SWI/SNF and Asf1p cooperate to displace histones during induction of the saccharomyces cerevisiae HO promoter

Overlapping coactivator function is required for transcriptional activation by the Candida glabrata Pdr1 transcription factor

Azole resistance in the pathogenic yeast Candida glabrata is a serious clinical complication and increasing in frequency. The majority of resistant organisms have been found to contain a substitution mutation in the Zn2Cys6 zinc cluster-containing transcription factor Pdr1. These mutations typically lead to this factor driving high, constitutive expression of target genes like the ATP-binding cassette transporter-encoding gene CDR1 . Overexpression of Cdr1 is required for the observed elevated fluconazole resistance exhibited by strains containing one of these hyperactive PDR1 alleles. While the identity of hyperactive PDR1 alleles has been extensively documented, the mechanisms underlying how these gain-of-function (GOF) forms of Pdr1 lead to elevated target gene transcription are not well understood. We have used a tandem affinity purification (TAP)-tagged form of Pdr1 to identify coactivator proteins that biochemically purify with the wild-type and two different GOF forms of Pdr1. Three coactivator proteins were found to associate with Pdr1: the SWI/SNF complex Snf2 chromatin remodeling protein and two different components of the SAGA complex, Spt7 and Ngg1. We found that deletion mutants lacking either SNF2 or SPT7 exhibited growth defects, even in the absence of fluconazole challenge. To overcome these issues, we employed a conditional degradation system to acutely deplete these coactivators and determined that loss of either coactivator complex, SWI/SNF or SAGA, caused defects in Pdr1-dependent transcription. A double degron strain that could be depleted for both SWI/SNF and SAGA exhibited a profound defect in PDR1 autoregulation, revealing that these complexes work together to ensure high level Pdr1-dependent gene transcription.

SWI/SNF and the histone chaperone Rtt106 drive expression of the Pleiotropic Drug Resistance network genes

The Pleiotropic Drug Resistance (PDR) network is central to the drug response in fungi, and its overactivation is associated with drug resistance. However, gene regulation of the PDR network is not well understood. Here, we show that the histone chaperone Rtt106 and the chromatin remodeller SWI/SNF control expression of the PDR network genes and confer drug resistance. In Saccharomyces cerevisiae, Rtt106 specifically localises to PDR network gene promoters dependent on transcription factor Pdr3, but not Pdr1, and is essential for Pdr3-mediated basal expression of the PDR network genes, while SWI/SNF is essential for both basal and drug-induced expression. Also in the pathogenic fungus Candida glabrata, Rtt106 and SWI/SNF regulate drug-induced PDR gene expression. Consistently, loss of Rtt106 or SWI/SNF sensitises drug-resistant S. cerevisiae mutants and C. glabrata to antifungal drugs. Since they cooperatively drive PDR network gene expression, Rtt106 and SWI/SNF represent potential therapeutic targets to combat antifungal resistance.

Targeting epigenetic machinery: Emerging novel allosteric inhibitors

Epigenetics has emerged as an extremely exciting fast-growing area of biomedical research in post genome era. Epigenetic dysfunction is tightly related with various diseases such as cancer and aging related degeneration, potentiating epigenetics modulators as important therapeutics targets. Indeed, inhibitors of histone deacetylase and DNA methyltransferase have been approved for treating blood tumor malignancies, whereas inhibitors of histone methyltransferase and histone acetyl-lysine recognizer bromodomain are in clinical stage. However, it remains a great challenge to discover potent and selective inhibitors by targeting catalytic site, as the same subfamily of epigenetic enzymes often share high sequence identity and very conserved catalytic core pocket. It is well known that epigenetic modifications are usually carried out by multi-protein complexes, and activation of catalytic subunit is often tightly regulated by other interactive protein component, especially in disease conditions. Therefore, it is not unusual that epigenetic complex machinery may exhibit allosteric regulation site induced by protein-protein interactions. Targeting allosteric site emerges as a compelling alternative strategy to develop epigenetic drugs with enhanced druggability and pharmacological profiles. In this review, we highlight recent progress in the development of allosteric inhibitors for epigenetic complexes through targeting protein-protein interactions. We also summarized the status of clinical applications of those inhibitors. Finally, we provide perspectives of future novel allosteric epigenetic machinery modulators emerging from otherwise undruggable single protein target.

SWI/SNF and RSC cooperate to reposition and evict promoter nucleosomes at highly expressed genes in yeast

The nucleosome remodeling complex RSC functions throughout the yeast genome to set the positions of -1 and +1 nucleosomes and thereby determines the widths of nucleosome-depleted regions (NDRs). The related complex SWI/SNF participates in nucleosome remodeling/eviction and promoter activation at certain yeast genes, including those activated by transcription factor Gcn4, but did not appear to function broadly in establishing NDRs. By analyzing the large cohort of Gcn4-induced genes in mutants lacking the catalytic subunits of SWI/SNF or RSC, we uncovered cooperation between these remodelers in evicting nucleosomes from different locations in the promoter and repositioning the +1 nucleosome downstream to produce wider NDRs-highly depleted of nucleosomes-during transcriptional activation. SWI/SNF also functions on a par with RSC at the most highly transcribed constitutively expressed genes, suggesting general cooperation by these remodelers for maximal transcription. SWI/SNF and RSC occupancies are greatest at the most highly expressed genes, consistent with their cooperative functions in nucleosome remodeling and transcriptional activation. Thus, SWI/SNF acts comparably with RSC in forming wide nucleosome-free NDRs to achieve high-level transcription but only at the most highly expressed genes exhibiting the greatest SWI/SNF occupancies.

A role for histone acetylation in regulating transcription elongation

Recently, we reported that a major function of histone acetylation at the yeast FLO1 gene was to regulate transcription elongation. Here, we discuss possible mechanisms by which histone acetylation might regulate RNA polymerase II processivity, and comment on the contribution to transcription of chromatin remodelling at gene coding regions and promoters.

Sas3 and Ada2(Gcn5)-dependent histone H3 acetylation is required for transcription elongation at the de-repressed FLO1 gene

The Saccharomyces cerevisiae FLO1 gene encodes a cell wall protein that imparts cell-cell adhesion. FLO1 transcription is regulated via the antagonistic activities of the Tup1-Cyc8 co-repressor and Swi-Snf co-activator complexes. Tup1-Cyc8 represses transcription through the organization of strongly positioned, hypoacetylated nucleosomes across gene promoters. Swi-Snf catalyzes remodeling of these nucleosomes in a mechanism involving histone acetylation that is poorly understood. Here, we show that FLO1 de-repression is accompanied by Swi-Snf recruitment, promoter histone eviction and Sas3 and Ada2(Gcn5)-dependent histone H3K14 acetylation. In the absence of H3K14 acetylation, Swi-Snf recruitment and histone eviction proceed, but transcription is reduced, suggesting these processes, while essential, are not sufficient for de-repression. Further analysis in the absence of H3K14 acetylation reveals RNAP II recruitment at the FLO1 promoter still occurs, but RNAP II is absent from the gene-coding region, demonstrating Sas3 and Ada2-dependent histone H3 acetylation is required for transcription elongation. Analysis of the transcription kinetics at other genes reveals shared mechanisms coupled to a distinct role for histone H3 acetylation, essential at FLO1, downstream of initiation. We propose histone H3 acetylation in the coding region provides rate-limiting control during the transition from initiation to elongation which dictates whether the gene is permissive for transcription.

Establishing nucleosome architecture and stability at promoters: Roles of pioneer transcription factors and the RSC chromatin remodeler

Improvements in deep sequencing, together with methods to rapidly deplete essential transcription factors (TFs) and chromatin remodelers, have recently led to a more detailed picture of promoter nucleosome architecture in yeast and its relationship to transcriptional regulation. These studies revealed that ∼40% of all budding yeast protein-coding genes possess a unique promoter structure, where we propose that an unusually unstable nucleosome forms immediately upstream of the transcription start site (TSS). This "fragile" nucleosome (FN) promoter architecture relies on the combined action of the essential RSC (Remodels Structure of Chromatin) nucleosome remodeler and pioneer transcription factors (PTFs). FNs are associated with genes whose expression is high, coupled to cell growth, and characterized by low cell-to-cell variability (noise), suggesting that they may promote these features. Recent studies in metazoans suggest that the presence of dynamic nucleosomes upstream of the TSS at highly expressed genes may be conserved throughout evolution.

Coordinated Action of Nap1 and RSC in Disassembly of Tandem Nucleosomes

The SWI/SNF and RSC family of ATP-dependent chromatin remodelers disassembles nucleosomes by moving nucleosomes into the vicinity of adjoining nucleosomes. We found that the histone chaperone Nap1 efficiently promotes disassembly of adjacent nucleosomes with which RSC collides and not the disassembly of nucleosomes mobilized by RSC. Nap1 is specific to RSC, as it does not target SWI/SNF, its paralog in Saccharomyces cerevisiae Extensive mutational analysis of Nap1 has revealed that Nap1 affinity for histones H2A-H2B and H3-H4 and its ability to displace histones from DNA are required for Nap1 to enhance RSC-mediated disassembly. Other histone chaperones, such as Vps75, that also bind histones are not able to enhance RSC-mediated disassembly. Our study suggests a mechanism by which Nap1 is recruited to actively transcribed regions and assists in the passage of the transcription complex through chromatin, and it provides a novel mechanism for the coordinated action of RSC and Nap1.

Mechanisms of ATP-Dependent Chromatin Remodeling Motors

Chromatin remodeling motors play essential roles in all DNA-based processes. These motors catalyze diverse outcomes ranging from sliding the smallest units of chromatin, known as nucleosomes, to completely disassembling chromatin. The broad range of actions carried out by these motors on the complex template presented by chromatin raises many stimulating mechanistic questions. Other well-studied nucleic acid motors provide examples of the depth of mechanistic understanding that is achievable from detailed biophysical studies. We use these studies as a guiding framework to discuss the current state of knowledge of chromatin remodeling mechanisms and highlight exciting open questions that would continue to benefit from biophysical analyses.

Genome-wide cooperation by HAT Gcn5, remodeler SWI/SNF, and chaperone Ydj1 in promoter nucleosome eviction and transcriptional activation

Chaperones, nucleosome remodeling complexes, and histone acetyltransferases have been implicated in nucleosome disassembly at promoters of particular yeast genes, but whether these cofactors function ubiquitously, as well as the impact of nucleosome eviction on transcription genome-wide, is poorly understood. We used chromatin immunoprecipitation of histone H3 and RNA polymerase II (Pol II) in mutants lacking single or multiple cofactors to address these issues for about 200 genes belonging to the Gcn4 transcriptome, of which about 70 exhibit marked reductions in H3 promoter occupancy on induction by amino acid starvation. Examining four target genes in a panel of mutants indicated that SWI/SNF, Gcn5, the Hsp70 cochaperone Ydj1, and chromatin-associated factor Yta7 are required downstream from Gcn4 binding, whereas Asf1/Rtt109, Nap1, RSC, and H2AZ are dispensable for robust H3 eviction in otherwise wild-type cells. Using ChIP-seq to interrogate all 70 exemplar genes in single, double, and triple mutants implicated Gcn5, Snf2, and Ydj1 in H3 eviction at most, but not all, Gcn4 target promoters, with Gcn5 generally playing the greatest role and Ydj1 the least. Remarkably, these three cofactors cooperate similarly in H3 eviction at virtually all yeast promoters. Defective H3 eviction in cofactor mutants was coupled with reduced Pol II occupancies for the Gcn4 transcriptome and the most highly expressed uninduced genes, but the relative Pol II levels at most genes were unaffected or even elevated. These findings indicate that nucleosome eviction is crucial for robust transcription of highly expressed genes but that other steps in gene activation are more rate-limiting for most other yeast genes.

ASF1 and the SWI/SNF complex interact functionally during nucleosome displacement, while FACT is required for nucleosome reassembly at yeast heat shock gene promoters during sustained stress

Histone chaperones are an integral part of the transcription regulatory machinery. We investigated the involvement of histone chaperones and their functional interactions with ATP-dependent chromatin remodeling complexes in the regulation of yeast heat shock genes. Strong functional interaction between the histone chaperone ASF1 and the ATP-dependent chromatin remodeling complex SWI/SNF is exhibited in synergistic diminishment of nucleosome displacement during heat shock in the ΔASF1/ΔSNF2 strain in comparison to individual ASF1 or SNF2 inactivation. A similar but less pronounced effect was observed for ISW1/ASF1 inactivation but not for ASF1/STH1 (RSC complex) combinatorial inactivation. The depletion of Spt16, which is a major subunit of the FACT histone chaperone complex, leads to a severe growth defect phenotype associated with unusual thermotolerance. The acquired thermotolerance in the Spt16-depleted strain is associated with a defect in the reassembly of nucleosomes at the promoters of heat shock genes during sustained heat stress, leading to increased recruitment of the transcriptional activator HSF and RNA polymerase II. The defect in nucleosome assembly associated with Spt16 depletion also leads to an increased tolerance to stress due to an increased concentration of NaCl.

Histone-modifying enzymes, histone modifications and histone chaperones in nucleosome assembly: Lessons learned from Rtt109 histone acetyltransferases

During DNA replication, nucleosomes ahead of replication forks are disassembled to accommodate replication machinery. Following DNA replication, nucleosomes are then reassembled onto replicated DNA using both parental and newly synthesized histones. This process, termed DNA replication-coupled nucleosome assembly (RCNA), is critical for maintaining genome integrity and for the propagation of epigenetic information, dysfunctions of which have been implicated in cancers and aging. In recent years, it has been shown that RCNA is carefully orchestrated by a series of histone modifications, histone chaperones and histone-modifying enzymes. Interestingly, many features of RCNA are also found in processes involving DNA replication-independent nucleosome assembly like histone exchange and gene transcription. In yeast, histone H3 lysine K56 acetylation (H3K56ac) is found in newly synthesized histone H3 and is critical for proper nucleosome assembly and for maintaining genomic stability. The histone acetyltransferase (HAT) regulator of Ty1 transposition 109 (Rtt109) is the sole enzyme responsible for H3K56ac in yeast. Much research has centered on this particular histone modification and histone-modifying enzyme. This Critical Review summarizes much of our current understanding of nucleosome assembly and highlights many important insights learned from studying Rtt109 HATs in fungi. We highlight some seminal features in nucleosome assembly conserved in mammalian systems and describe some of the lingering questions in the field. Further studying fungal and mammalian chromatin assembly may have important public health implications, including deeper understandings of human cancers and aging as well as the pursuit of novel anti-fungal therapies.

Regulating the chromatin landscape: structural and mechanistic perspectives

A large family of chromatin remodelers that noncovalently modify chromatin is crucial in cell development and differentiation. They are often the targets of cancer, neurological disorders, and other human diseases. These complexes alter nucleosome positioning, higher-order chromatin structure, and nuclear organization. They also assemble chromatin, exchange out histone variants, and disassemble chromatin at defined locations. We review aspects of the structural organization of these complexes, the functional properties of their protein domains, and variation between complexes. We also address the mechanistic details of these complexes in mobilizing nucleosomes and altering chromatin structure. A better understanding of these issues will be vital for further analyses of subunits of these chromatin remodelers, which are being identified as targets in human diseases by NGS (next-generation sequencing).

Decoupling nucleosome recognition from DNA binding dramatically alters the properties of the Chd1 chromatin remodeler

Chromatin remodelers can either organize or disrupt nucleosomal arrays, yet the mechanisms specifying these opposing actions are not clear. Here, we show that the outcome of nucleosome sliding by Chd1 changes dramatically depending on how the chromatin remodeler is targeted to nucleosomes. Using a Chd1–streptavidin fusion remodeler, we found that targeting via biotinylated DNA resulted in directional sliding towards the recruitment site, whereas targeting via biotinylated histones produced a distribution of nucleosome positions. Remarkably, the fusion remodeler shifted nucleosomes with biotinylated histones up to 50 bp off the ends of DNA and was capable of reducing negative supercoiling of plasmids containing biotinylated chromatin, similar to remodelling characteristics observed for SWI/SNF-type remodelers. These data suggest that forming a stable attachment to nucleosomes via histones, and thus lacking sensitivity to extranucleosomal DNA, seems to be sufficient for allowing a chromatin remodeler to possess SWI/SNF-like disruptive properties.

Chromatin and transcription in yeast

Understanding the mechanisms by which chromatin structure controls eukaryotic transcription has been an intense area of investigation for the past 25 years. Many of the key discoveries that created the foundation for this field came from studies of Saccharomyces cerevisiae, including the discovery of the role of chromatin in transcriptional silencing, as well as the discovery of chromatin-remodeling factors and histone modification activities. Since that time, studies in yeast have continued to contribute in leading ways. This review article summarizes the large body of yeast studies in this field.

The Transition of Poised RNA Polymerase II to an Actively Elongating State Is a "Complex" Affair

The initial discovery of the occupancy of RNA polymerase II at certain genes prior to their transcriptional activation occurred a quarter century ago in Drosophila. The preloading of these poised complexes in this inactive state is now apparent in many different organisms across the evolutionary spectrum and occurs at a broad and diverse set of genes. In this paper, we discuss the genetic and biochemical efforts in S. cerevisiae to describe the conversion of these poised transcription complexes to the active state for productive elongation. The accumulated evidence demonstrates that a multitude of coactivators and chromatin remodeling complexes are essential for this transition.

In vivo role for the chromatin-remodeling enzyme SWI/SNF in the removal of promoter nucleosomes by disassembly rather than sliding

Analysis of in vivo chromatin remodeling at the PHO5 promoter of yeast led to the conclusion that remodeling removes nucleosomes from the promoter by disassembly rather than sliding away from the promoter. The catalytic activities required for nucleosome disassembly remain unknown. Transcriptional activation of the yeast PHO8 gene was found to depend on the chromatin-remodeling complex SWI/SNF, whereas activation of PHO5 was not. Here, we show that PHO8 gene circles formed in vivo lose nucleosomes upon PHO8 induction, indicative of nucleosome removal by disassembly. Our quantitative analysis of expression noise and chromatin-remodeling data indicates that the dynamics of continual nucleosome removal and reformation at the activated promoters of PHO5 and PHO8 are closely similar. In contrast to PHO5, however, activator-stimulated transcription of PHO8 appears to be limited mostly to the acceleration of promoter nucleosome disassembly with little or no acceleration of promoter transitions following nucleosome disassembly, accounting for the markedly lower expression level of PHO8.

What do expression dynamics tell us about the mechanism of transcription?

Single-cell microscopy studies have the potential to provide an unprecedented view of gene expression with exquisite spatial and temporal sensitivity. However, there is a challenge to connect the holistic cellular view with a reductionist biochemical view. In particular, experimental efforts to characterize the in vivo regulation of transcription have focused primarily on measurements of the dynamics of transcription factors and chromatin modifying factors. Such measurements have elucidated the transient nature of many nuclear interactions. In the past few years, experimental approaches have emerged that allow for interrogation of the output of transcription at the single-molecule, single-cell level. Here, I summarize the experimental results and models that aim to provide an integrated view of transcriptional regulation.

Structure and mechanism of the Swi2/Snf2 remodeller Mot1 in complex with its substrate TBP

Swi2/Snf2-type ATPases regulate genome-associated processes such as transcription, replication and repair by catalysing the disruption, assembly or remodelling of nucleosomes or other protein-DNA complexes. It has been suggested that ATP-driven motor activity along DNA disrupts target protein-DNA interactions in the remodelling reaction. However, the complex and highly specific remodelling reactions are poorly understood, mostly because of a lack of high-resolution structural information about how remodellers bind to their substrate proteins. Mot1 (modifier of transcription 1 in Saccharomyces cerevisiae, denoted BTAF1 in humans) is a Swi2/Snf2 enzyme that specifically displaces the TATA box binding protein (TBP) from the promoter DNA and regulates transcription globally by generating a highly dynamic TBP pool in the cell. As a Swi2/Snf2 enzyme that functions as a single polypeptide and interacts with a relatively simple substrate, Mot1 offers an ideal system from which to gain a better understanding of this important enzyme family. To reveal how Mot1 specifically disrupts TBP-DNA complexes, we combined crystal and electron microscopy structures of Mot1-TBP from Encephalitozoon cuniculi with biochemical studies. Here we show that Mot1 wraps around TBP and seems to act like a bottle opener: a spring-like array of 16 HEAT (huntingtin, elongation factor 3, protein phosphatase 2A and lipid kinase TOR) repeats grips the DNA-distal side of TBP via loop insertions, and the Swi2/Snf2 domain binds to upstream DNA, positioned to weaken the TBP-DNA interaction by DNA translocation. A 'latch' subsequently blocks the DNA-binding groove of TBP, acting as a chaperone to prevent DNA re-association and ensure efficient promoter clearance. This work shows how a remodelling enzyme can combine both motor and chaperone activities to achieve functional specificity using a conserved Swi2/Snf2 translocase.

Chromatin remodelers clear nucleosomes from intrinsically unfavorable sites to establish nucleosome-depleted regions at promoters

Most promoters in yeast contain a nucleosome-depleted region (NDR), but the mechanisms by which NDRs are established and maintained in vivo are currently unclear. We have examined how genome-wide nucleosome placement is altered in the absence of two distinct types of nucleosome remodeling activity. In mutants of both SNF2, which encodes the ATPase component of the Swi/Snf remodeling complex, and ASF1, which encodes a histone chaperone, distinct sets of gene promoters carry excess nucleosomes in their NDRs relative to wild-type. In snf2 mutants, excess promoter nucleosomes correlate with reduced gene expression. In both mutants, the excess nucleosomes occupy DNA sequences that are energetically less favorable for nucleosome formation, indicating that intrinsic histone-DNA interactions are not sufficient for nucleosome positioning in vivo, and that Snf2 and Asf1 promote thermodynamic equilibration of nucleosomal arrays. Cells lacking SNF2 or ASF1 still accomplish the changes in promoter nucleosome structure associated with large-scale transcriptional reprogramming. However, chromatin reorganization in the mutants is reduced in extent compared to wild-type cells, even though transcriptional changes proceed normally. In summary, active remodeling is required for distributing nucleosomes to energetically favorable positions in vivo and for reorganizing chromatin in response to changes in transcriptional activity.

The SWI/SNF complex acts to constrain distribution of the centromeric histone variant Cse4

The SWI/SNF complex has an important role in regulating chromatin structure during transcriptional activation and DNA repair. Here, the SWI/SNF complex is also involved in the organisation of centromeric chromatin and prevention of the ectopic deposition of centromeric histone variants.

In order to gain insight into the function of the Saccharomyces cerevisiae SWI/SNF complex, we have identified DNA sequences to which it is bound genomewide. One surprising observation is that the complex is enriched at the centromeres of each chromosome. Deletion of the gene encoding the Snf2 subunit of the complex was found to cause partial redistribution of the centromeric histone variant Cse4 to sites on chromosome arms. Cultures of snf2Δ yeast were found to progress through mitosis slowly. This was dependent on the mitotic checkpoint protein Mad2. In the absence of Mad2, defects in chromosome segregation were observed. In the absence of Snf2, chromatin organisation at centromeres is less distinct. In particular, hypersensitive sites flanking the Cse4 containing nucleosomes are less pronounced. Furthermore, SWI/SNF complex was found to be especially effective in the dissociation of Cse4 containing chromatin in vitro. This suggests a role for Snf2 in the maintenance of point centromeres involving the removal of Cse4 from ectopic sites.

SWI/SNF has intrinsic nucleosome disassembly activity that is dependent on adjacent nucleosomes

The ATP-dependent chromatin remodeling complex SWI/SNF regulates transcription and has been implicated in promoter nucleosome eviction. Efficient nucleosome disassembly by SWI/SNF alone in biochemical assays, however, has not been directly observed. Employing a model system of dinucleosomes rather than mononucleosomes, we demonstrate that remodeling leads to ordered and efficient disassembly of one of the two nucleosomes. An H2A/H2B dimer is first rapidly displaced, and then, in a slower reaction, an entire histone octamer is lost. Nucleosome disassembly by SWI/SNF did not require additional factors such as chaperones or acceptors of histones. Observations in single molecules as well as bulk measurement suggest that a key intermediate in this process is one in which a nucleosome is moved toward the adjacent nucleosome. SWI/SNF recruited by the transcriptional activator Gal4-VP16 preferentially mobilizes the proximal nucleosome and destabilizes the adjacent nucleosome.

Tousled kinase TLK1B mediates chromatin assembly in conjunction with Asf1 regardless of its kinase activity

Background

The Tousled Like Kinases (TLKs) are involved in chromatin dynamics, including DNA replication and repair, transcription, and chromosome segregation. Indeed, the first two TLK1 substrates were identified as the histone H3 and Asf1 (a histone H3/H4 chaperone), which immediately suggested a function in chromatin remodeling. However, despite the straightforward assumption that TLK1 acts simply by phosphorylating its substrates and hence modifying their activity, TLK1 also acts as a chaperone. In fact, a kinase-dead (KD) mutant of TLK1B is functional in stimulating chromatin assembly in vitro. However, subtle effects of Asf1 phosphorylation are more difficult to probe in chromatin assembly assays. Not until very recently was the Asf1 site phosphorylated by TLK1 identified. This has allowed for probing directly the functionality of a site-directed mutant of Asf1 in chromatin assembly assays.

Findings

Addition of either wt or non-phosphorylatable mutant Asf1 to nuclear extract stimulates chromatin assembly on a plasmid. Similarly, TLK1B-KD stimulates chromatin assembly and it synergizes in reactions with supplemental Asf1 (wt or non-phosphorylatable mutant).

Conclusions

Although the actual function of TLKs as mediators of Asf1 activity cannot be easily studied in vivo, particularly since in mammalian cells there are two TLK genes and two Asf1 genes, we were able to study specifically the stimulation of chromatin assembly in vitro. In such assays, clearly the TLK1 kinase activity was not critical, as neither a non-phosphorylatable Asf1 nor use of the TLK1B-KD impaired the stimulation of nucleosome formation.

New functions for an old variant: no substitute for histone H3.3

Histone proteins often come in different variants serving specialized functions in addition to their fundamental role in packaging DNA. The metazoan histone H3.3 has been most closely associated with active transcription. Its role in histone replacement at active genes and promoters is conserved to the single histone H3 in yeast. However, recent genetic studies in flies have challenged its importance as a mark of active chromatin, and revealed unexpected insights into essential functions of H3.3 in the germline. With strikingly little amino acid sequence difference to the canonical H3, H3.3 therefore accomplishes a surprising variety of cellular and developmental processes.

Phosphorylation-mediated control of histone chaperone ASF1 levels by Tousled-like kinases

Histone chaperones are at the hub of a diverse interaction networks integrating a plethora of chromatin modifying activities. Histone H3/H4 chaperone ASF1 is a target for cell-cycle regulated Tousled-like kinases (TLKs) and both proteins cooperate during chromatin replication. However, the precise role of post-translational modification of ASF1 remained unclear. Here, we identify the TLK phosphorylation sites for both Drosophila and human ASF1 proteins. Loss of TLK-mediated phosphorylation triggers hASF1a and dASF1 degradation by proteasome-dependent and independent mechanisms respectively. Consistent with this notion, introduction of phosphorylation-mimicking mutants inhibits hASF1a and dASF1 degradation. Human hASF1b is also targeted for proteasome-dependent degradation, but its stability is not affected by phosphorylation indicating that other mechanisms are likely to be involved in control of hASF1b levels. Together, these results suggest that ASF1 cellular levels are tightly controlled by distinct pathways and provide a molecular mechanism for post-translational regulation of dASF1 and hASF1a by TLK kinases.