Phosphorylation of very-lysine-rich histone in Physarum polycephalum. Correlation with chromosome condensation

Helical chromonema coiling is conserved in eukaryotes

Efficient chromatin condensation is required to transport chromosomes during mitosis and meiosis, forming daughter cells. While it is well accepted that these processes follow fundamental rules, there has been a controversial debate for more than 140 years on whether the higher-order chromatin organization in chromosomes is evolutionarily conserved. Here, we summarize historical and recent investigations based on classical and modern methods. In particular, classical light microscopy observations based on living, fixed, and treated chromosomes covering a wide range of plant and animal species, and even in single-cell eukaryotes suggest that the chromatids of large chromosomes are formed by a coiled chromatin thread, named the chromonema. More recently, these findings were confirmed by electron and super-resolution microscopy, oligo-FISH, molecular interaction data, and polymer simulation. Altogether, we describe common and divergent features of coiled chromonemata in different species. We hypothesize that chromonema coiling in large chromosomes is a fundamental feature established early during the evolution of eukaryotes to handle increasing genome sizes.

Role of posttranslational modifications in memory and cognitive impairments caused by neonatal sevoflurane exposure

With the advancement of technology, increasingly many newborns are receiving general anesthesia at a young age for surgery, other interventions, or clinical assessment. Anesthetics cause neurotoxicity and apoptosis of nerve cells, leading to memory and cognitive impairments. The most frequently used anesthetic in infants is sevoflurane; however, it has the potential to be neurotoxic. A single, short bout of sevoflurane exposure has little impact on cognitive function, but prolonged or recurrent exposure to general anesthetics can impair memory and cognitive function. However, the mechanisms underlying this association remain unknown. Posttranslational modifications (PTMs), which can be described roughly as the regulation of gene expression, protein activity, and protein function, have sparked enormous interest in neuroscience. Posttranslational modifications are a critical mechanism mediating anesthesia-induced long-term modifications in gene transcription and protein functional deficits in memory and cognition in children, according to a growing body of studies in recent years. Based on these recent findings, our paper reviews the effects of sevoflurane on memory loss and cognitive impairment, discusses how posttranslational modifications mechanisms can contribute to sevoflurane-induced neurotoxicity, and provides new insights into the prevention of sevoflurane-induced memory and cognitive impairments.

Post-Translation Modifications and Mutations of Human Linker Histone Subtypes: Their Manifestation in Disease

Linker histones (LH) are a critical component of chromatin in addition to the canonical histones (H2A, H2B, H3, and H4). In humans, 11 subtypes (7 somatic and 4 germinal) of linker histones have been identified, and their diverse cellular functions in chromatin structure, DNA replication, DNA repair, transcription, and apoptosis have been explored, especially for the somatic subtypes. Delineating the unique role of human linker histone (hLH) and their subtypes is highly tedious given their high homology and overlapping expression patterns. However, recent advancements in mass spectrometry combined with HPLC have helped in identifying the post-translational modifications (PTMs) found on the different LH subtypes. However, while a number of PTMs have been identified and their potential nuclear and non-nuclear functions explored in cellular processes, there are very few studies delineating the direct relevance of these PTMs in diseases. In addition, recent whole-genome sequencing of clinical samples from cancer patients and individuals afflicted with Rahman syndrome have identified high-frequency mutations and therefore broadened the perspective of the linker histone mutations in diseases. In this review, we compile the identified PTMs of hLH subtypes, current knowledge of the relevance of hLH PTMs in human diseases, and the correlation of PTMs coinciding with mutations mapped in diseases.

Epigenetic Mechanisms in Memory and Cognitive Decline Associated with Aging and Alzheimer's Disease

Epigenetic mechanisms, which include DNA methylation, a variety of post-translational modifications of histone proteins (acetylation, phosphorylation, methylation, ubiquitination, sumoylation, serotonylation, dopaminylation), chromatin remodeling enzymes, and long non-coding RNAs, are robust regulators of activity-dependent changes in gene transcription. In the brain, many of these epigenetic modifications have been widely implicated in synaptic plasticity and memory formation. Dysregulation of epigenetic mechanisms has been reported in the aged brain and is associated with or contributes to memory decline across the lifespan. Furthermore, alterations in the epigenome have been reported in neurodegenerative disorders, including Alzheimer’s disease. Here, we review the diverse types of epigenetic modifications and their role in activity- and learning-dependent synaptic plasticity. We then discuss how these mechanisms become dysregulated across the lifespan and contribute to memory loss with age and in Alzheimer’s disease. Collectively, the evidence reviewed here strongly supports a role for diverse epigenetic mechanisms in memory formation, aging, and neurodegeneration in the brain.

Mitotic chromosomes

Our understanding of the structure and function of mitotic chromosomes has come a long way since these iconic objects were first recognized more than 140 years ago, though many details remain to be elucidated. In this chapter, we start with the early history of chromosome studies and then describe the path that led to our current understanding of the formation and structure of mitotic chromosomes. We also discuss some of the remaining questions. It is now well established that each mitotic chromatid consists of a central organizing region containing a so-called "chromosome scaffold" from which loops of DNA project radially. Only a few key non-histone proteins and protein complexes are required to form the chromosome: topoisomerase IIα, cohesin, condensin I and condensin II, and the chromokinesin KIF4A. These proteins are concentrated along the axis of the chromatid. Condensins I and II are primarily responsible for shaping the chromosome and the scaffold, and they produce the loops of DNA by an ATP-dependent process known as loop extrusion. Modelling of Hi-C data suggests that condensin II adopts a spiral staircase arrangement with an extruded loop extending out from each step in a roughly helical pattern. Condensin I then forms loops nested within these larger condensin II loops, thereby giving rise to the final compaction of the mitotic chromosome in a process that requires Topo IIα.

Mechanisms of Oocyte Maturation and Related Epigenetic Regulation

Meiosis is the basis of sexual reproduction. In female mammals, meiosis of oocytes starts before birth and sustains at the dictyate stage of meiotic prophase I before gonadotropins-induced ovulation happens. Once meiosis gets started, the oocytes undergo the leptotene, zygotene, and pachytene stages, and then arrest at the dictyate stage. During each estrus cycle in mammals, or menstrual cycle in humans, a small portion of oocytes within preovulatory follicles may resume meiosis. It is crucial for females to supply high quality mature oocytes for sustaining fertility, which is generally achieved by fine-tuning oocyte meiotic arrest and resumption progression. Anything that disturbs the process may result in failure of oogenesis and seriously affect both the fertility and the health of females. Therefore, uncovering the regulatory network of oocyte meiosis progression illuminates not only how the foundations of mammalian reproduction are laid, but how mis-regulation of these steps result in infertility. In order to provide an overview of the recently uncovered cellular and molecular mechanism during oocyte maturation, especially epigenetic modification, the progress of the regulatory network of oocyte meiosis progression including meiosis arrest and meiosis resumption induced by gonadotropins is summarized. Then, advances in the epigenetic aspects, such as histone acetylation, phosphorylation, methylation, glycosylation, ubiquitination, and SUMOylation related to the quality of oocyte maturation are reviewed.

Histone H1 Post-Translational Modifications: Update and Future Perspectives

Histone H1 is the most variable histone and its role at the epigenetic level is less characterized than that of core histones. In vertebrates, H1 is a multigene family, which can encode up to 11 subtypes. The H1 subtype composition is different among cell types during the cell cycle and differentiation. Mass spectrometry-based proteomics has added a new layer of complexity with the identification of a large number of post-translational modifications (PTMs) in H1. In this review, we summarize histone H1 PTMs from lower eukaryotes to humans, with a particular focus on mammalian PTMs. Special emphasis is made on PTMs, whose molecular function has been described. Post-translational modifications in H1 have been associated with the regulation of chromatin structure during the cell cycle as well as transcriptional activation, DNA damage response, and cellular differentiation. Additionally, PTMs in histone H1 that have been linked to diseases such as cancer, autoimmune disorders, and viral infection are examined. Future perspectives and challenges in the profiling of histone H1 PTMs are also discussed.

Site-specific regulation of histone H1 phosphorylation in pluripotent cell differentiation

BACKGROUND

Structural variation among histone H1 variants confers distinct modes of chromatin binding that are important for differential regulation of chromatin condensation, gene expression and other processes. Changes in the expression and genomic distributions of H1 variants during cell differentiation appear to contribute to phenotypic differences between cell types, but few details are known about the roles of individual H1 variants and the significance of their disparate capacities for phosphorylation. In this study, we investigated the dynamics of interphase phosphorylation at specific sites in individual H1 variants during the differentiation of pluripotent NT2 and mouse embryonic stem cells and characterized the kinases involved in regulating specific H1 variant phosphorylations in NT2 and HeLa cells.

RESULTS

Here, we show that the global levels of phosphorylation at H1.5-Ser18 (pS18-H1.5), H1.2/H1.5-Ser173 (pS173-H1.2/5) and H1.4-Ser187 (pS187-H1.4) are regulated differentially during pluripotent cell differentiation. Enrichment of pS187-H1.4 near the transcription start site of pluripotency factor genes in pluripotent cells is markedly reduced upon differentiation, whereas pS187-H1.4 levels at housekeeping genes are largely unaltered. Selective inhibition of CDK7 or CDK9 rapidly diminishes pS187-H1.4 levels globally and its enrichment at housekeeping genes, and similar responses were observed following depletion of CDK9. These data suggest that H1.4-S187 is a bona fide substrate for CDK9, a notion that is further supported by the significant colocalization of CDK9 and pS187-H1.4 to gene promoters in reciprocal re-ChIP analyses. Moreover, treating cells with actinomycin D to inhibit transcription and trigger the release of active CDK9/P-TEFb from 7SK snRNA complexes induces the accumulation of pS187-H1.4 at promoters and gene bodies. Notably, the levels of pS187-H1.4 enrichment after actinomycin D treatment or cell differentiation reflect the extent of CDK9 recruitment at the same loci. Remarkably, the global levels of H1.5-S18 and H1.2/H1.5-S173 phosphorylation are not affected by these transcription inhibitor treatments, and selective inhibition of CDK2 does not affect the global levels of phosphorylation at H1.4-S187 or H1.5-S18.

CONCLUSIONS

Our data provide strong evidence that H1 variant interphase phosphorylation is dynamically regulated in a site-specific and gene-specific fashion during pluripotent cell differentiation, and that enrichment of pS187-H1.4 at genes is positively related to their transcription. H1.4-S187 is likely to be a direct target of CDK9 during interphase, suggesting the possibility that this particular phosphorylation may contribute to the release of paused RNA pol II. In contrast, the other H1 variant phosphorylations we investigated appear to be mediated by distinct kinases and further analyses are needed to determine their functional significance.

Epigenetic characteristics of the mitotic chromosome in 1D and 3D

While chromatin characteristics in interphase are widely studied, characteristics of mitotic chromatin and their inheritance through mitosis are still poorly understood. During mitosis, chromatin undergoes dramatic changes: transcription stalls, chromatin-binding factors leave the chromatin, histone modifications change and chromatin becomes highly condensed. Many key insights into mitotic chromosome state and conformation have come from extensive microscopy studies over the last century. Over the last decade, the development of 3C-based techniques has enabled the study of higher order chromosome organization during mitosis in a genome-wide manner. During mitosis, chromosomes lose their cell type-specific and locus-dependent chromatin organization that characterizes interphase chromatin and fold into randomly positioned loop arrays. Upon exit of mitosis, cells are capable of quickly rearranging the chromosome conformation to form the cell type-specific interphase organization again. The information that enables this rearrangement after mitotic exit is thought to be encoded at least in part in mitotic bookmarks, e.g. histone modifications and variants, histone remodelers, chromatin factors, and non-coding RNA. Here we give an overview of the chromosomal organization and epigenetic characteristics of interphase and mitotic chromatin in vertebrates. Second, we describe different ways in which mitotic bookmarking enables epigenetic memory of the features of interphase chromatin through mitosis. And third, we explore the role of epigenetic modifications and mitotic bookmarking in cell differentiation.

Interphase H1 phosphorylation: Regulation and functions in chromatin

Many metazoan cell types differentially express multiple non-allelic amino acid sequence variants of histone H1. Although early work revealed that H1 variants, collectively, are phosphorylated during interphase and mitosis, differences between individual H1 variants in the sites they possess for mitotic and interphase phosphorylation have been elucidated only relatively recently. Here, we review current knowledge on the regulation and function of interphase H1 phosphorylation, with a particular emphasis on how differences in interphase phosphorylation among the H1 variants of mammalian cells may enable them to have differential effects on transcription and other chromatin processes.

Quantitative Mass Spectrometry Reveals that Intact Histone H1 Phosphorylations are Variant Specific and Exhibit Single Molecule Hierarchical Dependence

Breast cancer was the second leading cause of cancer related mortality for females in 2014. Recent studies suggest histone H1 phosphorylation may be useful as a clinical biomarker of breast and other cancers because of its ability to recognize proliferative cell populations. Although monitoring a single phosphorylated H1 residue is adequate to stratify high-grade breast tumors, expanding our knowledge of how H1 is phosphorylated through the cell cycle is paramount to understanding its role in carcinogenesis. H1 analysis by bottom-up MS is challenging because of the presence of highly homologous sequence variants expressed by most cells. These highly basic proteins are difficult to analyze by LC-MS/MS because of the small, hydrophilic nature of peptides produced by tryptic digestion. Although bottom-up methods permit identification of several H1 phosphorylation events, these peptides are not useful for observing the combinatorial post-translational modification (PTM) patterns on the protein of interest. To complement the information provided by bottom-up MS, we utilized a top-down MS/MS workflow to permit identification and quantitation of H1 proteoforms related to the progression of breast cells through the cell cycle. Histones H1.2 and H1.4 were observed in MDA-MB-231 metastatic breast cells, whereas an additional histone variant, histone H1.3, was identified only in nonneoplastic MCF-10A cells. Progressive phosphorylation of histone H1.4 was identified in both cell lines at mitosis (M phase). Phosphorylation occurred first at S172 followed successively by S187, T18, T146, and T154. Notably, phosphorylation at S173 of histone H1.2 and S172, S187, T18, T146, and T154 of H1.4 significantly increases during M phase relative to S phase, suggesting that these events are cell cycle-dependent and may serve as markers for proliferation. Finally, we report the observation of the H1.2 SNP variant A18V in MCF-10A cells.

Transient expression of RAD51 in the late G2-phase is required for cell cycle progression in synchronous Physarum cells

The homologous recombination factor RAD51 is highly conserved. This criterion enabled us to identify a RAD51 ortholog in Physarum polycephalum. We found that the Physarum protein presents a high homology to the human protein and cross-reacted with antibodies directed against the human RAD51. Taking advantage of the natural synchrony of millions of nuclei within a single cell of Physarum, we investigated the fluctuation of the amount of the PpRAD51 throughout the cell cycle. Our results showed that in the late G2-phase, RAD51 was transiently expressed in a large quantity. Furthermore, knocking-down RAD51 in the G2-phase abolished this transient expression before mitosis and affected cell cycle progression. These results support the idea that RAD51 plays a role in the progression of the cell cycle in the late G2-phase.

Histone H1 phosphorylation in breast cancer

Breast cancer is the second leading cause of cancer-related deaths in women. The need for new clinical biomarkers in breast cancer is necessary to further predict prognosis and therapeutic response. In this article, the LC-MS histone H1 phosphorylation profiles were established for three distinct breast cancer cell lines. The results show that the extent of H1 phosphorylation can distinguish between the different cell lines. The histone H1 from the metastatic cell line, MDA-MB-231, was subjected to chemical derivitization and LC-MS/MS analysis. The results suggest that the phosphorylation at threonine 146 is found on both histone H1.2 and histone H1.4. Cell lines were then treated with an extracellular stimulus, estradiol or kinase inhibitor LY294002, to monitor changes in histone H1 phosphorylation. The data show that histone H1 phosphorylation can increase and decrease in response to extracellular stimuli. Finally, primary breast tissues were stained for the histone H1 phosphorylation at threonine 146. Variable staining patterns across tumor grades and subtypes were observed with pT146 labeling correlating with tumor grade. These results establish the potential for histone H1 phosphorylation at threonine 146 as a clinical biomarker in breast cancer.

Isolation and analysis of linker histones across cellular compartments

Analysis of histones, especially histone H1, is severely limited by immunological reagent availability. This paper describes the application of cellular fractionation with LC-MS for profiling histones in the cytosol and upon chromatin. First, we show that linker histones enriched by cellular fractionation gives less nuclear contamination and higher histone content than when prepared by nuclei isolation. Second, we profiled the soluble linker histones throughout the cell cycle revealing phosphorylation increases as cells reach mitosis. Finally, we monitored histone H1.2-H1.5 translocation to the cytosol in response to the CDK inhibitor flavopiridol in primary CLL cells treated ex vivo. Data shows that all H1 variants translocate in response to drug treatment with no specific order to their cytosolic appearance. The results illustrate the utility of cellular fractionation in conjunction with LC-MS for the analysis of histone H1 throughout the cell.

BIOLOGICAL SIGNIFICANCE

This paper demonstrates the first time application of cellular fractionation to characterize cytosolic histone H1 by liquid chromatography mass spectrometry (LC-MS). Using the Ramos Burkitt's lymphoma cell line, cellular fractionation was shown to give less nuclear contamination and higher histone content than preparations by nuclei isolation. Further application of the cellular fractionation approach was shown by using primary chronic lymphocytic leukemia (CLL) cells to monitor the movement of histone H1 across cellular compartments in response to the cyclin dependent kinase inhibitor flavopiridol. Collectively, these data establish a mass spectrometric method for exploration into the function of cytosolic histone H1.

H1 histones: current perspectives and challenges

H1 and related linker histones are important both for maintenance of higher-order chromatin structure and for the regulation of gene expression. The biology of the linker histones is complex, as they are evolutionarily variable, exist in multiple isoforms and undergo a large variety of posttranslational modifications in their long, unstructured, NH₂- and COOH-terminal tails. We review recent progress in understanding the structure, genetics and posttranslational modifications of linker histones, with an emphasis on the dynamic interactions of these proteins with DNA and transcriptional regulators. We also discuss various experimental challenges to the study of H1 and related proteins, including limitations of immunological reagents and practical difficulties in the analysis of posttranslational modifications by mass spectrometry.

H4 replication-dependent diacetylation and Hat1 promote S-phase chromatin assembly in vivo

This study examined the function of H3 and H4 tail domains in replication-dependent chromatin assembly. Results show distinct functions of H3 and H4 tails in nuclear import and chromatin assembly. Further investigations show that H4 diacetylation is essential but not sufficient for nuclear import, as preventing Hat1 binding impedes histone transport in nuclei.

While specific posttranslational modification patterns within the H3 and H4 tail domains are associated with the S-phase, their actual functions in replication-dependent chromatin assembly have not yet been defined. Here we used incorporation of trace amounts of recombinant proteins into naturally synchronous macroplasmodia of Physarum polycephalum to examine the function of H3 and H4 tail domains in replication-coupled chromatin assembly. We found that the H3/H4 complex lacking the H4 tail domain was not efficiently recovered in nuclei, whereas depletion of the H3 tail domain did not impede nuclear import but chromatin assembly failed. Furthermore, our results revealed that the proper pattern of acetylation on the H4 tail domain is required for nuclear import and chromatin assembly. This is most likely due to binding of Hat1, as coimmunoprecipitation experiments showed Hat1 associated with predeposition histones in the cytoplasm and with replicating chromatin. These results suggest that the type B histone acetyltransferase assists in shuttling the H3/H4 complex from cytoplasm to the replication forks.

Histone H1 and its isoforms: contribution to chromatin structure and function

The lysine-rich H1 histone family in mammals includes eleven different subtypes, and thus it is the most divergent class of histone proteins. The central globular H1 domain asymmetrically interacts with DNA at the exit or entry end of the nucleosomal core DNA, and the C-terminal domain has a major impact on the linker DNA conformation and chromatin condensation. H1 histones are thus involved in the formation of higher order chromatin structures, and they modulate the accessibility of regulatory proteins, chromatin remodeling factors and histone modification enzymes to their target sites. The major posttranslational modification of H1 histones is phosphorylation, which reaches a peak during G2 and mitosis. Phosphorylation is, however, also involved in the control of DNA replication and it contributes to the regulation of gene expression. Disruption of linker histone genes, initially performed in order to delineate subtype-specific functions, revealed that disruption of one or two H1 subtype genes is quantitatively compensated by an increased expression of other subtypes. This suggests a functional redundancy among H1 subtypes. However, the inactivation of three subtypes and the reduction of the H1 moiety in half finally resulted in a phenotypic effect. On the other hand, studies on the role of particular subtypes at specific developmental stages in lower eukaryotes, but also in vertebrates suggest that specific subtypes of H1 participate in particular systems of gene regulation.

Phosphorylation of the carboxy-terminal domain of histone H1: effects on secondary structure and DNA condensation

Linker histone H1 plays an important role in chromatin folding. Phosphorylation by cyclin-dependent kinases is the main post-translational modification of histone H1. We studied the effects of phosphorylation on the secondary structure of the DNA-bound H1 carboxy-terminal domain (CTD), which contains most of the phosphorylation sites of the molecule. The effects of phosphorylation on the secondary structure of the DNA-bound CTD were site-specific and depended on the number of phosphate groups. Full phosphorylation significantly increased the proportion of beta-structure and decreased that of alpha-helix. Partial phosphorylation increased the amount of undefined structure and decreased that of alpha-helix without a significant increase in beta-structure. Phosphorylation had a moderate effect on the affinity of the CTD for the DNA, which was proportional to the number of phosphate groups. Partial phosphorylation drastically reduced the aggregation of DNA fragments by the CTD, but full phosphorylation restored to a large extent the aggregation capacity of the unphosphorylated domain. These results support the involvement of H1 hyperphosphorylation in metaphase chromatin condensation and of H1 partial phosphorylation in interphase chromatin relaxation. More generally, our results suggest that the effects of phosphorylation are mediated by specific structural changes and are not simply a consequence of the net charge.

Conformations and flexibilities of histones and high mobility group (HMG) proteins in chromatin structure and function

The packaging of the enormous lengths of eukaryotic DNA into the different conformational states of chromosomes is controlled very largely by an equal total mass of the histones, H1, H2A, H2B, H3 and H4. Histone sequences, sequence conservations, postsynthetic chemical modifications and studies of histone conformations and interactions show clearly that histones are multi-domain proteins. The N-terminal domains of all histones and the C-terminal domains of H1, H2A and H2B are flexible random coils, while the C-terminal regions of H3 and H4 and the central regions of H1, H2A and H2B are structured. Histones H3 and H4 are essential for nucleosome structure and interact with DNA to give the nucleus of the nucleosome structure, which is completed by interactions of the conserved structured regions of (H2A, H2B) dimers and H1. The flexible domains of all the histones are very basic and contain all the sites of reversible chemical modifications: acetylation of lysines in the core histones and phosphorylation of serines and threonines in histone H1. Strict correlations have been observed (i) between acetylation and DNA processing and (ii) between H1 phosphorylation and chromosome condensation. In addition to histone acetylation, active chromatin is also associated with high mobility group (HMG) proteins 14 and 17. These proteins are completely flexible under all solution conditions and their native structures must be imposed by their binding sites in active chromatin. The function of flexibility in these chromosomal proteins is not understood but is probably related to the enormous lengths of DNA which have to be controlled in the structures and function of chromosomes.

Proper Chromatin Condensation and Maintenance of Histone H3 Phosphorylation During Mouse Oocyte Meiosis Requires Protein Phosphatase Activity1

We have shown okadaic acid (OA) and calyculin-A (CLA) inhibition of mouse oocyte phosphoprotein phosphatase 1 (PPP1C) and/or phosphoprotein phosphatase 2A (PPP2CA) results in aberrant chromatin condensation, as evidenced by the inability to resolve bivalents. Phosphorylation of histone H3 at specific residues is thought to regulate chromatin condensation. Therefore, we examined changes in histone H3 phosphorylation during oocyte meiosis and the potential regulation by protein PPPs. Western blot and immunocytochemical analysis revealed histone H3 phosphorylation changed during mouse oocyte meiosis, with changes in chromatin condensation. Germinal vesicle-intact (GV-intact; 0 h) oocytes had no phospho-Ser10 but did have phospho-Ser28 histone H3. Oocytes that had undergone germinal vesicle breakdown (GVBD; 2 h) and progressed to metaphase I (MI; 7 h) and MII (16 h) had phosphorylated Ser10 and Ser28 histone H3 associated with condensed chromatin. To determine whether OA-induced aberrations in chromatin condensation were due to alterations in levels of histone H3 phosphorylation, we assessed phosphorylation of Ser10 and Ser28 residues following PPP inhibition. Oocytes treated with OA (1 microM) displayed increased phosphorylation of histone H3 at both Ser10 and Ser28 compared with controls. To begin to elucidate which OA-sensitive PPP is responsible for regulating chromatin condensation and histone H3 phosphorylation, we examined spatial and temporal localization of OA-sensitive PPPs, PPP1C, and PPP2CA. PPPC2A did not localize to condensed chromatin, whereas PPP1beta (PPP1CB) associated with condensing chromatin in GVBD, MI, and MII oocytes. Additionally, Western blot and immunocytochemistry confirmed presence of the PPP1C regulatory inhibitor subunit 2 (PPP1R2) in oocytes at condensed chromatin during meiosis and indicated a change in PPP1R2 phosphorylation. Inhibition of oocyte glycogen synthase kinase 3 (GSK3) appeared to regulate phosphorylation of PPP1R2. Furthermore, inhibition of GSK3 resulted in aberrant oocyte bivalent formation similar to that observed following PPP inhibition. These data suggest that PPP1CB is the OA/CLA-sensitive PPP that regulates oocyte chromatin condensation through regulation of histone H3 phosphorylation. Furthermore, GSK3 inhibition results in aberrant chromatin condensation and appears to regulate phosphorylation of PPP1R2.

The Ste20-like kinase SLK is required for cell cycle progression through G2

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein that can regulate actin reorganization during cell adhesion and spreading (Wagner, S., Flood, T. A., O'Reilly, P., Hume, K., and Sabourin, L. A. (2002) J. Biol. Chem. 277, 37685-37692). Because of its association with the microtubule network, we investigated whether SLK plays a role in cell cycle progression, a process that requires microtubule dynamics during mitosis. Consistent with microtubule association in exponentially growing cells, our results showed that SLK co-localizes with the mitotic spindle in cells undergoing mitosis. Expression of a kinase-inactive mutant or SLK small interfering RNAs inhibited cell proliferation and resulted in an accumulation of quiescent cells stimulated to re-enter the cell cycle in the G2 phase. Cultures expressing the mutant SLK displayed a normal pattern of cyclin D, E, and B expression but failed to down-regulate cyclin A levels, suggesting that they cannot proceed through M phase. In addition, these cultures displayed low levels of both phospho-H3 and active p34/cdc2 kinase. Overexpression of active SLK resulted in ectopic spindle assembly and the induction of cell cycle re-entry of Xenopus oocytes, suggesting that SLK is required for progression through G2 upstream of H1 kinase activation.

Dynamic distribution of Ser-10 phosphorylated histone H3 in cytoplasm of MCF-7 and CHO cells during mitosis

The dynamic distribution of phosphorylated Histone H3 on Ser10 (phospho-H3) in cells was investigated to determine its function during mitosis. Human breast adenocarcinoma cells MCF-7, and Chinese hamster cells CHO were analyzed by indirect immunofluorescence staining with an antibody against phospho-H3. We found that the phosphorylation begins at early prophase, and spreads throughout the chromosomes at late prophase. At metaphase, most of the phospho-H3 aggregates at the end of the condensed entity of chromosomes at equatorial plate. During anaphase and telophase, the fluorescent signal of phospho-H3 is detached from chromosomes into cytoplasm. At early anaphase, phospho-H3 shows ladder bands between two sets of separated chromosome, and forms "sandwich-like structure" when the chromosomes condensed. With the cleavage progressing, the "ladders" of the histone contract into a bigger bright dot. Then the histone aggregates and some of compacted microtubules in the midbody region are composed into a "bar-like" complex to separate daughter cells. The daughter cells seal their plasma membrane along with the ends of the "bar", inside which locates microtubules and modified histones, to finish the cytokinesis and keep the "bar complex" out of the cells. The specific distribution and kinetics of phospho-H3 in cytoplasm suggest that the modified histones may take part in the formation of midbody and play a crucial role in cytokinesis.

Epigenetic mechanisms in memory formation

Discoveries concerning the molecular mechanisms of cell differentiation and development have dictated the definition of a new sub-discipline of genetics known as epigenetics. Epigenetics refers to a set of self-perpetuating, post-translational modifications of DNA and nuclear proteins that produce lasting alterations in chromatin structure as a direct consequence, and lasting alterations in patterns of gene expression as an indirect consequence. The area of epigenetics is a burgeoning subfield of genetics in which there is considerable enthusiasm driving new discoveries. Neurobiologists have only recently begun to investigate the possible roles of epigenetic mechanisms in behaviour, physiology and neuropathology. Strikingly, the relevant data from the few extant neurobiology-related studies have already indicated a theme - epigenetic mechanisms probably have an important role in synaptic plasticity and memory formation.

Topoisomerase inhibitor induced dephosphorylation of H1 and H3 histones as a consequence of cell cycle arrest

Posttranslational modifications of histones have an integral function in the structural and functional organization of chromatin. Several changes in the modification state of histones could be observed after induction of apoptosis with topoisomerase inhibitors and other inducers. Most of these studies include the analysis of the state of phosphorylation of histones, and the results are to some extent controversial, depending on cell lines and agents used. In the present study we compared the kinetics of the dephosphorylation of H1 and H3 histones with apoptosis markers after treatment of leukemic cell lines with topoisomerase inhibitors. In parallel, we determined cell cycle parameters in detail. Dephosphorylation of both histone classes started within 1 h of induction, and no direct correlation with timing and intensity of the investigated apoptotic features could be observed. In contrast, we show that the effect of topoisomerase inhibitors on the state of H1 and H3 phosphorylation is not directly related to apoptosis, but reflects the changes in the cell cycle distribution of cells treated with these inducers.

The N-terminus of histone H2B, but not that of histone H3 or its phosphorylation, is essential for chromosome condensation

We have studied the role of individual histone N-termini and the phosphorylation of histone H3 in chromosome condensation. Nucleosomes, reconstituted with histone octamers containing different combinations of recombinant full-length and tailless histones, were used as competitors for chromosome assembly in Xenopus egg extracts. Nucleosomes reconstituted with intact octamers inhibited chromosome condensation as efficiently as the native ones, while tailless nucleosomes were unable to affect this process. Importantly, the addition to the extract of particles containing only intact histone H2B strongly interfered with chromosome formation while such an effect was not observed with particles lacking the N-terminal tail of H2B. This demonstrates that the inhibition effect observed in the presence of competitor nucleosomes is mainly due to the N-terminus of this histone, which, therefore, is essential for chromosome condensation. Nucleosomes in which all histones but H3 were tailless did not impede chromosome formation. In addition, when competitor nucleosome particles were reconstituted with full-length H2A, H2B and H4 and histone H3 mutated at the phosphorylable serine 10 or serine 28, their inhibiting efficiency was identical to that of the native particles. Hence, the tail of H3, whether intact or phosphorylated, is not important for chromosome condensation. A novel hypothesis, termed 'the ready production label' was suggested to explain the role of histone H3 phosphorylation during cell division.

Cyclin A-CDK phosphorylates Sp1 and enhances Sp1-mediated transcription

Cyclin A-mediated activation of cyclin-dependent kinases (CDKs) is essential for cell cycle transversal. Cyclin A activity is regulated on several levels and cyclin A elevation in a number of cancers suggests a role in tumorigenesis. In the present study, we used a modified DNA binding site selection and PCR amplification procedure to identify DNA binding proteins that are potential substrates of cyclin A-CDK. One of the sequences identified is the Sp1 transcription factor binding site. Co-immunoprecipitation experiments show that cyclin A and Sp1 can interact physically. In vitro and in vivo phosphorylation studies indicate that cyclin A-CDK complexes can phosphorylate Sp1. The phosphorylation site is located in the N-terminal region of the protein. Cells overexpressing cyclin A have elevated levels of Sp1 DNA binding activity, suggesting that cyclin A-CDK-mediated phosphorylation augments Sp1 DNA binding properties. In co-transfection studies, cyclin A expression stimulated transcription from an Sp1-regulated promoter. Mutation of the phosphorylation site abrogated cyclin A-CDK-dependent phosphorylation, augmentation of Sp1 transactivation function and DNA binding activity.

pEg2 aurora-A kinase, histone H3 phosphorylation, and chromosome assembly in Xenopus egg extract

In eukaryotes cell division is accompanied by phosphorylation of histone H3 at serine 10. In this work we have studied the kinase activity responsible for this histone H3 modification by using cell-free extracts prepared from Xenopus eggs. We have found that the Xenopus aurora-A kinase pEg2, immunoprecipitated from the extract, is able to phosphorylate specifically histone H3 at serine 10. The enzyme is incorporated into chromatin during in vitro chromosome assembly, and the kinetics of this incorporation parallels that of histone H3 phosphorylation. Recombinant pEg2 phosphorylates efficiently histone H3 at serine 10 in reconstituted nucleosomes and in sperm nuclei decondensed in heated extracts. These data identify pEg2 as a good candidate for mitotic histone H3 kinase. However, immunodepletion of pEg2 does not interfere with the chromosome assembly properties of the extract nor with the pattern of H3 phosphorylation, suggesting the existence of multiple kinases involved in this H3 modification in Xenopus eggs. This hypothesis is supported by in gel activity assay experiments using extracts from Saccharomyces cerevisiae.

Histone H3 phosphorylation and cell division

Histone H3 is specifically phosphorylated during both mitosis and meiosis in patterns that are specifically coordinated in both space and time. Histone H3 phosphorylation may initiate at different phases of the cell division in different organisms, but metaphase chromosomes are always found to be heavily phosphorylated. Upon exit of mitosis/meiosis a global dephosphorylation of H3 takes place. Potential candidates for H3 kinases are described and their hypothetical mechanism of action on highly condensed chromatin templates is discussed. In addition, a novel hypothesis for the role of histone H3 phosphorylation during cell division is proposed. This hypothesis, termed the 'ready production label' model, explains the results in the literature and suggests that phosphorylation of histone H3 is a part of a complex signaling mechanism.

Mitotic phosphorylation of histone H3 is governed by Ipl1/aurora kinase and Glc7/PP1 phosphatase in budding yeast and nematodes

Phosphorylation of histone H3 at serine 10 occurs during mitosis and meiosis in a wide range of eukaryotes and has been shown to be required for proper chromosome transmission in Tetrahymena. Here we report that Ipl1/aurora kinase and its genetically interacting phosphatase, Glc7/PP1, are responsible for the balance of H3 phosphorylation during mitosis in Saccharomyces cerevisiae and Caenorhabditis elegans. In these models, both enzymes are required for H3 phosphorylation and chromosome segregation, although a causal link between the two processes has not been demonstrated. Deregulation of human aurora kinases has been implicated in oncogenesis as a consequence of chromosome missegregation. Our findings reveal an enzyme system that regulates chromosome dynamics and controls histone phosphorylation that is conserved among diverse eukaryotes.

Identification of a novel phosphorylation site on histone H3 coupled with mitotic chromosome condensation

Histone H3 (H3) phosphorylation at Ser(10) occurs during mitosis in eukaryotes and was recently shown to play an important role in chromosome condensation in Tetrahymena. When producing monoclonal antibodies that recognize glial fibrillary acidic protein phosphorylation at Thr(7), we obtained some monoclonal antibodies that cross-reacted with early mitotic chromosomes. They reacted with 15-kDa phosphoprotein specifically in mitotic cell lysate. With microsequencing, this phosphoprotein was proved to be H3. Mutational analysis revealed that they recognized H3 Ser(28) phosphorylation. Then we produced a monoclonal antibody, HTA28, using a phosphopeptide corresponding to phosphorylated H3 Ser(28). This antibody specifically recognized the phosphorylation of H3 Ser(28) but not that of glial fibrillary acidic protein Thr(7). Immunocytochemical studies with HTA28 revealed that Ser(28) phosphorylation occurred in chromosomes predominantly during early mitosis and coincided with the initiation of mitotic chromosome condensation. Biochemical analyses using (32)P-labeled mitotic cells also confirmed that H3 is phosphorylated at Ser(28) during early mitosis. In addition, we found that H3 is phosphorylated at Ser(28) as well as Ser(10) when premature chromosome condensation was induced in tsBN2 cells. These observations suggest that H3 phosphorylation at Ser(28), together with Ser(10), is a conserved event and is likely to be involved in mitotic chromosome condensation.

Phosphorylation of histone H3 is required for proper chromosome condensation and segregation

Phosphorylation of histone H3 at serine 10 occurs during mitosis in diverse eukaryotes and correlates closely with mitotic and meiotic chromosome condensation. To better understand the function of H3 phosphorylation in vivo, we created strains of Tetrahymena in which a mutant H3 gene (S10A) was the only gene encoding the major H3 protein. Although both micronuclei and macronuclei contain H3 in typical nucleosomal structures, defects in nuclear divisions were restricted to mitotically dividing micronuclei; macronuclei, which are amitotic, showed no defects. Strains lacking phosphorylated H3 showed abnormal chromosome segregation, resulting in extensive chromosome loss during mitosis. During meiosis, micronuclei underwent abnormal chromosome condensation and failed to faithfully transmit chromosomes. These results demonstrate that H3 serine 10 phosphorylation is causally linked to chromosome condensation and segregation in vivo and is required for proper chromosome dynamics.

Histone H3 phosphorylation is required for the initiation, but not maintenance, of mammalian chromosome condensation

The temporal and spatial patterns of histone H3 phosphorylation implicate a specific role for this modification in mammalian chromosome condensation. Cells arrest in late G2 when H3 phosphorylation is competitively inhibited by microinjecting excess substrate at mid-S-phase, suggesting a requirement for activity of the kinase that phosphorylates H3 during the initiation of chromosome condensation and entry into mitosis. Basal levels of phosphorylated H3 increase primarily in late-replicating/early-condensing heterochromatin both during G2 and when premature chromosome condensation is induced. The prematurely condensed state induced by okadaic acid treatment during S-phase culminates with H3 phosphorylation throughout the chromatin, but in an absence of mitotic chromosome morphology, indicating that the phosphorylation of H3 is not sufficient for complete condensation. Mild hypotonic treatment of cells arrested in mitosis results in the dephosphorylation of H3 without a cytological loss of chromosome compaction. Hypotonic-treated cells, however, complete mitosis only when H3 is phosphorylated. These observations suggest that H3 phosphorylation is required for cell cycle progression and specifically for the changes in chromatin structure incurred during chromosome condensation.

Entry into mitosis without Cdc2 kinase activation

Mouse FT210 cells at 39 degreesC cannot enter mitosis but arrest in G2 phase, because they lack Cdc2 kinase activity as a result of a temperature-sensitive lesion in the cdc2 gene. Incubation of arrested cells with the protein phosphatase 1 and 2A inhibitor okadaic acid induces morphologically normal chromosome condensation. We now show that okadaic acid also induces two other landmark events of early mitosis, nuclear lamina depolymerization and centrosome separation, in the absence of Cdc2 kinase activity. Okadaic acid-induced entry into mitosis is accompanied by partial activation of Cdc25C and may be prevented by tyrosine phosphatase inhibitors and by the protein kinase inhibitor staurosporine, suggesting that Cdc25C and kinases distinct from Cdc2 are required for these mitotic events. Using in-gel assays, we show that a 45-kDa protein kinase normally activated at mitosis is also activated by okadaic acid independently of Cdc2 kinase. The 45-kDa kinase can utilize GTP, is stimulated by spermine and is inhibited by heparin. These properties are characteristic of the kinase CK2, but immunoprecipitation studies indicate that it is not CK2. The data underline the importance of a tyrosine phosphatase, possibly Cdc25C, and of kinases other than Cdc2 in the structural changes the cell undergoes at mitosis, and indicate that entry into mitosis involves the activation of multiple kinases working in concert with Cdc2 kinase.

Four distinct cyclin-dependent kinases phosphorylate histone H1 at all of its growth-related phosphorylation sites

In mammalian cells, up to six serines and threonines in histone H1 are phosphorylated in vivo in a cell cycle dependent manner that has long been linked with chromatin condensation. Growth-associated H1 kinases, now known as cyclin-dependent kinases (CDKs), are thought to be the enzymes responsible for this process. This paper describes the phosphorylation of histone H1 by four different purified CDKs. The four CDKs phosphorylate only the cell cycle specific phosphorylation sites of H1, indicating that they belong to the kinase class responsible for growth-related H1 phosphorylation in vivo. All four CDKs phosphorylate all of the interphase and mitotic-specific H1 sites. In addition to the (S/T)PXK consensus phosphorylation sites, these four CDKs also phosphorylate a mitotic-specific in vivo H1 phosphorylation site that lacks this sequence. There is no site selectivity among the growth-related phosphorylation sites by any of the four CDKs because all four CDKs phosphorylate all relevant sites. The results imply that the cell cycle dependent H1 phosphorylations observed in vivo must involve differential accessibility of H1 sites at different stages of the cell cycle.

In vivo phosphorylation of histone H1 variants during the cell cycle

In vivo phosphorylation of the five histone H1 variants H1a-H1e including H1(0) in NIH 3T3 mouse fibroblasts was examined during the cell cycle by using a combination of HPLC techniques and conventional AU gel electrophoresis. Phosphorylation starts during the late G1 phase and increases throughout the S phase. In the late S phase, the H1 variants exist as a combination of molecules containing 0 or 1 (H1a, H1c), 0-2 (H1d), or 0-3 (H1b, H1e) phosphate groups with a share of unphosphorylated protein ranging between 35% and 75%, according to the particular subtype. Pulse-chase experiments show that phosphorylation during the S phase is a dynamic phosphorylation process with a limited phosphorylation maximum. In most H1 subtypes, phosphorylation occurs very rapidly at the G2/M transition with only small amounts of intermediate phosphorylated molecules. Phosphorylation of mouse H1c, however, occurs stepwise during this transition. Phosphorylated mouse histone subtypes from cells in mitosis contain four phosphate groups in the case of H1a, H1c, and H1e and five in the case of H1b and H1d. Comparison of the mouse phosphorylation pattern to that in rat C-6 glioma cells showed differences for H1e and H1d. By comparing the different phosphorylation patterns of the individual H1 variants during the cell cycle, we were able to classify the H1 histones into subtypes with low (H1a, H1c, H1(0)) and high (H1b, H1d, H1e) phosphorylation levels.

Characterization of the mitotic specific phosphorylation site of histone H1. Absence of a consensus sequence for the p34cdc2/cyclin B kinase

32P-Labeled histone H1 was isolated from synchronized Chinese hamster (line CHO) cells, subjected to tryptic digestion, and fractionated into 15 phosphopeptides by high performance liquid chromatography. These phosphopeptides were grouped into five classes having different cell cycle phosphorylation kinetics: 1) peptides reaching a maximum phosphorylation rate in S and then declining in G2 and M, 2) peptides reaching a maximum phosphorylation rate in G2 and then remaining constant or declining in M, 3) peptides with increasing phosphorylation throughout S and G2 and reaching a maximum in M, 4) one peptide that was phosphorylated only in M, and 5) peptides that had low levels of phosphorylation that remained constant throughout the cell cycle. Amino acid analysis and sequencing demonstrated that the mitotic specific H1 phosphopeptide was the 16-amino acid, N-terminal, tryptic peptide Ac-SETAPAAPAAAPPAEK of the H1-1 class. This peptide, which is phosphorylated on both the Ser and Thr, does not contain the consensus sequence (S/T)PXZ (where X is any amino acid and Z is a basic amino acid). This sequence is thought to be required by the p34cdc2/cyclin B kinase that has maximum phosphorylating activity in mitosis. These data indicate that this kinase either does not have an obligatory requirement for the consensus sequence in vivo as generally believed or that it is not the enzyme responsible for the mitotic specific H1 phosphorylation.

Development of a high-performance liquid chromatographic method for the analysis of staurosporine

Staurosporine (Stsp), a protein kinase inhibitor, has been found to have a differential effect on the proliferation of normal and transformed cells in vitro. Hence, Stsp might be used in cancer therapy to arrest normal proliferating cells in G1, while permitting tumor cells to continue proliferation. The patient could then be treated with a therapeutic agent of maximum toxicity for actively proliferating tumor cells. To facilitate investigations of Stsp in vivo, we have developed an HPLC method for measuring the levels of Stsp in blood. Using a rat model, plasma containing Stsp is treated with acetone to precipitate proteins and extract the Stsp. The acetone extract is then subjected to reversed-phase HPLC on a mu Bondapak C18 column. Using a linear elution gradient of acetonitrile containing trifluoroacetic acid, Stsp elutes as a sharp peak at ca. 35 min which can be detected by UV absorption at 292 nm. No blood or reagent components interfere with its quantification. The calibration curve, ranging from 0.1 to 2.0 micrograms Stsp, demonstrated a linear response to Stsp concentration having a correlation coefficient (r2) of 0.990. Precision analysis demonstrated that the method will yield results that are +/- 11.6% from the mean 95% (two standard deviations) of the time. This method was used to measure Stsp levels in plasma after administering an injection of 0.2 mg Stsp into the jugular vein of rats. No Stsp could be detected in the plasma 5 min after injection, even though enough Stsp was administered to be easily detectable if it was simply contained in the plasma. Thus, it is concluded that some compartment other than the plasma must adsorb the Stsp from the plasma and sequester it in vivo.

Chromosome condensation induced by fostriecin does not require p34cdc2 kinase activity and histone H1 hyperphosphorylation, but is associated with enhanced histone H2A and H3 phosphorylation

Chromosome condensation at mitosis correlates with the activation of p34cdc2 kinase, the hyperphosphorylation of histone H1 and the phosphorylation of histone H3. Chromosome condensation can also be induced by treating interphase cells with the protein phosphatase 1 and 2A inhibitors okadaic acid and fostriecin. Mouse mammary tumour FT210 cells grow normally at 32 degrees C, but at 39 degrees C they lose p34cdc2 kinase activity and arrest in G2 because of a temperature-sensitive lesion in the cdc2 gene. The treatment of these G2-arrested FT210 cells with fostriecin or okadaic acid resulted in full chromosome condensation in the absence of p34cdc2 kinase activity or histone H1 hyperphosphorylation. However, phosphorylation of histones H2A and H3 was strongly stimulated, partly through inhibition of histone H2A and H3 phosphatases, and cyclins A and B were degraded. The cells were unable to complete mitosis and divide. In the presence of the protein kinase inhibitor starosporine, the addition of fostriecin did not induce histone phosphorylation and chromosome condensation. The results show that chromosome condensation can take place without either the histone H1 hyperphosphorylation or the p34cdc2 kinase activity normally associated with mitosis, although it requires a staurosporine-sensitive protein kinase activity. The results further suggest that protein phosphatases 1 and 2A may be important in regulating chromosome condensation by restricting the level of histone phosphorylation during interphase, thereby preventing premature chromosome condensation.

Protein kinases and phosphatases that act on histidine, lysine, or arginine residues in eukaryotic proteins: a possible regulator of the mitogen-activated protein kinase cascade

Phosphohistidine goes undetected in conventional studies of protein phosphorylation, although it may account for 6% of total protein phosphorylation in eukaryotes. Procedures for studying protein N- kinases are described. Genes whose products are putative protein histidine kinases occur in a yeast and a plant. In rat liver plasma membranes, activation of the small G-protein, Ras, causes protein histidine phosphorylation. Cellular phosphatases dephosphorylate phosphohistidine. One eukaryotic protein histidine kinase has been purified, and specific proteins phosphorylated on histidine have been observed. There is a protein arginine kinase in mouse and protein lysine kinases in rat. Protein phosphohistidine may regulate the mitogen-activated protein kinase cascade.

The chromatin of trypanosomes

The nuclear chromatin of trypanosomes is organised in the form of nucleosome filaments. When soluble chromatin is prepared under suitable conditions, a regular array of nucleosomes can be shown by electron microscopy. Chromatin of blood stream as well as procyclic culture forms of Trypanosoma brucei brucei and of T. cruzi shows limited compaction at salt concentrations increasing from 1 to 100 mM. No 30 nm fibres, typical for higher eukaryotes, are formed. Digestion of the nuclear chromatin with micrococcal nuclease and analysis of the histone proteins with various techniques reveal that the basic organisation of the trypanosome chromatin is similar but not identical as compared to that of higher eukaryotes. Distinct differences are present with respect to biochemical properties of the histones as well as to their interaction with the DNA. The primary structure of the histones also differs significantly from that found in other lower and higher eukaryotes. The function of the recently described H1-like proteins in trypanosomes is currently being investigated. The differences that have already been found in the structure and compaction of the trypanosome chromatin compared to that of higher eukaryotes lead us to expect differences of gene expression which, in turn, might offer targets for the control of trypanosomiasis.

Polyamines, chromatin structure and transcription

Nucleosomes are the basic elements of chromatin structure. Polyamines, such as spermine and spermidine, are small ubiquitous molecules absolutely required for cell growth. Photoaffinity polyamines bind to specific locations in nucleosomes and can change the helical twist of DNA in nucleosomes. Acetylation of polyamines reduces their affinity for DNA and nucleosomes, thus the helical twist of DNA in nucleosomes could be regulated by cells through acetylation. I suggest that histone and polyamine acetylation act synergistically to modulate chromatin structure. On naked DNA, the photoaffinity spermine bound preferentially to a specific 'TATA' sequence element, suggesting that polyamines may be involved in the unusual chromatin structure in this region. Further work is needed to test whether the specificities shown by photoaffinity polyamines are also shown by cellular polyamines; such experiments are now feasible.