Histone H2B ubiquitylation is associated with elongating RNA polymerase II

Control of V(D)J Recombination through Transcriptional Elongation and Changes in Locus Chromatin Structure and Nuclear Organization

V(D)J recombination is the assembly of gene segments at the antigen receptor loci to generate antigen receptor diversity in T and B lymphocytes. This process is regulated, according to defined developmental programs, by the action of a single specific recombinase complex formed by the recombination antigen gene (RAG-1/2) proteins that are expressed in immature lymphocytes. V(D)J recombination is strictly controlled by RAG-1/2 accessibility to specific recombination signal sequences in chromatin at several levels: cellular lineage, temporal regulation, gene segment order, and allelic exclusion. DNA cleavage by RAG-1/2 is regulated by the chromatin structure, transcriptional elongation, and three-dimensional architecture and position of the antigen receptor loci in the nucleus. Cis-elements specifically direct transcription and V(D)J recombination at these loci through interactions with transacting factors that form molecular machines that mediate a sequence of structural events. These events open chromatin to activate transcriptional elongation and to permit the access of RAG-1/2 to their recombination signal sequences to drive the juxtaposition of the V, D, and J segments and the recombination reaction itself. This chapter summarizes the advances in this area and the important role of the structure and position of antigen receptor loci within the nucleus to control this process.

The RING finger protein MSL2 in the MOF complex is an E3 ubiquitin ligase for H2B K34 and is involved in crosstalk with H3 K4 and K79 methylation

We demonstrate that RING finger protein MSL2 in the MOF-MSL complex is a histone ubiquitin E3 ligase. MSL2, together with MSL1, has robust histone ubiquitylation activity that mainly targets nucleosomal H2B on lysine 34 (H2B K34ub), a site within a conserved basic patch on H2B tail. H2B K34ub by MSL1/2 directly regulates H3 K4 and K79 methylation through trans-tail crosstalk both in vitro and in cells. The significance of MSL1/2-mediated histone H2B ubiquitylation is underscored by the facts that MSL1/2 activity is important for transcription activation at HOXA9 and MEIS1 loci and that this activity is evolutionarily conserved in the Drosophila dosage compensation complex. Altogether, these results indicate that the MOF-MSL complex possesses two distinct chromatin-modifying activities (i.e., H4 K16 acetylation and H2B K34 ubiquitylation) through MOF and MSL2 subunits. They also shed light on how an intricate network of chromatin-modifying enzymes functions coordinately in gene activation.

The Paf1 complex represses SER3 transcription in Saccharomyces cerevisiae by facilitating intergenic transcription-dependent nucleosome occupancy of the SER3 promoter

Previous studies have shown that repression of the Saccharomyces cerevisiae SER3 gene is dependent on transcription of SRG1 from noncoding DNA initiating within the intergenic region 5' of SER3 and extending across the SER3 promoter region. By a mechanism dependent on the activities of the Swi/Snf chromatin remodeling factor, the HMG-like factor Spt2, and the Spt6 and Spt16 histone chaperones, SRG1 transcription deposits nucleosomes over the SER3 promoter to prevent transcription factors from binding and activating SER3. In this study, we uncover a role for the Paf1 transcription elongation complex in SER3 repression. We find that SER3 repression is primarily dependent on the Paf1 and Ctr9 subunits of this complex, with minor contributions by the Rtf1, Cdc73, and Leo1 subunits. We show that the Paf1 complex localizes to the SRG1 transcribed region under conditions that repress SER3, consistent with it having a direct role in mediating SRG1 transcription-dependent SER3 repression. Importantly, we show that the defect in SER3 repression in strains lacking Paf1 subunits is not a result of reduced SRG1 transcription or reduced levels of known Paf1 complex-dependent histone modifications. Rather, we find that strains lacking subunits of the Paf1 complex exhibit reduced nucleosome occupancy and reduced recruitment of Spt16 and, to a lesser extent, Spt6 at the SER3 promoter. Taken together, our results suggest that Paf1 and Ctr9 repress SER3 by maintaining SRG1 transcription-dependent nucleosome occupancy.

RNF20 inhibits TFIIS-facilitated transcriptional elongation to suppress pro-oncogenic gene expression

hBRE1/RNF20 is the major E3 ubiquitin ligase for histone H2B. RNF20 depletion causes a global reduction of monoubiquitylated H2B (H2Bub) levels and augments the expression of growth-promoting, pro-oncogenic genes. Those genes reside preferentially in compact chromatin and are inefficiently transcribed under basal conditions. We now report that RNF20, presumably via H2Bub, selectively represses those genes by interfering with chromatin recruitment of TFIIS, a factor capable of relieving stalled RNA polymerase II. RNF20 inhibits the interaction between TFIIS and the PAF1 complex and hinders transcriptional elongation. TFIIS ablation selectively abolishes the upregulation of those genes upon RNF20 depletion and attenuates the cellular response to EGF. Consistent with its positive role in transcription of pro-oncogenic genes, TFIIS expression is elevated in various human tumors. Our findings provide a molecular mechanism for selective gene repression by RNF20 and position TFIIS as a key target of RNF20's tumor suppressor activity.

The specificity and topology of chromatin interaction pathways in yeast

Packaging of DNA into chromatin has a profound impact on gene expression. To understand how changes in chromatin influence transcription, we analyzed 165 mutants of chromatin machinery components in Saccharomyces cerevisiae. mRNA expression patterns change in 80% of mutants, always with specific effects, even for loss of widespread histone marks. The data are assembled into a network of chromatin interaction pathways. The network is function based, has a branched, interconnected topology, and lacks strict one-to-one relationships between complexes. Chromatin pathways are not separate entities for different gene sets, but share many components. The study evaluates which interactions are important for which genes and predicts additional interactions, for example between Paf1C and Set3C, as well as a role for Mediator in subtelomeric silencing. The results indicate the presence of gene-dependent effects that go beyond context-dependent binding of chromatin factors and provide a framework for understanding how specificity is achieved through regulating chromatin.

RNF20-RNF40: A ubiquitin-driven link between gene expression and the DNA damage response

The DNA damage response (DDR) is emerging as a vast signaling network that temporarily modulates numerous aspects of cellular metabolism in the face of DNA lesions, especially critical ones such as the double strand break (DSB). The DDR involves extensive dynamics of protein post-translational modifications, most notably phosphorylation and ubiquitylation. The DSB response is mobilized primarily by the ATM protein kinase, which phosphorylates a plethora of key players in its various branches. It is based on a core of proteins dedicated to the damage response, and a cadre of proteins borrowed temporarily from other cellular processes to help meet the challenge. A recently identified novel component of the DDR pathway - histone H2B monoubiquitylation - exemplifies this principle. In mammalian cells, H2B monoubiquitylation is driven primarily by an E3 ubiquitin ligase composed of the two RING finger proteins RNF20 and RNF40. Generation of monoubiquitylated histone H2B (H2Bub) has been known to be coupled to gene transcription, presumably modulating chromatin decondensation at transcribed regions. New evidence indicates that the regulatory function of H2Bub on gene expression can selectively enhance or suppress the expression of distinct subsets of genes through a mechanism involving the hPAF1 complex and the TFIIS protein. This delicate regulatory process specifically affects genes that control cell growth and genome stability, and places RNF20 and RNF40 in the realm of tumor suppressor proteins. In parallel, it was found that following DSB induction, the H2B monoubiquitylation module is recruited to damage sites where it induces local H2Bub, which in turn is required for timely recruitment of DSB repair protein and, subsequently, timely DSB repair. This pathway represents a crossroads of the DDR and chromatin organization, and is a typical example of how the DDR calls to action functional modules that in unprovoked cells regulate other processes.

The tightly controlled deubiquitination activity of the human SAGA complex differentially modifies distinct gene regulatory elements

The multisubunit SAGA coactivator complex facilitates access of general transcription factors to DNA through histone acetylation mediated by GCN5. USP22 (ubiquitin-specific protease 22) was recently described as a subunit of the human SAGA complex that removes ubiquitin from monoubiquitinated histone H2B and H2A in vitro. Here we demonstrate an allosteric regulation of USP22 through multiple interactions with different domains of other subunits of the SAGA deubiquitination module (ATXN7, ATXN7L3, and ENY2). Downregulation of ATXN7L3 by short hairpin RNA (shRNA) specifically inactivated the SAGA deubiquitination activity, leading to a strong increase of global H2B ubiquitination and a moderate increase of H2A ubiquitination. Thus, SAGA is the major H2Bub deubiquitinase in human cells, and this activity cannot be fully compensated by other deubiquitinases. Here we show that the deubiquitination activity of SAGA is required for full activation of SAGA-dependent inducible genes. Interestingly, the reduction of the SAGA deubiquitination activity and the parallel increase in H2B ubiquitation at inducible target genes before activation do not induce aberrant gene expression. Our data together indicate that different dynamic equilibriums of H2B ubiquitination/deubiquitination are established at different gene regulatory elements and that H2B ubiquitination changes are necessary but not sufficient to trigger parallel activation of gene expression.

The United States of histone ubiquitylation and methylation

In this issue of Molecular Cell, Wu et al. (2011) reveal that ubiquitylation of histone 2B lysine 34 stimulates histone methyltransferase activity on nucleosomes, a finding with implications for the general mechanism by which monoubiquitylation may influence subsequent modification activities.

WAC, a functional partner of RNF20/40, regulates histone H2B ubiquitination and gene transcription

Histone H2B ubiquitination plays an important role in regulating chromatin organization during gene transcription. It has been shown that RNF20/40 regulates H2B ubiquitination. Here, using protein affinity purification, we have identified WAC as a functional partner of RNF20/40. Depletion of WAC abolishes H2B ubiquitination. WAC interacts with RNF20/40 through its C-terminal coiled-coil region and promotes RNF20/40 s E3 ligase activity for H2B ubiquitination. The N-terminal WW domain of WAC recognizes RNA polymerase II. During gene transcription, WAC targets RNF20/40 to associate with RNA polymerase II complex for H2B ubiquitination at active transcription sites, which regulates transcription. Moreover, WAC-dependent transcription is important for cell-cycle checkpoint activation in response to genotoxic stress. Taken together, our results demonstrate an important regulator for transcription-coupled histone H2B ubiquitination.

Chatting histone modifications in mammals

Eukaryotic chromatin can be highly dynamic and can continuously exchange between an open transcriptionally active conformation and a compacted silenced one. Post-translational modifications of histones have a pivotal role in regulating chromatin states, thus influencing all chromatin dependent processes. Methylation is currently one of the best characterized histone modification and occurs on arginine and lysine residues. Histone methylation can regulate other modifications (e.g. acetylation, phosphorylation and ubiquitination) in order to define a precise functional chromatin environment. In this review we focus on histone methylation and demethylation, as well as on the enzymes responsible for setting these marks. In particular we are describing novel concepts on the interdependence of histone modifications marks and discussing the molecular mechanisms governing this cross-talks.

The Paf1 complex represses ARG1 transcription in Saccharomyces cerevisiae by promoting histone modifications

The conserved multifunctional Paf1 complex is important for the proper transcription of numerous genes, and yet the exact mechanisms by which it controls gene expression remain unclear. While previous studies indicate that the Paf1 complex is a positive regulator of transcription, the repression of many genes also requires the Paf1 complex. In this study we used ARG1 as a model gene to study transcriptional repression by the Paf1 complex in Saccharomyces cerevisiae. We found that several members of the Paf1 complex contribute to ARG1 repression and that the complex localizes to the ARG1 promoter and coding region in repressing conditions, which is consistent with a direct repressive function. Furthermore, Paf1 complex-dependent histone modifications are enriched at the ARG1 locus in repressing conditions, and histone H3 lysine 4 methylation contributes to ARG1 repression. Consistent with previous reports, histone H2B monoubiquitylation, the mark upstream of histone H3 lysine 4 methylation, is also important for ARG1 repression. To begin to identify the mechanistic basis for Paf1 complex-mediated repression of ARG1, we focused on the Rtf1 subunit of the complex. Through an analysis of RTF1 mutations that abrogate known Rtf1 activities, we found that Rtf1 mediates ARG1 repression primarily by facilitating histone modifications. Other members of the Paf1 complex, such as Paf1, appear to repress ARG1 through additional mechanisms. Together, our results suggest that Rtf1-dependent histone H2B ubiquitylation and H3 K4 methylation repress ARG1 expression and that histone modifications normally associated with active transcription can occur at repressed loci and contribute to transcriptional repression.

A chromatin perspective of plant cell cycle progression

The finely regulated series of events that span from the birth of a cell to the production of two new born cells encompass the cell cycle. Cell cycle progression occurs in a unidirectional manner and requires passing through a number of stages in response to cellular, developmental and environmental cues. In addition to these signaling cascades, transcriptional regulation plays a major role and acts coordinately with genome duplication during S-phase and chromosome segregation during mitosis. In this context, chromatin is revealing as a highly dynamic and major player in cell cycle regulation not only owing to the changes that occur as a consequence of cell cycle progression but also because some specific chromatin modifications are crucial to move across the cell cycle. These are particularly relevant for controlling transcriptional activation and repression as well as initiation of DNA replication and chromosome compaction. As a consequence the epigenetic landscape of a proliferating cell is very complex throughout the cell cycle. These aspects of chromatin dynamics together with the impact of epigenetic modifications on cell proliferation will be discussed in this article. This article is part of a Special Issue entitled: Epigenetic Control of cellular and developmental processes in plants.

The histone chaperone FACT: structural insights and mechanisms for nucleosome reorganization

Changes in chromatin architecture induced by epigenetic mechanisms are essential for normal cellular processes such as gene expression, DNA repair, and cellular division. Compact chromatin presents a barrier to these processes and is highly regulated by epigenetic markers binding to components of the nucleosome. Histone modifications directly influence chromatin dynamics and facilitate recruitment of additional factors such as chromatin remodelers and histone chaperones. One member of this last class of factors, FACT (facilitates chromatin transcription), is categorized as a histone chaperone critical for nucleosome reorganization during replication, transcription, and DNA repair. Significant discoveries regarding the role of histone chaperones and specifically FACT have come over the past dozen years from a number of independent laboratories. Here, we review the structural and biophysical basis for FACT-mediated nucleosome reorganization and discuss up-to-date models for FACT function.

Requirement of TFIIH kinase subunit Mat1 for RNA Pol II C-terminal domain Ser5 phosphorylation, transcription and mRNA turnover

The relevance of serine 5 phosphorylation of RNA polymerase II carboxy-terminal domain during initiation has been difficult to determine in mammalian cells as no general in vivo Ser5 kinase has been identified. Here, we demonstrate that deletion of the TFIIH kinase subunit Mat1 in mouse fibroblasts leads to dramatically reduced Pol II Ser5 phosphorylation. This is associated with defective capping and reduced Ser2 phosphorylation, decreased Pol II progression into elongation and severely attenuated transcription detected through analysis of nascent mRNAs, establishing a general requirement for mammalian Mat1 in transcription. Surprisingly, the general defect in Pol II transcription in Mat1^−/− fibroblasts is not reflected in the majority of steady-state mRNAs. This indicates widespread stabilization of mRNAs and points to the existence of a regulatory mechanism to stabilize mRNAs following transcriptional attenuation, thus revealing a potential caveat in similar studies limited to analysis of steady-state mRNAs.

Decoding the trans-histone crosstalk: methods to analyze H2B ubiquitination, H3 methylation and their regulatory factors

Regulation of histone H3 lysine 4 and 79 methylation by histone H2B lysine 123 monoubiquitination is an evolutionarily conserved trans-histone crosstalk mechanism, which demonstrates a functional role for histone ubiquitination within the cell. The regulatory enzymes, factors and processes involved in the establishment and dynamic modulation of these modifications and their genome-wide distribution patterns have been determined in many model systems. Rapid progress in understanding this trans-histone crosstalk has been made using the standard experimental tools of chromatin biology in budding yeast (Saccharomyces cerevisiae), a highly tractable model organism. Here, we provide a set of modified and refined experimental procedures that can be used to gain further insights into the underlying mechanisms that govern this crosstalk in budding yeast. Importantly, the improved procedures and their underlying principles can also be applied to other model organisms. Methods presented here provide a rapid and efficient means to prepare enriched protein extracts to better preserve and assess the steady state levels of histones, non-histone proteins and their modifications. Improved chromatin immunoprecipitation and double immunoprecipitation protocols are provided to measure the occupancy and distribution of proteins and their modified forms at specific chromatin regions or loci. A quick and easy method to measure overall protein abundance and changes in protein-protein and protein-DNA interactions on native chromatin is also described.

A genetic and molecular toolbox for analyzing histone ubiquitylation and sumoylation in yeast

Combinations of phosphorylation, acetylation, methylation, ubiquitylation, and sumoylation of histones comprise what is referred to as the "histone code". These marks influence processes from transcription to DNA replication, where gaining access to DNA organized in chromatin is necessary. Much emphasis has been placed on the role of histone ubiquitylation and sumoylation during the process of transcription. Histone H2B is monoubiquitylated at lysine 123 in budding yeast and influences gene activation. All four of the core histones are sumoylated on their amino terminal tails in this organism, and this serves to negatively regulate gene expression. Because antibodies specific for ubiquitylated or sumoylated yeast histones are not commercially available, and these marks are highly sensitive to proteolysis in native cell extracts, special genetic and molecular tools have been developed to monitor these dynamic and often rare modifications in vivo. Here, we describe some of these tools, with emphasis on how they can be used for transcriptional studies.

Histone H2B ubiquitylation disrupts local and higher-order chromatin compaction

Regulation of chromatin structure involves histone post-translational modifications which can modulate intrinsic properties of the chromatin fiber to change the chromatin state. We used chemically defined nucleosome arrays to demonstrate that H2B ubiquitylation (uH2B), a modification associated with transcription, interferes with chromatin compaction and leads to an open and biochemically accessible fiber conformation. Importantly, these effects were specific for ubiquitin, as compaction of chromatin modified with a similar ubiquitin-sized protein, Hub1, was only weakly affected. Applying a fluorescence based method we found that uH2B acts through a mechanism distinct from H4 tail acetylation (acH4), a modification known to disrupt chromatin folding. Finally, incorporation of both uH2B and acH4 in nucleosomes resulted in synergistic inhibition of higher order chromatin structure formation, possibly a result of their distinct mode of action.

Glucose metabolism induces mono-ubiquitination of histone H2B in mammalian cells

Histone modifications play an important role in transcriptional regulation and are intimately involved in important biological and disease processes. Despite their functional significance, whether and how extracellular signals modulate histone modifications are not well defined. Using mono-ubiquitination of histone H2B as a model system, we have previously shown that mono-ubiquitination of histone H2B is induced by glucose through glycolysis in budding yeast Saccharomyces cerevisiae. Because histones are well conserved proteins among eukaryotes and glycolysis is the most conserved metabolic pathway, we reasoned that the glucose-glycolysis-uH2B signal pathway originally discovered in yeast may be conserved in human cells. Using cultured human glioma cells as a model, we show in this report that extracellular media modulated global levels of mono-ubiquitination of histone H2B at K120 (uH2B). Nutrient deprivation removed the ubiquitin moiety of uH2B. Glucose-containing media induced uH2B in the cells while media lacking glucose had no effect on the induction of uH2B, suggesting that glucose was required for inducing uH2B in the cells. In contrast, non-metabolic glucose analogs were defective in inducing uH2B, suggesting that glucose metabolism was required for glucose-induced mono-ubiquitination in the cultured glioma cells. Moreover, shRNA knockdown of PKM2, an essential enzyme for glycolysis in malignant tumors, inhibited mono-ubiquitination of histone H2B in these cells. Taken together, our previous and current results demonstrate that the novel glucose-glycolysis-uH2B signal pathway is well conserved from yeast to mammalian cells, providing an evolutionarily-conserved regulatory mechanism of histone modifications.

Temporal and spatial regulatory functions of the V(D)J recombinase

In developing lymphocytes, V(D)J recombination is subject to tight spatial and temporal regulation. An emerging body of evidence indicates that some of these constraints, particularly with respect to locus specificity and cell cycle phase, are enforced by regulatory cues that converge directly on the RAG proteins themselves. Active chromatin is bound by RAG-2 through a specific histone modification that may serve the recombinase as an allosteric activator as well as a docking site. RAG-1 possesses intrinsic histone ubiquitin ligase activity, suggesting that the recombinase not only responds to chromatin modification but is itself able to modify chromatin. The cyclin A/Cdk2 component of the cell cycle clock triggers periodic destruction of RAG-2, thereby restricting V(D)J recombination to the G0/G1 cell cycle phases. These examples illustrate that the RAG proteins, in addition to their direct actions on DNA, are able to detect and respond to intracellular signals, thereby coordinating recombinase activity with intracellular processes such as cell division and transcription.

The role of deubiquitinating enzymes in chromatin regulation

Post-translational modifications of the histones are centrally involved in the regulation of all DNA-templated processes, including gene transcription, DNA replication, recombination, and repair. These modifications are often dynamic, and their removal is just as important as their addition in proper regulation of cellular functions. Although histone acetylation/deacetylation and histone methylation/demethylation are highly studied, the functions and regulation of histone ubiquitination and deubiquitination are less well understood. This review highlights our current understanding of how histone ubiquitination impacts gene transcription, DNA repair, and cell cycle progression, and stresses the importance of deubiquitinases to normal cellular functions as well as to disease states such as cancer.

Menin and RNF20 recruitment is associated with dynamic histone modifications that regulate signal transducer and activator of transcription 1 (STAT1)-activated transcription of the interferon regulatory factor 1 gene (IRF1)

BACKGROUND

Signal transducer and activator of transcription (STAT) activation of gene expression is both rapid and transient, and when properly executed it affects growth, differentiation, homeostasis and the immune response, but when dysregulated it contributes to human disease. Transcriptional activation is regulated by alterations to the chromatin template. However, the role of histone modification at gene loci that are activated for transcription in response to STAT signaling is poorly defined.

RESULTS

Using chromatin immunoprecipitation, we profiled several histone modifications during STAT1 activation of the interferon regulatory factor 1 gene (IRF1). Methylated lysine histone proteins H3K4me2, H3K4me3, H3K79me3, H3K36me3 and monoubiquitinated histone ubH2B are dynamic and correlate with interferon (IFN)γ induction of STAT1 activity. Chemical inhibition of H3K4 methylation downregulates IRF1 transcription and decreases RNA polymerase II (Pol II) occupancy at the IRF1 promoter. MEN1, a component of a complex proteins associated with Set1 (COMPASS)-like complex and the hBRE1 component, RNF20, are localized to IRF1 in the uninduced state and are further recruited when IRF1 is activated. RNAi-mediated depletion of RNF20 lowers both ubH2B and H3K4me3, but surprisingly, upregulates IFNγ induced IRF1 transcription. The dynamics of phosphorylation in the C-terminal domain (CTD) of Pol II are disrupted during gene activation as well.

CONCLUSIONS

H2B monoubiquitination promotes H3K4 methylation, but the E3 ubiquitin ligase, RNF20, is repressive of inducible transcription at the IRF1 gene locus, suggesting that ubH2B can, directly or indirectly, affect Pol II CTD phosphorylation cycling to exert control on ongoing transcription.

The proteasome and its regulatory roles in gene expression

Cumulative evidence indicates that the proteasome, which is mainly known as a protein-degrading machine, is very essential for gene expression. Destructive functions of the proteasome, i.e., ubiquitin-dependent proteolytic activity, are significant for activator localization, activator destruction, co-activator/repressor destruction and PIC disassembly. Non-proteolytic functions of the proteasome are important for recruitment of activators and co-activators to promoters, ubiquitin-dependent histone modification, transcription elongation and possibly maturation of mRNA via the facilitation of mRNA export from the nucleus to the cytoplasm. In this review, we discuss how the proteasome regulates transcription at numerous stages during gene expression. This article is part of a Special Issue entitled The 26S Proteasome: When degradation is just not enough!

Histone H2B ubiquitination and beyond: Regulation of nucleosome stability, chromatin dynamics and the trans-histone H3 methylation

Regulation of Set1-COMPASS-mediated H3K4 methylation and Dot1-mediated H3K79 methylation by H2BK123 ubiquitination (H2Bub1) is an evolutionarily conserved trans-histone crosstalk mechanism. How H2Bub1 impacts chromatin structure and affects Set1-COMPASS/Dot1 functions has not been fully defined. Ubiquitin was proposed to bind proteins to physically bridge H2Bub1 with Set1-COMPASS/Dot1. Alternatively, the bulky ubiquitin was thought to be a "wedge" that loosens the nucleosome for factor access. Contrary to the latter possibility, recent discoveries provide evidence for nucleosome stabilization by H2Bub1 via preventing the constant H2A-H2B eviction. Recent data has also uncovered a "docking-site" on H2B for Set1-COMPASS. Collectively, these findings invoke a model, where ubiquitin acts as a "glue" to bind the nucleosome together for supporting Set1-COMPASS/Dot1 functions. This review provides an overview of these novel findings. Additionally, how H2Bub1 and its deubiquitination might alter the chromatin dynamics during transcription is discussed. Possible models for nucleosome stabilization by ubiquitin are also provided.

Chromatin dynamics mediated by histone modifiers and histone chaperones in postreplicative recombination

Eukaryotic chromatin is regulated by chromatin factors such as histone modification enzymes, chromatin remodeling complexes and histone chaperones in a variety of DNA-dependent reactions. Among these reactions, transcription in the chromatin context is well studied. On the other hand, how other DNA-dependent reactions, including postreplicative homologous recombination, are regulated in the chromatin context remains elusive. Here, histone H3 Lys56 acetylation, mediated by the histone acetyltransferase Rtt109 and the histone chaperone Cia1/Asf1, is shown to be required for postreplicative sister chromatid recombination. This recombination did not occur in the cia1/asf1-V94R mutant, which lacks histone binding and histone chaperone activities and which cannot promote the histone acetyltransferase activity of Rtt109. A defect in another histone chaperone, CAF-1, led to an increase in acetylated H3-K56 (H3-K56-Ac)-dependent postreplicative recombination. Some DNA lesions recognized by the putative ubiquitin ligase complex Rtt101-Mms1-Mms22, which is reported to act downstream of the H3-K56-Ac signaling pathway, seem to be increased in CAF-1 defective cells. Taken together, these data provide the framework for a postreplicative recombination mechanism controlled by histone modifiers and histone chaperones in multiple ways.

Bre1p-mediated histone H2B ubiquitylation regulates apoptosis in Saccharomyces cerevisiae

BRE1 encodes an E3 ubiquitin protein ligase that is required for the ubiquitylation of histone H2B at lysine 123 (K123). Ubiquitylation of this histone residue is involved in a variety of cellular processes including gene activation and gene silencing. Abolishing histone H2B ubiquitylation also confers X-ray sensitivity and abrogates checkpoint activation after DNA damage. Here we show that Saccharomyces cerevisiae Bre1p exhibits anti-apoptotic activity in yeast and that this is linked to histone H2B ubiquitylation. We found that enhanced levels of Bre1p protect from hydrogen-peroxide-induced cell death, whereas deletion of BRE1 enhances cell death. Moreover, cells lacking Bre1p show reduced lifespan during chronological ageing, a physiological apoptotic condition in yeast. Importantly, the resistance against apoptosis is conferred by histone H2B ubiquitylation mediated by the E3 ligase activity of Bre1p. Furthermore, we found that the death of Δbre1 cells depends on the yeast caspase Yca1p, because Δbre1 cells exhibit increased caspase activity when compared with wild-type cells, and deletion of YCA1 leads to reduced apoptosis sensitivity of cells lacking Bre1p.

Novel trans-tail regulation of H2B ubiquitylation and H3K4 methylation by the N terminus of histone H2A

Chromatin is regulated by cross talk among different histone modifications, which can occur between residues within the same tail or different tails in the nucleosome. The latter is referred to as trans-tail regulation, and the best-characterized example of this is the dependence of H3 methylation on H2B ubiquitylation. Here we describe a novel form of trans-tail regulation of histone modifications involving the N-terminal tail of histone H2A. Mutating or deleting residues in the N-terminal tail of H2A reduces H2B ubiquitylation and H3K4 methylation but does not affect the recruitment of the modifying enzymes, Rad6/Bre1 and COMPASS, to genes. The H2A tail is required for the incorporation of Cps35 into COMPASS, and increasing the level of ubiquitylated H2B in H2A tail mutants suppresses the H3K4 methylation defect, suggesting that the H2A tail regulates H2B-H3 cross talk. We mapped the region primarily responsible for this regulation to the H2A repression domain, HAR. The HAR and K123 of H2B are in close proximity to each other on the nucleosome, suggesting that they form a docking site for the ubiquitylation machinery. Interestingly, the HAR is partially occluded by nucleosomal DNA, suggesting that the function of the H2A cross talk pathway is to restrict histone modifications to nucleosomes altered by transcription.

Role of ubiquitination in meiotic recombination repair

Programmed and unprogrammed double-strand breaks (DSBs) often arise from such physiological requirements as meiotic recombination, and exogenous insults, such as ionizing radiation (IR). Due to deleterious impacts on genome stability, DSBs must be appropriately processed and repaired in a regulatory manner. Recent investigations have indicated that ubiquitination is a critical factor in DNA damage response and meiotic recombination repair. This review summarizes the effects of proteins and complexes associated with ubiquitination with regard to homologous recombination (HR)-dependent DSB repair.

Inducible gene expression: diverse regulatory mechanisms

The rapid activation of gene expression in response to stimuli occurs largely through the regulation of RNA polymerase II-dependent transcription. In this Review, we discuss events that occur during the transcription cycle in eukaryotes that are important for the rapid and specific activation of gene expression in response to external stimuli. In addition to regulated recruitment of the transcription machinery to the promoter, it has now been shown that control steps can include chromatin remodelling and the release of paused polymerase. Recent work suggests that some components of signal transduction cascades also play an integral part in activating transcription at target genes.

The human PAF1 complex acts in chromatin transcription elongation both independently and cooperatively with SII/TFIIS

Genetic and cell-based studies have implicated the PAF1 complex (PAF1C) in transcription-associated events, but there has been no evidence showing a direct role in facilitating transcription of a natural chromatin template. Here, we demonstrate an intrinsic ability of human PAF1C (hPAF1C) to facilitate activator (p53)- and histone acetyltransferase (p300)-dependent transcription elongation from a recombinant chromatin template in a biochemically defined RNA polymerase II transcription system. This represents a PAF1C function distinct from its established role in histone ubiquitylation and methylation. Importantly, we further demonstrate a strong synergy between hPAF1C and elongation factor SII/TFIIS and an underlying mechanism involving direct hPAF1C-SII interactions and cooperative binding to RNA polymerase II. Apart from a distinct PAF1C function, the present observations provide a molecular mechanism for the cooperative function of distinct transcription elongation factors in chromatin transcription.

Global analysis of mutual interaction surfaces of nucleosomes with comprehensive point mutants

The surfaces of core histones in nucleosome are exposed as required for factor recognition, or buried for histone-DNA and histone-histone interactions. To understand the mechanisms by which nucleosome structure and function are coordinately altered in DNA-mediated reactions, it is essential to define the roles of both exposed and buried residues and their functional relationships. For this purpose, we developed GLASP (GLobal Analysis of Surfaces by Point mutation) and GLAMP (GLobal Analysis of Mutual interaction surfaces of multi-subunit protein complex by Point mutation) strategies, both of which are comprehensive analyses by point mutagenesis of exposed and buried residues in nucleosome, respectively. Four distinct DNA-mediated reactions evaluated by Ty suppression (the Spt(-) phenotype), and sensitivities to 6-azauracil (6AU), hydroxyurea (HU), and methyl methanesulfonate (MMS), require common and different GLAMP residues. Mutated GLAMP residues at the interface between histones H2A and H2B mainly affect the Spt(-) phenotype but not HU and MMS sensitivities. Interestingly, among the mutated GLAMP residues surrounding the histone H3-H3' interface, some equally affect the Spt(-) phenotype, and HU and MMS sensitivities, whereas others differentially affect the Spt(-) phenotype, and HU and MMS sensitivities. Based on these and other results, the functional relationships among chromatin factors and GLASP and GLAMP residues provide insights into nucleosome disassembly/assembly processes in DNA-mediated reactions.

The Paf1 complex: platform or player in RNA polymerase II transcription?

The Paf1 complex (Paf1C), composed of the proteins Paf1, Ctr9, Cdc73, Rtf1, and Leo1, accompanies RNA polymerase II (pol II) from the promoter to the 3' end formation site of mRNA and snoRNA encoding genes; it is also found associated with RNA polymerase I (pol I) on rDNA. The Paf1C is found in simple and complex eukaryotes; in human cells hSki8 is also part of the complex. The Paf1C has been linked to a large and growing list of transcription related processes including: communication with transcriptional activators; recruitment and activation of histone modification factors; facilitation of elongation on chromatin templates; and the recruitment of 3' end-processing factors necessary for accurate termination of transcription. Absence of, or mutations in, Paf1C factors result in alterations in gene expression that can result in misregulation of developmental programs and loss of control of cell division leading to cancer in humans. This review considers recent information that may help to resolve whether the Paf1C is primarily a "platform" on pol II that coordinates the association of many critical transcription factors, or if the complex itself plays a more direct role in one or more steps in transcription.

Transcript Elongation by RNA Polymerase II

Until recently, it was generally assumed that essentially all regulation of transcription takes place via regions adjacent to the coding region of a gene--namely promoters and enhancers--and that, after recruitment to the promoter, the polymerase simply behaves like a machine, quickly "reading the gene." However, over the past decade a revolution in this thinking has occurred, culminating in the idea that transcript elongation is extremely complex and highly regulated and, moreover, that this process significantly affects both the organization and integrity of the genome. This review addresses basic aspects of transcript elongation by RNA polymerase II (RNAPII) and how it relates to other DNA-related processes.

Histones: annotating chromatin

Chromatin is a highly regulated nucleoprotein complex through which genetic material is structured and maneuvered to elicit cellular processes, including transcription, cell division, differentiation, and DNA repair. In eukaryotes, the core of this structure is composed of nucleosomes, or repetitive histone octamer units typically enfolded by 147 base pairs of DNA. DNA is arranged and indexed through these nucleosomal structures to adjust local chromatin compaction and accessibility. Histones are subject to multiple covalent posttranslational modifications, some of which alter intrinsic chromatin properties, others of which present or hinder binding modules for non-histone, chromatin-modifying complexes. Although certain histone marks correlate with different biological outputs, we have yet to fully appreciate their effects on transcription and other cellular processes. Tremendous advancements over the past years have uncovered intriguing histone-related matters and raised important related questions. This review revisits past breakthroughs and discusses novel developments that pertain to histone posttranslational modifications and the affects they have on transcription and DNA packaging.

A Bre1-associated protein, large 1 (Lge1), promotes H2B ubiquitylation during the early stages of transcription elongation

Transcription activation has been proposed to require both ubiquitylation and deubiquitylation of histone H2B. Here, we show that Lge1 (Large 1) is found in a complex containing Rad6.Bre1 and that it controls the recruitment of Bre1, a ubiquitin ligase, and Ubp8, a deubiquitylase, to promote ubiquitylation during the early steps in elongation. Chromatin immunoprecipitation experiments showed that Lge1 associates with promoter and coding regions of actively transcribed genes in a transcription-dependent manner. Disruption of Lge1 abolished ubiquitylation of histone H2B on lysine 123 and H3 methylation on lysines 4 and 79 and resulted in significant sensitivity to 6-azauracil and mycophenolic acid. In particular, in Lge1-deficient cells, Bre1 recruitment was attenuated, whereas recruitment of Ubp8 was facilitated. These alterations were coincident with changes in the interaction between Bre1.Ubp8 and RNA polymerase II phosphorylated at serine 5 of the C-terminal domain. We propose that Lge1 has a novel function in disrupting the balance between the recruitment of Bre1 and Ubp8, thus promoting transcription elongation.

Histone H2B ubiquitylation is not required for histone H3 methylation at lysine 4 in tetrahymena

Ubiquitylation of histone H2B and/or a component of the system that ubiquitylates H2B is required for methylation of histone H3 at lysine 4 (H3K4) in yeasts and probably in humans. In this study, the single ubiquitylation site was mapped to conserved lysine 115 of the C-terminal region of histone H2B in the single-cell model organism Tetrahymena thermophila. In strains lacking H2B ubiquitylation, H3K4 methylation was not detectably affected. As in other organisms, the E2 ubiquitin-conjugating enzyme Ubc2 and the E3 ubiquitin ligase Bre1 were required for H2B ubiquitylation. However, neither enzyme was required for H3K4 methylation. These studies argue that, in T. thermophila, the histone ubiquitylation mechanism is not required for H3K4 methylation, demonstrating that different organisms can speak different languages in the "cross-talk" among post-translational modifications on different histones.

Ubiquitination of histone H2B regulates chromatin dynamics by enhancing nucleosome stability

The mechanism by which ubiquitination of histone H2B (H2Bub1) regulates H3-K4 and -K79 methylation and the histone H2A-H2B chaperone Spt16-mediated nucleosome dynamics during transcription is not fully understood. Upon investigating the effect of H2Bub1 on chromatin structure, we find that contrary to the supposed role for H2Bub1 in opening up chromatin, it is important for nucleosome stability. First, we show that H2Bub1 does not function as a "wedge" to non-specifically unfold chromatin, as replacement of ubiquitin with a bulkier SUMO molecule conjugated to the C-terminal helix of H2B cannot functionally support H3-K4 and -K79 methylation. Second, using a series of biochemical analyses, we demonstrate that nucleosome stability is reduced or enhanced, when the levels of H2Bub1 are abolished or increased, respectively. Besides transcription elongation, we show that H2Bub1 regulates initiation by stabilizing nucleosomes positioned over the promoters of repressed genes. Collectively, our study reveals an intrinsic difference in the property of chromatin assembled in the presence or absence of H2Bub1 and implicates the regulation of nucleosome stability as the mechanism by which H2Bub1 modulates nucleosome dynamics and histone methylation during transcription.

Ubiquitination of histone H2B regulates chromatin dynamics by enhancing nucleosome stability

The mechanism by which ubiquitination of histone H2B (H2Bub1) regulates H3-K4 and -K79 methylation and the histone H2A-H2B chaperone Spt16-mediated nucleosome dynamics during transcription is not fully understood. Upon investigating the effect of H2Bub1 on chromatin structure, we find that contrary to the supposed role for H2Bub1 in opening up chromatin, it is important for nucleosome stability. First, we show that H2Bub1 does not function as a "wedge" to non-specifically unfold chromatin, as replacement of ubiquitin with a bulkier SUMO molecule conjugated to the C-terminal helix of H2B cannot functionally support H3-K4 and -K79 methylation. Second, using a series of biochemical analyses, we demonstrate that nucleosome stability is reduced or enhanced, when the levels of H2Bub1 are abolished or increased, respectively. Besides transcription elongation, we show that H2Bub1 regulates initiation by stabilizing nucleosomes positioned over the promoters of repressed genes. Collectively, our study reveals an intrinsic difference in the property of chromatin assembled in the presence or absence of H2Bub1 and implicates the regulation of nucleosome stability as the mechanism by which H2Bub1 modulates nucleosome dynamics and histone methylation during transcription.

New mutant versions of yeast FACT subunit Spt16 affect cell integrity

Transcription by RNA polymerase II is impeded by the nucleosomal organization of DNA; these negative effects are modulated at several stages of nucleosomal DNA transcription by FACT, a heterodimeric transcription factor. At promoters, FACT facilitates the binding of TATA-binding factor, while during transcription elongation FACT mediates the necessary destabilization of nucleosomes and subsequent restoration of nucleosome structure in the wake of the transcription elongation complex. Altered FACT activity can impair the fidelity of transcription initiation and affect transcription patterns. Using reporter genes we have identified new mutant versions of the Spt16 subunit of yeast FACT with dominant negative effects on the fidelity of transcription initiation. Two of these spt16 mutant alleles also affect cell integrity. Cells relying on these spt16 mutant alleles display sorbitol-remediated temperature sensitivity, altered sensitivity to detergent, and abnormal morphologies, and are further inhibited by the ssd1-d mutation. The overexpression of components of protein kinase C (Pkc1) signaling diminishes this spt16 ssd1-d temperature sensitivity, whereas gene deletions eliminating components of Pkc1 signaling further impair these spt16 mutant cells. Thus, the FACT subunit Spt16 and Pkc1 signaling have an overlapping essential function, with an unexpected role for FACT in the maintenance of cell integrity.

Histone H2BK123 monoubiquitination is the critical determinant for H3K4 and H3K79 trimethylation by COMPASS and Dot1

Histone H2B monoubiquitination by Rad6/Bre1 is required for the trimethylation of both histone H3K4 and H3K79 by COMPASS and Dot1 methyltransferases, respectively. The dependency of methylation at H3K4 and H3K79 on the monoubiquitination of H2BK123 was recently challenged, and extragenic mutations in the strain background used for previous studies or epitope-tagged proteins were suggested to be the sources of this discrepancy. In this study, we show that H3K4 and H3K79 methylation is solely dependent on H2B monoubiquitination regardless of any additional alteration to the H2B sequence or genome. Furthermore, we report that Y131, one of the yeast histone H2A/H2B shuffle strains widely used for the last decade in the field of chromatin and transcription biology, carries a wild-type copy of each of the HTA2 and HTB2 genes under the GAL1/10 promoter on chromosome II. Therefore, we generated the entire histone H2A and H2B alanine-scanning mutant strains in another background, which does not express wild-type histones.

RAD6-Mediated transcription-coupled H2B ubiquitylation directly stimulates H3K4 methylation in human cells

H2B ubiquitylation has been implicated in active transcription but is not well understood in mammalian cells. Beyond earlier identification of hBRE1 as the E3 ligase for H2B ubiquitylation in human cells, we now show (1) that hRAD6 serves as the cognate E2-conjugating enzyme; (2) that hRAD6, through direct interaction with hPAF-bound hBRE1, is recruited to transcribed genes and ubiquitylates chromatinized H2B at lysine 120; (3) that hPAF-mediated transcription is required for efficient H2B ubiquitylation as a result of hPAF-dependent recruitment of hBRE1-hRAD6 to the Pol II transcription machinery; (4) that H2B ubiquitylation per se does not affect the level of hPAF-, SII-, and p300-dependent transcription and likely functions downstream; and (5) that H2B ubiquitylation directly stimulates hSET1-dependent H3K4 di- and trimethylation. These studies establish the natural H2B ubiquitylation factors in human cells and also detail the mechanistic basis for H2B ubiquitylation and function during transcription.

The cap binding complex influences H2B ubiquitination by facilitating splicing of the SUS1 pre-mRNA

Pre-messenger RNA splicing is carried out by a large ribonucleoprotein complex called the spliceosome. Despite the striking evolutionary conservation of the spliceosomal components and their functions, controversy persists about the relative importance of splicing in Saccharomyces cerevisiae-particularly given the paucity of intron-containing genes in yeast. Here we show that splicing of one pre-messenger RNA, SUS1, a component of the histone H2B ubiquitin protease machinery, is essential for establishing the proper modification state of chromatin. One protein complex that is intimately involved in pre-mRNA splicing, the yeast cap-binding complex, appears to be particularly important, as evidenced by its extensive and unique genetic interactions with enzymes that catalyze histone H2B ubiquitination. Microarray studies show that cap binding complex (CBC) deletion has a global effect on gene expression, and for approximately 20% of these genes, this effect is suppressed when ubiquitination of histone H2B is eliminated. Consistent with this finding of histone H2B dependent effects on gene expression, deletion of the yeast cap binding complex leads to overubiquitination of histone H2B. A key component of the ubiquitin-protease module of the SAGA complex, Sus1, is encoded by a gene that contains two introns and is misspliced when the CBC is deleted, leading to destabilization of the ubiquitin protease complex and defective modulation of cellular H2B levels. These data demonstrate that pre-mRNA splicing plays a critical role in histone H2B ubiquitination and that the CBC in particular helps to establish the proper state of chromatin and proper expression of genes that are regulated at the level of histone H2B ubiquitination.

CDK9 directs H2B monoubiquitination and controls replication-dependent histone mRNA 3'-end processing

Post-translational histone modifications have essential roles in controlling nuclear processes; however, the specific mechanisms regulating these modifications and their combinatorial activities remain elusive. Cyclin-dependent kinase 9 (CDK9) regulates gene expression by phosphorylating transcriptional regulatory proteins, including the RNA polymerase II carboxy-terminal domain. Here, we show that CDK9 activity is essential for maintaining global and gene-associated levels of histone H2B monoubiquitination (H2Bub1). Furthermore, CDK9 activity and H2Bub1 help to maintain correct replication-dependent histone messenger RNA (mRNA) 3'-end processing. CDK9 knockdown consistently resulted in inefficient recognition of the correct mRNA 3'-end cleavage site and led to increased read-through of RNA polymerase II to an alternative downstream polyadenylation signal. Thus, CDK9 acts to integrate phosphorylation during transcription with chromatin modifications to control co-transcriptional histone mRNA processing.

FACT and the proteasome promote promoter chromatin disassembly and transcriptional initiation

The packaging of the eukaryotic genome into chromatin represses gene expression by blocking access of the general transcription machinery to the underlying DNA sequences. Accordingly, eukaryotes have developed a variety of mechanisms to disrupt, alter, or disassemble nucleosomes from promoter regions and open reading frames to allow transcription to occur. Although we know that chromatin disassembly from the yeast PHO5 promoter is triggered by the Pho4 activator, the mechanism is far from clear. Here we show that the Pho4 activator can occupy its nucleosome-bound DNA binding site within the PHO5 promoter. In contrast to the role of Saccharomyces cerevisiae FACT (facilitates chromatin transcription) complex in assembling chromatin within open reading frames, we find that FACT is involved in the disassembly of histones H2A/H2B from the PHO5 promoter during transcriptional induction. We have also discovered that the proteasome is required for efficient chromatin disassembly and transcriptional induction from the PHO5 promoter. Mutants of the degradation function of the proteasome have a defect in recruitment of the Pho4 activator, whereas mutants of the ATPase cap of the proteasome do recruit Pho4 but are still delayed for chromatin assembly. Finally, we rule out the possibility that the proteasome or ATPase cap is driving chromatin disassembly via a potential ATP-dependent chromatin remodeling activity.

Direct Bre1-Paf1 complex interactions and RING finger-independent Bre1-Rad6 interactions mediate histone H2B ubiquitylation in yeast

Recent yeast genetic studies have implicated the ubiquitin-conjugating enzyme and ubiquitin ligase functions of yRad6 and yBre1, respectively, in H2B ubiquitylation. However, there have been no corresponding biochemical analyses demonstrating intrinsic enzyme activities of yRad6 and yBre1 or related mechanistic details. Here, we describe a robust in vitro chromatin ubiquitylation assay that involves purified H2B ubiquitylation factors and natural nucleosomes. Our results indicate that yRad6 has an in vitro ability to nonspecifically ubiquitylate all core histones in the absence of an ubiquitin ligase but that yBre1 functions, through direct interactions with yRad6, to direct the ubiquitin conjugating activity of yRad6 toward the physiological H2B ubiquitylation site. Moreover, a yRad6 domain mapping analysis shows that an intact UBC domain is required for binding to yBre1, whereas the C-terminal acidic tail domain that is not required for a stable yBre1-yRad6 interaction is necessary for full enzyme activity of yRad6. We also find that, analogous to heteromeric complex formation by BRE1 paralogues in other organisms, yBre1 forms a homo-multimeric complex. Of special significance, our detailed biochemical analyses further show that the yBre1 RING finger domain is essential for H2B ubiquitylation but, surprisingly, dispensable for interaction of yBre1 with yRad6. In further support of the genetically identified requirement of the RNA polymerase II-associated yPaf1 complex for H2B ubiquitylation, protein interaction studies reveal that a purified yPaf1 complex directly and selectively interacts with yBre1 and thus serves to link the H2B ubiquitylation and general transcription machineries. These studies provide a more detailed mechanistic basis for H2B ubiquitylation in yeast.

Control of transcriptional elongation and cotranscriptional histone modification by the yeast BUR kinase substrate Spt5

Elongation by RNA polymerase II (RNAPII) is a finely regulated process in which many elongation factors contribute to gene regulation. Among these factors are the polymerase-associated factor (PAF) complex, which associates with RNAPII, and several cyclin-dependent kinases, including positive transcription elongation factor b (P-TEFb) in humans and BUR kinase (Bur1-Bur2) and C-terminal domain (CTD) kinase 1 (CTDK1) in Saccharomyces cerevisiae. An important target of P-TEFb and CTDK1, but not BUR kinase, is the CTD of the Rpb1 subunit of RNAPII. Although the essential BUR kinase phosphorylates Rad6, which is required for histone H2B ubiquitination on K123, Rad6 is not essential, leaving a critical substrate(s) of BUR kinase unidentified. Here we show that BUR kinase is important for the phosphorylation in vivo of Spt5, a subunit of the essential yeast RNAPII elongation factor Spt4/Spt5, whose human orthologue is DRB sensitivity-inducing factor. BUR kinase can also phosphorylate the C-terminal region (CTR) of Spt5 in vitro. Like BUR kinase, the Spt5 CTR is important for promoting elongation by RNAPII and recruiting the PAF complex to transcribed regions. Also like BUR kinase and the PAF complex, the Spt5 CTR is important for histone H2B K123 monoubiquitination and histone H3 K4 and K36 trimethylation during transcription elongation. Our results suggest that the Spt5 CTR, which contains 15 repeats of a hexapeptide whose consensus sequence is S[T/A]WGG[A/Q], is a substrate of BUR kinase and a platform for the association of proteins that promote both transcription elongation and histone modification in transcribed regions.

HISTONE MONOUBIQUITINATION1 interacts with a subunit of the mediator complex and regulates defense against necrotrophic fungal pathogens in Arabidopsis

This work examines the role of the Arabidopsis thaliana RING E3 ligase, HISTONE MONOUBIQUITINATION1 (HUB1) in disease resistance. Loss-of-function alleles of HUB1 show increased susceptibility to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola, whereas HUB1 overexpression conferred resistance to B. cinerea. By contrast, responses to the bacterial pathogen Pseudomonas syringae are unaltered in hub1 plants. hub1 mutants have thinner cell walls but increased callose around an infection site. HUB1 acts independently of jasmonate, but ethylene (ET) responses and salicylate modulate the resistance of hub1 mutants to necrotrophic fungi. The ET response factor ETHYLENE INSENSITIVE2 is epistatic to HUB1 for A. brassicicola resistance but additive to HUB1 for B. cinerea resistance. HUB1 interacts with MED21, a subunit of the Arabidopsis Mediator, a conserved complex that regulates RNA polymerase II. RNA interference lines with reduced MED21 expression are highly susceptible to A. brassicicola and B. cinerea, whereas T-DNA insertion alleles are embryonic lethal, suggesting an essential role for MED21. However, HUB1-mediated histone H2B modification is independent of histone H3 and DNA methylation. In sum, histone H2B monoubiquitination is an important chromatin modification with regulatory roles in plant defense against necrotrophic fungi most likely through modulation of gene expression.

HISTONE MONOUBIQUITINATION1 interacts with a subunit of the mediator complex and regulates defense against necrotrophic fungal pathogens in Arabidopsis

This work examines the role of the Arabidopsis thaliana RING E3 ligase, HISTONE MONOUBIQUITINATION1 (HUB1) in disease resistance. Loss-of-function alleles of HUB1 show increased susceptibility to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola, whereas HUB1 overexpression conferred resistance to B. cinerea. By contrast, responses to the bacterial pathogen Pseudomonas syringae are unaltered in hub1 plants. hub1 mutants have thinner cell walls but increased callose around an infection site. HUB1 acts independently of jasmonate, but ethylene (ET) responses and salicylate modulate the resistance of hub1 mutants to necrotrophic fungi. The ET response factor ETHYLENE INSENSITIVE2 is epistatic to HUB1 for A. brassicicola resistance but additive to HUB1 for B. cinerea resistance. HUB1 interacts with MED21, a subunit of the Arabidopsis Mediator, a conserved complex that regulates RNA polymerase II. RNA interference lines with reduced MED21 expression are highly susceptible to A. brassicicola and B. cinerea, whereas T-DNA insertion alleles are embryonic lethal, suggesting an essential role for MED21. However, HUB1-mediated histone H2B modification is independent of histone H3 and DNA methylation. In sum, histone H2B monoubiquitination is an important chromatin modification with regulatory roles in plant defense against necrotrophic fungi most likely through modulation of gene expression.

Mutants of the Paf1 complex alter phenotypic expression of the yeast prion [PSI+]

The yeast [PSI+] prion is an epigenetic modifier of translation termination fidelity that causes nonsense suppression. The prion [PSI+] forms when the translation termination factor Sup35p adopts a self-propagating conformation. The presence of the [PSI+] prion modulates survivability in a variety of growth conditions. Nonsense suppression is essential for many [PSI+]-mediated phenotypes, but many do not appear to be due to read-through of a single stop codon, but instead are multigenic traits. We hypothesized that other global mechanisms act in concert with [PSI+] to influence [PSI+]-mediated phenotypes. We have identified one such global regulator, the Paf1 complex (Paf1C). Paf1C is conserved in eukaryotes and has been implicated in several aspects of transcriptional and posttranscriptional regulation. Mutations in Ctr9p and other Paf1C components reduced [PSI+]-mediated nonsense suppression. The CTR9 deletion also alters nonsense suppression afforded by other genetic mutations but not always to the same extent as the effects on [PSI+]-mediated read-through. Our data suggest that the Paf1 complex influences mRNA translatability but not solely through changes in transcript stability or abundance. Finally, we demonstrate that the CTR9 deletion alters several [PSI+]-dependent phenotypes. This provides one example of how [PSI+] and genetic modifiers can interact to uncover and regulate phenotypic variability.