasing its transcriptional output. Studies on polytene chromosomes have demonstrated that when both roX1 and roX2 are absent, the MSL-complex becomes less abundant on the male X-chromosome and is relocated to the chromocenter and the 4^th chromosome. Here we address the role of roX RNAs in MSL-complex targeting and the evolution of dosage compensation in Drosophila. We performed ChIP-seq experiments which showed that MSL-complex recruitment to high affinity sites (HAS) on the X-chromosome is independent of roX and that the HAS sequence motif is conserved in D. simulans. Additionally, a complete and enzymatically active MSL-complex is recruited to six specific genes on the 4^th chromosome. Interestingly, our sequence analysis showed that in the absence of roX RNAs, the MSL-complex has an affinity for regions enriched in Hoppel transposable elements and repeats in general. We hypothesize that roX mutants reveal the ancient targeting of the MSL-complex and propose that the role of roX RNAs is to prevent the binding of the MSL-complex to heterochromatin.

In both fruit flies and humans, males and females have different sets of sex chromosomes. This generates differences in gene dosage that must be compensated for by adjusting the transcriptional output of most genes located on the X-chromosome. The specific recognition and targeting of the X-chromosome is essential for such dosage compensation. In fruit flies, dosage compensation is mediated by the male-specific lethal (MSL) complex, which upregulates gene transcription on the male X-chromosome. The MSL-complex consists of five proteins and two non-coding RNAs, roX1 and roX2. While non-coding RNAs are known to be critical for dosage compensation in both flies and mammals, their precise functions remain elusive. Here we present a study on the targeting and function of the MSL-complex in the absence of roX RNAs. The results obtained suggest that the dosage compensating MSL-complex has an intrinsic tendency to target repeat-rich regions and that the function of roX RNAs is to prevent its binding to such targets. Our findings reveal an ancient targeting and regulatory function of the MSL-complex that has been adapted for use in dosage compensation and modified by the rapidly evolving noncoding roX RNAs.

Stochastic cellular fate decision making by multiple infecting lambda phage

Bacteriophage lambda is a classic system for the study of cellular decision making. Both experiments and mathematical models have demonstrated the importance of viral concentration in the lysis-lysogeny decision outcome in lambda phage. However, a recent experimental study using single cell and single phage resolution reported that cells with the same viral concentrations but different numbers of infecting phage (multiplicity of infection) can have markedly different rates of lysogeny. Thus the decision depends on not only viral concentration, but also directly on the number of infecting phage. Here, we attempt to provide a mechanistic explanation of these results using a simple stochastic model of the lambda phage genetic network. Several potential factors including intrinsic gene expression noise, spatial dynamics and cell-cycle effects are investigated. We find that interplay between the level of intrinsic noise and viral protein decision threshold is a major factor that produces dependence on multiplicity of infection. However, simulations suggest spatial segregation of phage particles does not play a significant role. Cellular image processing is used to re-analyse the original time-lapse movies from the recent study and it is found that higher numbers of infecting phage reduce the cell elongation rate. This could also contribute to the observed phenomena as cellular growth rate can affect transcription rates. Our model further predicts that rate of lysogeny is dependent on bacterial growth rate, which can be experimentally tested. Our study provides new insight on the mechanisms of individual phage decision making. More generally, our results are relevant for the understanding of gene-dosage compensation in cellular systems.

H2B ubiquitylation promotes RNA Pol II processivity via PAF1 and pTEFb

Histone H2B ubiquitination plays an important role in transcription regulation. It has been shown that H2B ubiquitination is regulated by multiple upstream events associated with elongating RNA polymerase. Here we demonstrate that H2B K34 ubiquitylation by the MOF-MSL complex is part of the protein networks involved in early steps of transcription elongation. Knocking down MSL2 in the MOF-MSL complex affects not only global H2BK34ub, but also multiple cotranscriptionally regulated histone modifications. More importantly, we show that the MSL, PAF1, and RNF20/40 complexes are recruited and stabilized at active gene promoters by direct binary interactions. The stabilized complexes serve to regulate chromatin association of pTEFb through a positive feedback loop and facilitate Pol II transition during early transcription elongation. Results from our biochemical studies are underscored by genome-wide analyses that show high RNA Pol II processivity and transcription activity at MSL target genes.

Transcriptional control of a whole chromosome: emerging models for dosage compensation

Males and females of many animal species differ in their sex-chromosome karyotype, and this creates imbalances between X-chromosome and autosomal gene products that require compensation. Although distinct molecular mechanisms have evolved in three highly studied systems, they all achieve coordinate regulation of an entire chromosome by differential RNA-polymerase occupancy at X-linked genes. High-throughput genome-wide methods have been pivotal in driving the latest progress in the field. Here we review the emerging models for dosage compensation in mammals, flies and nematodes, with a focus on mechanisms affecting RNA polymerase II activity on the X chromosome.

"Jump start and gain" model for dosage compensation in Drosophila based on direct sequencing of nascent transcripts

Dosage compensation in Drosophila is mediated by the MSL complex, which increases male X-linked gene expression approximately 2-fold. The MSL complex preferentially binds the bodies of active genes on the male X, depositing H4K16ac with a 3' bias. Two models have been proposed for the influence of the MSL complex on transcription: one based on promoter recruitment of RNA polymerase II (Pol II), and a second featuring enhanced transcriptional elongation. Here, we utilize nascent RNA sequencing to document dosage compensation during transcriptional elongation. We also compare X and autosomes from published data on paused and elongating polymerase in order to assess the role of Pol II recruitment. Our results support a model for differentially regulated elongation, starting with release from 5' pausing and increasing through X-linked gene bodies. Our results highlight facilitated transcriptional elongation as a key mechanism for the coordinated regulation of a diverse set of genes.

Male X-linked genes in Drosophila melanogaster are compensated independently of the Male-Specific Lethal complex

Background

In organisms where the two sexes have unequal numbers of X-chromosomes, the expression of X-linked genes needs to be balanced not only between the two sexes, but also between X and the autosomes. In Drosophila melanogaster, the Male-Specific Lethal (MSL) complex is believed to produce a 2-fold increase in expression of genes on the male X, thus restoring this balance.

Results

Here we show that almost all the genes on the male X are effectively compensated. However, many genes are compensated without any significant recruitment of the MSL-complex. These genes are very weakly, if at all, affected by mutations or RNAi against MSL-complex components. In addition, even the genes that are strongly bound by MSL rely on mechanisms other than the MSL-complex for proper compensation. We find that long, non-ubiquitously expressed genes tend to rely less on the MSL-complex for their compensation and genes that in addition are far from High Affinity Sites tend to not bind the complex at all or very weakly.

Conclusions

We conclude that most of the compensation of X-linked genes is produced by an MSL-independent mechanism. Similar to the case of the MSL-mediated compensation we do not yet know the mechanism behind the MSL-independent compensation that appears to act preferentially on long genes. Even if we observe similarities, it remains to be seen if the mechanism is related to the buffering that is observed in autosomal aneuploidies.

Conservation and de novo acquisition of dosage compensation on newly evolved sex chromosomes in Drosophila

Dosage compensation has arisen in response to the evolution of distinct male (XY) and female (XX) karyotypes. In Drosophila melanogaster, the MSL complex increases male X transcription approximately twofold. X-specific targeting is thought to occur through sequence-dependent binding to chromatin entry sites (CESs), followed by spreading in cis to active genes. We tested this model by asking how newly evolving sex chromosome arms in Drosophila miranda acquired dosage compensation. We found evidence for the creation of new CESs, with the analogous sequence and spacing as in D. melanogaster, providing strong support for the spreading model in the establishment of dosage compensation.

Condensin controls recruitment of RNA polymerase II to achieve nematode X-chromosome dosage compensation

The X-chromosome gene regulatory process called dosage compensation ensures that males (1X) and females (2X) express equal levels of X-chromosome transcripts. The mechanism in Caenorhabditis elegans has been elusive due to improperly annotated transcription start sites (TSSs). Here we define TSSs and the distribution of transcriptionally engaged RNA polymerase II (Pol II) genome-wide in wild-type and dosage-compensation-defective animals to dissect this regulatory mechanism. Our TSS-mapping strategy integrates GRO-seq, which tracks nascent transcription, with a new derivative of this method, called GRO-cap, which recovers nascent RNAs with 5' caps prior to their removal by co-transcriptional processing. Our analyses reveal that promoter-proximal pausing is rare, unlike in other metazoans, and promoters are unexpectedly far upstream from the 5' ends of mature mRNAs. We find that C. elegans equalizes X-chromosome expression between the sexes, to a level equivalent to autosomes, by reducing Pol II recruitment to promoters of hermaphrodite X-linked genes using a chromosome-restructuring condensin complex. DOI:http://dx.doi.org/10.7554/eLife.00808.001.

Mammalian X upregulation is associated with enhanced transcription initiation, RNA half-life, and MOF-mediated H4K16 acetylation

X upregulation in mammals increases levels of expressed X-linked transcripts to compensate for autosomal biallelic expression. Here, we present molecular mechanisms that enhance X expression at transcriptional and posttranscriptional levels. Active mouse X-linked promoters are enriched in the initiation form of RNA polymerase II (PolII-S5p) and in specific histone marks, including histone H4 acetylated at lysine 16 (H4K16ac) and histone variant H2AZ. The H4K16 acetyltransferase males absent on the first (MOF), known to mediate the Drosophila X upregulation, is also enriched on the mammalian X. Depletion of MOF or male-specific lethal 1 (MSL1) in mouse ES cells causes a specific decrease in PolII-S5p and in expression of a subset of X-linked genes. Analyses of RNA half-life data sets show increased stability of mammalian X-linked transcripts. Both ancestral X-linked genes, defined as those conserved on chicken autosomes, and newly acquired X-linked genes are upregulated by similar mechanisms but to a different extent, suggesting that subsets of genes are distinctly regulated depending on their evolutionary history.

Chromatin proteins captured by ChIP-mass spectrometry are linked to dosage compensation in Drosophila

X-chromosome dosage compensation by the MSL (male-specific lethal) complex is required in Drosophila melanogaster to increase gene expression from the single male X to equal that of both female X chromosomes. Instead of focusing solely on protein complexes released from DNA, here we used chromatin-interacting protein MS (ChIP-MS) to identify MSL interactions on cross-linked chromatin. We identified MSL-enriched histone modifications, including histone H4 Lys16 acetylation and histone H3 Lys36 methylation, and CG4747, a putative Lys36-trimethylated histone H3 (H3K36me3)-binding protein. CG4747 is associated with the bodies of active genes, coincident with H3K36me3, and is mislocalized in the Set2 mutant lacking H3K36me3. CG4747 loss of function in vivo results in partial mislocalization of the MSL complex to autosomes, and RNA interference experiments confirm that CG4747 and Set2 function together to facilitate targeting of the MSL complex to active genes, validating the ChIP-MS approach.

Different chromatin interfaces of the Drosophila dosage compensation complex revealed by high-shear ChIP-seq

Transcriptional enhancement of X-linked genes to compensate for the sex chromosome monosomy in Drosophila males is brought about by a ribonucleoprotein assembly called Male-Specific-Lethal or Dosage Compensation Complex (MSL-DCC). This machinery is formed in male flies and specifically associates with active genes on the X chromosome. After assembly at dedicated high-affinity "entry" sites (HAS) on the X chromosome, the complex distributes to the nearby active chromatin. High-resolution, genome-wide mapping of the MSL-DCC subunits by chromatin immunoprecipitation (ChIP) on oligonucleotide tiling arrays suggests a rather homogenous spreading of the intact complex onto transcribed chromatin. Coupling ChIP to deep sequencing (ChIP-seq) promises to map the chromosomal interactions of the DCC with improved resolution. We present ChIP-seq binding profiles for all complex subunits, including the first description of the RNA helicase MLE binding pattern. Exploiting the preferential representation of direct chromatin contacts upon high-energy shearing, we report a surprising functional and topological separation of MSL protein contacts at three classes of chromosomal binding sites. Furthermore, precise determination of DNA fragment lengths by paired-end ChIP-seq allows decrypting of the local complex architecture. Primary contacts of MSL-2 and MLE define HAS for the DCC. In contrast, association of the DCC with actively transcribed gene bodies is mediated by MSL-3 binding to nucleosomes. We identify robust MSL-1/MOF binding at a fraction of active promoters genome-wide. Correlation analyses suggest that this association reflects a function outside dosage compensation. Our comprehensive analysis provides a new level of information on different interaction modes of a multiprotein complex at distinct regions within the genome.

Mutations in the transcription elongation factor SPT5 disrupt a reporter for dosage compensation in Drosophila

In Drosophila, the MSL (Male Specific Lethal) complex up regulates transcription of active genes on the single male X-chromosome to equalize gene expression between sexes. One model argues that the MSL complex acts upon the elongation step of transcription rather than initiation. In an unbiased forward genetic screen for new factors required for dosage compensation, we found that mutations in the universally conserved transcription elongation factor Spt5 lower MSL complex dependent expression from the miniwhite reporter gene in vivo. We show that SPT5 interacts directly with MSL1 in vitro and is required downstream of MSL complex recruitment, providing the first mechanistic data corroborating the elongation model of dosage compensation.

Drosophila males hypertranscribe most of the genes along their single X chromosome to match the output of females with two X chromosomes. It had been difficult to imagine how the MSL dosage compensation complex could impose a modest, but essential, ∼two-fold increase by interacting with hundreds of different factors that control transcription initiation for such a diverse collection of genes. An alternative model proposed that dosage compensation instead acted at some step of transcription elongation common to all genes. We performed a genetic screen for mutations that subtly reduce dosage compensation and recovered mutations in the Spt5 gene that encodes a universally conserved elongation factor. SPT5 closes the RNA polymerase II clamp around the DNA template to prevent pausing or premature termination. We find that the dosage compensation complex genetically and physically interacts with SPT5 on actively transcribed genes providing direct molecular support for the elongation model of dosage compensation.

Faster-X evolution of gene expression in Drosophila

DNA sequences on X chromosomes often have a faster rate of evolution when compared to similar loci on the autosomes, and well articulated models provide reasons why the X-linked mode of inheritance may be responsible for the faster evolution of X-linked genes. We analyzed microarray and RNA-seq data collected from females and males of six Drosophila species and found that the expression levels of X-linked genes also diverge faster than autosomal gene expression, similar to the "faster-X" effect often observed in DNA sequence evolution. Faster-X evolution of gene expression was recently described in mammals, but it was limited to the evolutionary lineages shortly following the creation of the therian X chromosome. In contrast, we detect a faster-X effect along both deep lineages and those on the tips of the Drosophila phylogeny. In Drosophila males, the dosage compensation complex (DCC) binds the X chromosome, creating a unique chromatin environment that promotes the hyper-expression of X-linked genes. We find that DCC binding, chromatin environment, and breadth of expression are all predictive of the rate of gene expression evolution. In addition, estimates of the intraspecific genetic polymorphism underlying gene expression variation suggest that X-linked expression levels are not under relaxed selective constraints. We therefore hypothesize that the faster-X evolution of gene expression is the result of the adaptive fixation of beneficial mutations at X-linked loci that change expression level in cis. This adaptive faster-X evolution of gene expression is limited to genes that are narrowly expressed in a single tissue, suggesting that relaxed pleiotropic constraints permit a faster response to selection. Finally, we present a conceptional framework to explain faster-X expression evolution, and we use this framework to examine differences in the faster-X effect between Drosophila and mammals.

Distinct contributions of MSL complex subunits to the transcriptional enhancement responsible for dosage compensation in Drosophila

The regulatory mechanism of dosage compensation is the paramount example of epigenetic regulation at the chromosomal level. In Drosophila, this mechanism, designed to compensate for the difference in the dosage of X-linked genes between the sexes, depends on the MSL complex that enhances the transcription of the single dose of these genes in males. We have investigated the function of various subunits of the complex in mediating dosage compensation. Our results confirm that the highly enriched specific acetylation of histone H4 at lysine 16 of compensated genes by the histone acetyl transferase subunit MOF induces a more disorganized state of their chromatin. We have determined that the association of the MSL complex reduces the level of negative supercoiling of the deoxyribonucleic acid of compensated genes, and we have defined the role that the other subunits of the complex play in this topological modification. Lastly, we have analyzed the potential contribution of ISWI-containing remodeling complexes to the architecture of compensated chromatin, and we suggest a role for this remodeling factor in dosage compensation.

Solution structure of MSL2 CXC domain reveals an unusual Zn3Cys9 cluster and similarity to pre-SET domains of histone lysine methyltransferases

The dosage compensation complex (DCC) binds to single X chromosomes in Drosophila males and increases the transcription level of X-linked genes by approximately twofold. Male-specific lethal 2 (MSL2) together with MSL1 mediates the initial recruitment of the DCC to high-affinity sites in the X chromosome. MSL2 contains a DNA-binding cysteine-rich CXC domain that is important for X targeting. In this study, we determined the solution structure of MSL2 CXC domain by NMR spectroscopy. We identified three zinc ions in the CXC domain and determined the metal-to-cysteine connectivities from ¹H-¹¹³Cd correlation experiments. The structure reveals an unusual zinc-cysteine cluster composed of three zinc ions coordinated by six terminal and three bridging cysteines. The CXC domain exhibits unexpected structural homology to pre-SET motifs of histone lysine methyltransferases, expanding the distribution and structural diversity of the CXC domain superfamily. Our findings provide novel structural insight into the evolution and function of CXC domains.

Identification of chromatin-associated regulators of MSL complex targeting in Drosophila dosage compensation

Sex chromosome dosage compensation in Drosophila provides a model for understanding how chromatin organization can modulate coordinate gene regulation. Male Drosophila increase the transcript levels of genes on the single male X approximately two-fold to equal the gene expression in females, which have two X-chromosomes. Dosage compensation is mediated by the Male-Specific Lethal (MSL) histone acetyltransferase complex. Five core components of the MSL complex were identified by genetic screens for genes that are specifically required for male viability and are dispensable for females. However, because dosage compensation must interface with the general transcriptional machinery, it is likely that identifying additional regulators that are not strictly male-specific will be key to understanding the process at a mechanistic level. Such regulators would not have been recovered from previous male-specific lethal screening strategies. Therefore, we have performed a cell culture-based, genome-wide RNAi screen to search for factors required for MSL targeting or function. Here we focus on the discovery of proteins that function to promote MSL complex recruitment to “chromatin entry sites,” which are proposed to be the initial sites of MSL targeting. We find that components of the NSL (Non-specific lethal) complex, and a previously unstudied zinc-finger protein, facilitate MSL targeting and display a striking enrichment at MSL entry sites. Identification of these factors provides new insight into how MSL complex establishes the specialized hyperactive chromatin required for dosage compensation in Drosophila.

Gene regulation is essential to all living things. For example, levels of gene expression in individual cells must be fine-tuned during development and in response to changing environmental conditions. Genes are regulated by DNA binding proteins and by factors that influence DNA packaging into chromatin. The MSL complex in Drosophila melanogaster is a chromatin-modifying complex that specifically regulates a large number of genes. The MSL complex targets active genes on the single male X chromosome to upregulate their output to match both female X chromosomes. How the MSL complex specifically targets the X chromosome and upregulates active genes is only partially understood. In order to increase our understanding of gene regulation at a mechanistic level, we performed a genome-wide genetic screen in male cells to identify factors that facilitate MSL targeting and function. Our results identify two chromatin-associated protein complexes and a new candidate DNA binding protein as key factors in MSL–based regulation. We also provide an extensive list of additional candidate genes to be examined in future studies.

POF regulates the expression of genes on the fourth chromosome in Drosophila melanogaster by binding to nascent RNA

In Drosophila, two chromosome-wide compensatory systems have been characterized: the dosage compensation system that acts on the male X chromosome and the chromosome-specific regulation of genes located on the heterochromatic fourth chromosome. Dosage compensation in Drosophila is accomplished by hypertranscription of the single male X chromosome mediated by the male-specific lethal (MSL) complex. The mechanism of this compensation is suggested to involve enhanced transcriptional elongation mediated by the MSL complex, while the mechanism of compensation mediated by the painting of fourth (POF) protein on the fourth chromosome has remained elusive. Here, we show that POF binds to nascent RNA, and this binding is associated with increased transcription output from chromosome 4. We also show that genes located in heterochromatic regions spend less time in transition from the site of transcription to the nuclear envelope. These results provide useful insights into the means by which genes in heterochromatic regions can overcome the repressive influence of their hostile environment.

Sequence signatures involved in targeting the Male-Specific Lethal complex to X-chromosomal genes in Drosophila melanogaster

Background

In Drosophila melanogaster, the dosage-compensation system that equalizes X-linked gene expression between males and females, thereby assuring that an appropriate balance is maintained between the expression of genes on the X chromosome(s) and the autosomes, is at least partially mediated by the Male-Specific Lethal (MSL) complex. This complex binds to genes with a preference for exons on the male X chromosome with a 3' bias, and it targets most expressed genes on the X chromosome. However, a number of genes are expressed but not targeted by the complex. High affinity sites seem to be responsible for initial recruitment of the complex to the X chromosome, but the targeting to and within individual genes is poorly understood.

Results

We have extensively examined X chromosome sequence variation within five types of gene features (promoters, 5' UTRs, coding sequences, introns, 3' UTRs) and intergenic sequences, and assessed its potential involvement in dosage compensation. Presented results show that: the X chromosome has a distinct sequence composition within its gene features; some of the detected variation correlates with genes targeted by the MSL-complex; the insulator protein BEAF-32 preferentially binds upstream of MSL-bound genes; BEAF-32 and MOF co-localizes in promoters; and that bound genes have a distinct sequence composition that shows a 3' bias within coding sequence.

Conclusions

Although, many strongly bound genes are close to a high affinity site neither our promoter motif nor our coding sequence signatures show any correlation to HAS. Based on the results presented here, we believe that there are sequences in the promoters and coding sequences of targeted genes that have the potential to direct the secondary spreading of the MSL-complex to nearby genes.

Autoregulation of the Drosophila Noncoding roX1 RNA Gene

Most genes along the male single X chromosome in Drosophila are hypertranscribed about two-fold relative to each of the two female X chromosomes. This is accomplished by the MSL (male-specific lethal) complex that acetylates histone H4 at lysine 16. The MSL complex contains two large noncoding RNAs, roX1 (RNA on X) and roX2, that help target chromatin modifying enzymes to the X. The roX RNAs are functionally redundant but differ in size, sequence, and transcriptional control. We wanted to find out how roX1 production is regulated. Ectopic DC can be induced in wild-type (roX1(+) roX2(+)) females if we provide a heterologous source of MSL2. However, in the absence of roX2, we found that roX1 expression failed to come on reliably. Using an in situ hybridization probe that is specific only to endogenous roX1, we found that expression was restored if we introduced either roX2 or a truncated but functional version of roX1. This shows that pre-existing roX RNA is required to positively autoregulate roX1 expression. We also observed massive cis spreading of the MSL complex from the site of roX1 transcription at its endogenous location on the X chromosome. We propose that retention of newly assembled MSL complex around the roX gene is needed to drive sustained transcription and that spreading into flanking chromatin contributes to the X chromosome targeting specificity. Finally, we found that the gene encoding the key male-limited protein subunit, msl2, is transcribed predominantly during DNA replication. This suggests that new MSL complex is made as the chromatin template doubles. We offer a model describing how the production of roX1 and msl2, two key components of the MSL complex, are coordinated to meet the dosage compensation demands of the male cell.

Dosage compensation in Drosophila melanogaster: epigenetic fine-tuning of chromosome-wide transcription

Dosage compensation is an epigenetic mechanism that normalizes gene expression from unequal copy numbers of sex chromosomes. Different organisms have evolved alternative molecular solutions to this task. In Drosophila melanogaster, transcription of the single male X chromosome is upregulated by twofold in a process orchestrated by the dosage compensation complex. Despite this conceptual simplicity, dosage compensation involves multiple coordinated steps to recognize and activate the entire X chromosome. We are only beginning to understand the intriguing interplay between multiple levels of local and long-range chromatin regulation required for the fine-tuned transcriptional activation of a heterogeneous gene population. This Review highlights the known facts and open questions of dosage compensation in D. melanogaster.

Roles of long, non-coding RNA in chromosome-wide transcription regulation: lessons from two dosage compensation systems

A large part of higher eukaryotic genomes is transcribed into RNAs lacking any significant open reading frame. This "non-coding part" has been shown to actively contribute to regulating gene expression, but the mechanisms are largely unknown. Particularly instructive examples are provided by the dosage compensation systems, which assure that the single X chromosome in male cells and the two X chromosomes in female cells give rise to similar amounts of gene product. Although this is achieved by very different strategies in mammals and fruit flies, long, non-coding RNAs (lncRNAs) are involved in both cases. Here we summarize recent progress towards unraveling the mechanisms, by which the Xist and roX RNAs mediate the selective association of regulators with individual target chromosomes, to initiate dosage compensation in mammals and fruit flies, respectively.

Balancing sex chromosome expression and satisfying the sexes

Equalizing sex chromosome expression between the sexes when they have largely differing gene content appears to be necessary, and across species, is accomplished in a variety of ways. Even in birds, where the process is less than complete, a mechanism to reduce the difference in gene dose between the sexes exists. In early development, while the dosage difference is unregulated and still in flux, it is frequently exploited by sex determination mechanisms. The Drosophila female sex determination process is one clear example, determining the sexes based on X chromosome dose. Recent data show that in Drosophila, the female sex not only reads this gene balance difference, but at the same time usurps the moment. Taking advantage of the transient default state of male dosage compensation, the sex determination master-switch Sex-lethal which resides on the X, has its expression levels enhanced before it works to correct the gene imbalance. Intriguingly, key developmental genes which could create developmental havoc if their levels were unbalanced show more exquisite regulation, suggesting nature distinguishes them and ensures their expression is kept in the desirable range.

The genomic binding sites of a noncoding RNA

Long noncoding RNAs (lncRNAs) have important regulatory roles and can function at the level of chromatin. To determine where lncRNAs bind to chromatin, we developed capture hybridization analysis of RNA targets (CHART), a hybridization-based technique that specifically enriches endogenous RNAs along with their targets from reversibly cross-linked chromatin extracts. CHART was used to enrich the DNA and protein targets of endogenous lncRNAs from flies and humans. This analysis was extended to genome-wide mapping of roX2, a well-studied ncRNA involved in dosage compensation in Drosophila. CHART revealed that roX2 binds at specific genomic sites that coincide with the binding sites of proteins from the male-specific lethal complex that affects dosage compensation. These results reveal the genomic targets of roX2 and demonstrate how CHART can be used to study RNAs in a manner analogous to chromatin immunoprecipitation for proteins.

X-chromosome hyperactivation in mammals via nonlinear relationships between chromatin states and transcription

Dosage compensation in mammals occurs at two levels. In addition to balancing X-chromosome dosage between males and females via X-inactivation, mammals also balance dosage of Xs and autosomes. It has been proposed that X-autosome equalization occurs by upregulation of Xa (active X). To investigate mechanism, we perform allele-specific ChIP-seq for chromatin epitopes and analyze RNA-seq data. The hypertranscribed Xa demonstrates enrichment of active chromatin marks relative to autosomes. We derive predictive models for relationships among POL-II, active mark densities, and gene expression, and suggest that Xa upregulation involves increased transcription initiation and elongation. Enrichment of active marks on Xa does not scale proportionally with transcription output, a disparity explained by nonlinear quantitative dependencies among active histone marks, POL-II occupancy, and transcription. Significantly, the trend of nonlinear upregulation also occurs on autosomes. Thus, Xa upregulation involves combined increases of active histone marks and POL-II occupancy, without invoking X-specific dependencies between chromatin states and transcription.

The MOF-containing NSL complex associates globally with housekeeping genes, but activates only a defined subset

The MOF (males absent on the first)-containing NSL (non-specific lethal) complex binds to a subset of active promoters in Drosophila melanogaster and is thought to contribute to proper gene expression. The determinants that target NSL to specific promoters and the circumstances in which the complex engages in regulating transcription are currently unknown. Here, we show that the NSL complex primarily targets active promoters and in particular housekeeping genes, at which it colocalizes with the chromatin remodeler NURF (nucleosome remodeling factor) and the histone methyltransferase Trithorax. However, only a subset of housekeeping genes associated with NSL are actually activated by it. Our analyses reveal that these NSL-activated promoters are depleted of certain insulator binding proteins and are enriched for the core promoter motif ‘Ohler 5’. Based on these results, it is possible to predict whether the NSL complex is likely to regulate a particular promoter. We conclude that the regulatory capacity of the NSL complex is highly context-dependent. Activation by the NSL complex requires a particular promoter architecture defined by combinations of chromatin regulators and core promoter motifs.

Evidence for compensatory upregulation of expressed X-linked genes in mammals, Caenorhabditis elegans and Drosophila melanogaster

Many animal species use a chromosome-based mechanism of sex determination, which has led to the coordinate evolution of dosage-compensation systems. Dosage compensation not only corrects the imbalance in the number of X chromosomes between the sexes but also is hypothesized to correct dosage imbalance within cells that is due to monoallelic X-linked expression and biallelic autosomal expression, by upregulating X-linked genes twofold (termed 'Ohno's hypothesis'). Although this hypothesis is well supported by expression analyses of individual X-linked genes and by microarray-based transcriptome analyses, it was challenged by a recent study using RNA sequencing and proteomics. We obtained new, independent RNA-seq data, measured RNA polymerase distribution and reanalyzed published expression data in mammals, C. elegans and Drosophila. Our analyses, which take into account the skewed gene content of the X chromosome, support the hypothesis of upregulation of expressed X-linked genes to balance expression of the genome.