Interaction of HMG 14 and 17 with actively transcribed genes

HMGNs: The enhancer charmers

The DNase I hypersensitive sites (DHSs) of chromatin constitute one of the best landmarks of eukaryotic genes that are poised and/or activated for transcription. For over 35 years, the high-mobility group nucleosome-binding chromosomal proteins HMGN1 and HMGN2 have been shown to play a role in the establishment of these chromatin-accessible domains at transcriptional regulatory elements, namely promoters and enhancers. The critical presence of HMGNs at enhancers, as highlighted by a recent publication, suggests a role for them in the structural and functional fine-tuning of the DHSs in vertebrates. As we review here, while preferentially out-competing histone H1 binding and invading neighbor nucleosomes, HMGNs may also modulate histone H3 at serine 10 (H3S10ph), which plays an important role in enhancer function and transcriptional initiation.

Transcriptional regulation by HMGN proteins

High mobility group nucleosomal proteins (HMGNs) are small non-histone proteins associated with chromatin. HMGNs have the unique ability to bind to nucleosomes with higher affinity than to naked DNA [1]. They have been studied extensively for their ability to modulate transcription. Although initially viewed as general transcriptional activators on chromatin templates, it is now appreciated that they are instead highly specific modulators of gene expression. We review the mechanisms for targeting HMGNs to specific genes and for how they subsequently regulate transcription.

Regulation of chromatin structure and function by HMGN proteins

High mobility group nucleosome-binding (HMGN) proteins are architectural non-histone chromosomal proteins that bind to nucleosomes and modulate the structure and function of chromatin. The interaction of HMGN proteins with nucleosomes is dynamic and the proteins compete with the linker histone H1 chromatin-binding sites. HMGNs reduce the H1-mediated compaction of the chromatin fiber and facilitate the targeting of regulatory factors to chromatin. They modulate the cellular epigenetic profile, affect gene expression and impact the biological processes such as development and the cellular response to environmental and hormonal signals. Here we review the role of HMGN in chromatin structure, the link between HMGN proteins and histone modifications, and discuss the consequence of this link on nuclear processes and cellular phenotype.

Chromosomal proteins HMGN3a and HMGN3b regulate the expression of glycine transporter 1

HMGN proteins promote chromatin unfolding, enhance access to nucleosomes, and modulate transcription from chromatin templates. It is not known whether they act indiscriminately as general modulators of transcription or whether they regulate specific gene expression. Here, we investigated the role of HMGN3, a recently discovered HMGN family member, in transcription in vivo. We created cell lines overexpressing HMGN3a or its splice variant, HMGN3b, and analyzed their gene expression profiles using microarrays and reverse transcriptase PCR. We found that ectopic expression of HMGN3a alters the expression of approximately 0.8% of genes. Both HMGN3a and HMGN3b upregulate the expression of the glycine transporter 1 gene (Glyt1). Glyt1 encodes a membrane transporter that regulates the glycine concentration in synaptic junctions. Both GLYT1 and HMGN3 are highly expressed in glia cells and the eye, and we show that both proteins are coexpressed in the retina. Chromatin immunoprecipitation assays showed that HMGN3 protein is recruited to a region of the Glyt1 gene encompassing the Glyt1a transcriptional start site. These results suggest that HMGN3 regulates Glyt1 expression and demonstrate that members of the HMGN family can regulate the transcription of specific genes.

Response of high mobility group proteins of human kidney T1 and murine L 929 cell lines to heat shock

High mobility group (HMG) proteins in human kidney T1 and murine L 929 cells have been investigated after exposure to heat shock at 41 degrees C and their influence on the organizational change of chromatin under heat shock condition has been examined. Results reveal that the two cell lines show differential response of the HMG proteins 1 & 2 and 14 & 17 to heat shock. Neither T1 nor L 929 cells show significant differences in response to heat shock with respect to the binding affinities of HMG proteins 1 & 2 or 14 & 17 to DNA, as revealed by DNase I sensitivity and chromatin reconstitution assays. Furthermore, the HMG proteins of both the non-heat shocked and the heat shocked T1 and L 929 cells can recover their chromatin activity following reconstitution. These findings suggest that although the HMG proteins might undergo some change in response to heat shock, their inherent potential of reassociation with DNA is still retained.

Phosphorylation and subcellular redistribution of high mobility group proteins 14 and 17, analyzed by mass spectrometry

High mobility group (HMG) proteins 14 and 17 are nonhistone nuclear proteins that have been implicated in control of transcription and chromatin structure. To examine the posttranslational modifications of HMG-14 and -17 in vivo, HMG proteins were prepared from nuclear vs. cytosolic fractions of human K562 cells treated with 12-O-tetradecanoylphorbol 13-acetate (TPA) or okadaic acid (OA) and examined by electrospray mass spectrometry. Analysis of full-length masses demonstrated mono-, di-, and triphosphorylation of HMG-14 and mono- and diphosphorylation of HMG-17 from OA treated cells, whereas HMG-14 and -17 from TPA treated cells were monophosphorylated. Peptide mass and sequence analysis showed major and minor phosphorylation sites, respectively, at Ser24 and Ser28 in HMG-17, and Ser20 and Ser24 in HMG-14. These sites were found in the consensus sequence RRSARLSAK, within the nucleosomal binding domain of each protein. A third phosphorylation site in HMG-14 was located at either Ser6 or Ser7. Interestingly, the proportion of HMG-14 and -17 found in cytosolic pools increased significantly after 1 h of treatment compared to control cells and showed preferential phosphorylation compared with proteins from nuclear fractions. These results suggest that phosphorylation of HMG-14 and -7 interferes with nuclear localization mechanisms in a manner favoring release from nuclei.

Stimulation of replication efficiency of a chromatin template by chromosomal protein HMG-17

The effect of chromosomal protein HMG-17 on the replication of a chromatin template was studied with minichromosomes containing the SV40 origin of replication. The minichromosomes were assembled from M13 DNA in Xenopus egg extracts in either the absence or presence of HMG-17. Structural data show that HMG-17 was efficiently incorporated into the chromatin and induced an extended chromatin structure. Using an in vitro SV40 replication system, we find that minichromosomes containing HMG-17 replicate with higher efficiency than minichromosomes deficient of HMG-17. The replicational potential of chromatin was enhanced only when HMG-17 was incorporated into the template during, but not after, chromatin assembly. HMG-17 stimulated replication only from a chromatin template, but not from protein-free DNA. Thus, HMG-17 protein enhances the rate of replication of a chromatin template by unfolding the higher order chromatin structure and increasing the accessibility of target sequences to components of the replication machinery.

Structure of active chromatin: isolation and characterization of transcriptionally active chromatin from rat liver

Rat liver nuclei were isolated in low-ionic-strength buffer in the absence of bi- and multi-valent cations. Digestion of these nuclei by endogenous nuclease, micrococcal nuclease and DNase I revealed that a minor chromatin fraction was preferentially digested into poly- and oligo-nucleosomes. Southern blot hybridization with various active gene probes confirmed that these chromatin fragments represent coding and 5' upstream regions of transcriptionally active chromatin. Active chromatin fragments were released selectively into the medium, with inactive chromatin remaining inside the nuclei, under the above ionic conditions. The inclusion of bivalent cations during the digestion of nuclei reversed the solubility behaviour of active chromatin. Rearrangement and exchange of histone H1 between chromatin fragments was prevented by using low-salt conditions in all steps in the absence of bivalent cations. All histones, including H1, were present in stoichiometric amounts in this active chromatin fraction. Active nucleosomes showed a lower electrophoretic mobility than bulk nucleosomes in an acrylamide/agarose composite gel in the absence of Mg2+, but were selectively bound to the gel in the presence of this ion.

Neither HMG-14a nor HMG-17 gene function is required for growth of chicken DT40 cells or maintenance of DNaseI-hypersensitive sites

HMG-14 and HMG-17 form a family of ubiquitous non-histone chromosomal proteins and have been reported to bind preferentially to regions of active chromatin structure. Our previous studies demonstrated that the chicken HMG-17 gene is dispensable for normal growth of the DT40 chicken lymphoid cell line. Here it is shown that the major chicken HMG-14 gene,HMG-14a, is also dispensable and, moreover, that DT40-derived cells lacking both HMG-17 and HMG-14a proteins show no obvious change in phenotype with respect to the parental DT40 cells. Furthermore, no compensatory changes in HMG-14b or histone protein levels were observed in cells lacking both HMG-14a and HMG-17, nor were any alterations detected in such hallmarks of chromatin structure as DNaseI-hypersensitive sites or micrococcal nuclease digestion patterns. It is concluded that the HMG-14a and HMG-17 proteins are not required for normal growth of avian cell linesin vitro, nor for the maintenance of DNaseI-hypersensitive sites in chromatin.

The chicken HMG-17 gene is dispensable for cell growth in vitro

HMG-17 is a highly conserved and ubiquitous nonhistone chromosomal protein that binds to nucleosome core particles. HMG-17 and HMG-14 form a family of chromosomal proteins that have been reported to bind preferentially to regions of active chromatin structure. To study the functional role of the single-copy chicken HMG-17 gene, null mutants were generated by targeted gene disruption in a chicken lymphoid cell line, DT40. Heterozygous and homozygous null mutant cell lines were generated by two independent selection strategies. Heterozygous null mutant lines produced about half the normal level of HMG-17 protein, and homozygous null lines produced no detectable HMG-17. No significant changes in cell phenotype were observed in cells harboring either singly or doubly disrupted HMG-17 genes, and no compensatory changes in HMG-14 or histone protein levels were observed. It is concluded that HMG-17 protein is not required for normal growth of avian cell lines in vitro, nor does the absence of HMG-17 protein lead to any major changes in cellular phenotype, at least in lymphoid cells.

Identification of cDNA clones encoding HMG 2, a major protein of the mexican axolotl hydrocortisone-sensitive thymocytes

We have identified and analyzed cDNA clones encoding a major 26 kDa protein of the HMG1-2 family which is abundant in the cytoplasm and nucleus of axolotl hydrocortisone-sensitive thymocytes. The axolotl HMG2 protein is very similar to proteins belonging to the HMG1-2 family, from teleost fish to mammals. All the molecular features of the HMG1-2 proteins are conserved, including the high proportion of basic and aromatic residues, and the characteristic acidic C-terminus tail. The 3'-untranslated region of the HMG2 axolotl cDNA is also similar to the avian and mammalian HMG2 3'-UT sequences, suggesting that some selective events have acted at the DNA level to conserve this region, which could be important in the differential expression of the HMG1 and HMG2 genes. The axolotl HMG2 protein contains the two well conserved HMG boxes which are thought to be the DNA-binding domains of the molecule. Axolotl thymocytes and spleen cells contain almost identical amounts of HMG2 mRNAs but HMG2 polypeptide is undetectable in spleen cells using anti-26 kDa antibodies. The reason for the accumulation of HMG1-2 molecules in vertebrate hydrocortisone-sensitive thymocytes is discussed, as well as their possible role in apoptosis.

Promoter-proximal pausing of RNA polymerase II defines a general rate-limiting step after transcription initiation

We have shown previously that the majority of RNA polymerase II complexes initiated at the c-myc gene are paused in the promoter-proximal region, similar to observations in the Drosophila hsp70 gene. Our analyses define the TATA box or initiator sequences in the c-myc gene as necessary components for the establishment of paused RNA polymerase II. Deletion of upstream sequences or even the TATA box does not influence significantly the degree of transcriptional initiation or pausing. Deletion of both the TATA box and sequences at the transcription initiation site, however, abolishes transcriptional pausing of transcription complexes but still allows synthesis of full-length RNA. Further analyses with synthetic promoter constructs reveal that the simple combination of upstream activator with TATA consensus sequences or initiator sequences act synergistically to recruit high levels of RNA polymerase II complexes. Only a minor fraction of these complexes escapes into regions further downstream. Several different trans-activation domains fused to GAL4-DNA-binding domains, including strong activators such as VP16, do not eliminate promoter-proximal pausing of RNA polymerase. Thus, we conclude that pausing of RNA polymerase II is a common phenomenon in eukaryotic transcription and does not require complex promoter structures. Further analyses reveal that enhancers have a modest influence on transcription initiation and on release of transcription complexes out of the pause site but may function primarily to increase the elongation competence of transcription complexes.

The non-histone chromosomal protein HMG-I(Y) contributes to repression of the immunoglobulin heavy chain germ-line epsilon RNA promoter

The rate of germ-line RNA transcription correlates with the rate of immunoglobulin heavy chain isotype switching. A promoter element for the transcription of RNA from the germ-line mouse immunoglobulin epsilon heavy chain constant region gene is induced by interleukin(IL)-4 and lipopolysaccharide, and is bound at its transcription initiation sites by an IL-4-inducible nuclear protein, NF-BRE. To examine the function of the binding site for this IL-4-inducible complex, substitution mutations were introduced in the promoter. These binding site mutations increased promoter activity and decreased binding of NF-BRE. To investigate the paradox of an IL-4-inducible protein binding to a repressor site in an IL-4-inducible promoter, we determined that the non-histone chromosomal protein HMG-I(Y) binds at the transcription initiation sites of the germ-line epsilon promoter. Assays with antisera against HMG-I(Y) revealed monomeric HMG-I(Y) in nuclear extracts. Cotransfection of an expression construct directing the synthesis of anti-sense HMG-I(Y) RNA also increased promoter activity, consistent with a repressor function of HMG-I(Y). Thus, the data are most consistent with a model in which HMG-I(Y) participates in repression of promoter activity. The effects of IL-4 may include derepression at this site.

Evolutionarily conserved motifs and protein binding elements in the 5' region of the chromosomal protein HMG-14 gene

Although the structure of several genes coding for chromosomal proteins HMG-14 and HMG-17 has been determined, the mechanisms regulating the expression of these genes has not yet been examined. Toward this goal, we have cloned and sequenced a fragment containing the first three exons and 956 bp upstream from the start of translation of the functional mouse HMG-14 gene. Comparison of this sequence to the known sequence of the human HMG-14 gene revealed the presence of five distinct blocks of high sequence identity flanking the start of transcription and the CAAT box. DNase I and mobility-shift analysis identified a DNA region, downstream from the start of transcription, which may be important for the formation of a stable protein-DNA complex. Affinity chromatography on columns containing oligonucleotides corresponding to this sequence indicates that this region is a protein binding site.

Poly (adenosine diphosphate-ribose) synthesis in the anterior pituitary of the female rat throughout the estrous cycle: study of possible relation to cell proliferation and prolactin gene expression

To investigate whether cell proliferation and PRL gene expression in female rat pituitary during estrous cycle were mediated by the poly (ADP)-ribosylation of chromatin proteins, anterior pituitaries at different estrous cycle were obtained from female Wistar rats, and poly (ADP-ribose) synthesis, DNA synthesis, PRL and GH messenger RNAs and PRL content in the pituitary, and serum concentrations of PRL and estradiol were analyzed. From diestrus to proestrus, poly (ADP-ribose) synthesis, the contents of PRL messenger RNA and PRL in the pituitary increased significantly, and decreased at estrus. However, DNA synthesis and serum concentration of PRL showed a significant increase from proestrus to estrus. Serum estradiol concentration increased from diestrus to proestrus. No significant change was observed in the pituitary GH messenger RNA content during estrous cycle. The increase of PRL messenger RNA from diestrus to proestrus was abolished completely by the administration of nicotinamide, an inhibitor of poly (ADP-ribose) synthesis, to rats at diestrus. These results indicate that poly (ADP)-ribosylation of chromatin proteins may play some role in cell proliferation and transcription of PRL gene during rat estrous cycle.

Intracellular lectin-binding sites in symbiont-bearing Crithidia species

Crithidia oncopelti, C. deanei, and C. desouzai are flagellates of the Trypanosomatidae family that present bacterium-like endosymbionts in their cytoplasm. Direct and indirect lectin-gold labeling techniques were used at the electron microscopic level in Lowicryl K4M-embedded cells to demonstrate the presence of intracellular lectin-binding sites. We used the lectins Ulex europaeus I, Griffonia simplicifolia II, Ricinus communis I, Arachis hypogaea, G. simplicifolia I, Wistaria floribunda, Limulus polyphemus, and Canavalia ensiformis, which recognize alpha-L-fucose, alpha- and beta-N-acetylglucosamine, beta-galactose and beta-N-acetylgalactosamine, beta-galactose, alpha-galactose, beta-N-acetylgalactosamine, sialic acid and alpha-D-mannose, and alpha-D-glucose residues, respectively. The nucleus was the cellular structure most frequently labeled by the lectins. The Golgi complex was seldom labeled, whereas the endoplasmic reticulum and the flagellar pocket presented a large number of binding sites. Symbionts had their two unit membranes weakly labeled by the different lectins but displayed no labeling of the space between the membranes.

The gene encoding chicken chromosomal protein HMG-14a is transcribed into multiple mRNAs

The sequence and structure of the chicken HMG14a gene encoding HMG-14a non-histone chromosomal protein suggest that it may be a unique member of the HMG (high mobility group) gene family with properties intermediate to those of the typical HMG-14 and HMG-17 genes. Genomic clones were isolated which together contain the complete chicken HMG-14a gene. The gene covers about 10 kb while coding for an mRNA of about 1000 nt in size. Primer extension, S1 mapping and further cDNA clone analysis suggest that HMG-14a codes for multiple mRNAs arising from two or more transcription start points with alternative splicing and utilization of two or more polyadenylation sites. However, no variation in the coding portion of the mRNA has been observed. The sequence of the promoter region of HMG-14a is similar to that of chicken HMG-14b and human HMG-14 in that it is very G+C rich, contains several putative Sp1-binding sequences and has an unusually high density of CpG dinucleotides. Expression studies confirm earlier results suggesting that the gene is expressed at low levels in most tissue types.

Heparin increases chromatin accessibility by binding the trypsin-sensitive basic residues in histones

Recent evidence indicates that chromatin accessibility to transcription factors is of regulatory significance. The polyanion heparin is known to increase chromatin accessibility to DNAase I and to stimulate both RNA and DNA synthesis. In the present study, chromatin structure and its modification by polyanions were examined by using trypsin and micrococcal nuclease as probes. Both heparin and poly(glutamic acid) were found to be equivalent to trypsin digestion of histones in their ability to increase nuclease accessibility in chromatin. However, no increase in nuclease accessibility was observed when trypsin-digested chromatin was further treated with heparin, indicating that polyanions and trypsin are not additive in their effects on chromatin accessibility. Moreover, sucrose-gradient analysis demonstrated that heparin binds tightly to intact nucleosomes but not to trypsin-digested nucleosomes. These data suggest that polyanions interact predominantly with the trypsin-sensitive lysine and arginine residues in histone H1 and the N-terminal segments of the core histones. The possible relevance of these results to the chromatin structure of actively transcribed regions is discussed.

Lectin binding sites on head structures of the spermatid and spermatozoon of the mosquito Culex quinquefasciatus (Diptera, Culicidae)

The presence of intranuclear and acrosomal lectin binding sites in spermatids and spermatozoa of the mosquito Culex quinquefasciatus was analysed. Direct and indirect lectin-gold techniques were used on LR White-embedded cells. The nuclear compartment was the structure most intensely labelled. Early spermatid nucleus showed moderate labelling for peanut agglutinin (PNA), Griffonia simplicifolia IB4 (GS-IB4) and Ricinus communis agglutinin (RCA), and light labelling for the other lectins tested. The sperm nucleus was intensely labelled by all lectins. The acrosome, an enzyme-containing structure, was labelled by some lectins. The anterior acrosomal region was labelled by PNA, while the proximal acrosomal region was labelled by PNA and G. simplicifolia II (GS II) lectins, and showed the presence of fucose residues with the use of Ulex europaeus I (UEA-I) lectin. The spermatozoa stored in the spermatheca showed the same pattern of labelling as that observed in spermatozoa localized in testis and seminal vesicles for all lectins tested. Carbohydrate residues in the nuclear compartment may be involved with the process of chromatin condensation. In the acrosomal region these residues may play a role in the process of sperm-oocyte interaction.

Demethylation of the constant region genes of immunoglobulins reflects the differentiation state of the B cell

Previous results showed a developmentally regulated, strong linkage between demethylation and transcriptional activity for the light chain kappa locus in the mouse (Kelley et al., Molec. cell. Biol. 8, 930-937, 1988). These results indicate the existence of a stage of development of the B cell in which permanent expression (which may be enhancer independent) of a gene is associated with its demethylation. According to this result, demethylation could mirror terminal differentiation of a cell. We tested this hypothesis by analyzing the methylation status of immunoglobulin (Ig) genes in normal B cells before and after their activation with lipopolysaccharide (LPS) to induce IgM secretion and an immunoglobulin class switch. This pattern of methylation has been compared with that of Ig genes in nonlymphoid tissues and in transformed cell lines. In general, transformed cells are terminally differentiated cells. Our results show, that in normal splenic B cells only regions proximal to the heavy chain enhancer are demethylated. The coding regions of the c mu, c delta and the c gamma 1 genes remain methylated regardless of transcription. Demethylation of the coding regions is only detectable in transformed cell lines. Hence demethylation of immunoglobulin genes may reflect a stage of terminal differentiation in which the transcription pattern of the cell is fixed. Methylation of the genes before terminal differentiation may be necessary to allow controlled expression of genes on the transcriptional level, such as by splicing and differential termination.

HMG proteins 14 and 17 become cross-linked to the globular domain of histone H3 near the nucleosome core particle dyad

HMG proteins were derivatized with the photoactivatable cross-linker N-succinimidyl 3-((4-azidophenyl)dithio)propionate and then allowed to associate with nucleosome core particles. Following photolysis, peptide mapping of the principal dimeric adducts was carried out. Cross-linking occurred primarily from a central location in the HMGs to a central location in H3. The positions of these cross-links, considered along with other data from the literature, show that HMG proteins 14 and 17 make important contacts to H3 near the front face of the nucleosome. This raises the possibility that HMGs 14 and 17 participate in the reported conformational transition which exposes the H3 sulfhydryls of active nucleosomes.

Comparison of total cellular DNA, mRNA, and rRNA levels between normals and Down syndrome patients

Two recent studies have suggested that the absolute cellular RNA and/or DNA levels in Down syndrome (DS) may be unusual compared to normals and may be linked to phenotypic expression (Pash et al., 1990; Dahl et al. 1988). We have extended these two studies by directly quantitating and comparing the total cellular mRNA, rRNA, and DNA levels in fibroblasts and lymphocytes derived from normal and DS individuals. The assay methods used, which allowed us to present the various nucleic acids levels in picograms per cell, did not reveal any significant difference between the two groups.

Resistance of adult keratinocytes to differentiation-induced decrease in Ha-ras mRNA levels observed in newborn keratinocytes

During two-stage mouse skin tumorigenesis, the mouse c-Ha-ras oncogene undergoes activation by point mutation after initiation with polycyclic aromatic hydrocarbons. Furthermore, initiated epidermal cells containing an activated Ha-ras oncogene have been shown to be resistant to calcium-induced terminal differentiation. However, the relationship between Ha-ras expression and the differentiation process is not well understood in either normal or initiated cells. Before attempting to explore the role of Ha-ras expression in epidermal differentiation during tumorigenesis, we felt that investigation of Ha-ras gene expression in normal primary epidermal cells undergoing differentiation was warranted, since primary cultures of normal newborn and adult keratinocytes presumably contain the stem cells from which skin tumors arise. In the present studies, northern blot analysis was used to compare Ha-ras expression in normal newborn and adult epidermal cells undergoing differentiation. Steady-state levels of Ha-ras mRNA remained unchanged in primary cultures of normal adult epidermal cells during calcium-induced differentiation, whereas steady-state levels of Ha-ras transcripts decreased during calcium-induced differentiation in primary newborn epidermal cells. Differentiation was induced by switching the adult and newborn keratinocytes from medium containing 0.05 mM Ca2+ to medium containing one of three different calcium concentrations (0.15, 0.5, or 1.2 mM Ca2+). The decrease in Ha-ras mRNA levels observed during differentiation in newborn keratinocytes occurred as an intermediate event in the differentiation process, was specific for the Ha-ras gene, and was not due to a general decrease in transcriptional activity during differentiation. Characteristic patterns of keratin 14 gene expression and cornified envelope formation were observed, verifying that the differentiation process had been induced in both the primary adult and newborn epidermal cells. That adult keratinocytes are resistant to the differentiation-induced reduction in Ha-ras mRNA expression observed in newborn keratinocytes may explain the difference in in vivo tumorigenic potentials of newborn and adult skin.

The response of B cells in spleen, Peyer's patches, and lymph nodes to LPS and IL-4

The vast majority of B lymphocytes in the Peyer's patches (PP) and lymph nodes (LN) are memory cells or activated cells. Hence, in comparison to B lymphocytes in the spleen (SP), most B cells in these lymphoid organs have already encountered antigen. To further examine the ability of B cells in these peripheral lymphoid organs to respond to mitogens and interleukins in vitro, we have analyzed the ability of these cells (as compared to splenic B cells) to respond to LPS and LPS plus IL-4. Our results indicate that B cells from PPs and LNs proliferate poorly to LPS during the first 3 days of culture. In contrast, at later times, PP and LN B cells show enhanced proliferation as compared to splenic B cells. Furthermore, the addition of Interleukin-4 (IL-4) changes the proliferative activity of B cells from PPs and LNs, had only a minimal effect on splenic B cells. Hence, high doses of IL-4 (100 units/ml) enhance the proliferative rate of B cells from PPs and LNs early after activation, and have a suppressive effect at later times. The enhanced response of cells in PPs and LNs is further manifested by the presence of larger numbers of sIgG1+ cells 4 days after activation with LPS plus IL-4 and at 5 days these cells also secrete proportionally more IgG1 than splenic B cells. Enhanced IgG1 secretion is reflected in the methylation pattern of the s gamma 1 switch region of these cells. In cells from PP and LN cultured with LPS plus IL-4, most alleles containing the s gamma 1 region are demethylated or partly deleted, reflecting activation of this region of the Ig gene complex. In contrast, in splenic B cells, half the alleles remain in germline configuration. Our results suggest the presence of larger numbers of "preactivated" B cells in PPs and LNs as compared to spleen. These cells more rapidly secrete Ig following stimulation with LPS plus IL-4 in the absence of significant proliferation.

Assessment of the transcriptional activation potential of the HMG chromosomal proteins

Chromosomal proteins HMG-14, HMG-17, and HMG-1 are among the most abundant, ubiquitous, and evolutionarily conserved nonhistone proteins. Analysis of their structure reveals features which are similar to those of certain transcription factors. The distribution of charged amino acid residues along the polypeptide chains is asymmetric: positive charges are clustered toward the N-terminal region, while negative charges are clustered toward the C-terminal region. The residues in the C-terminal region have the potential to form alpha helices with negatively charged surfaces. The abilities of HMG-14, -17, and -1 to function as transcriptional activators were studied in Saccharomyces cerevisiae cells expressing LexA-HMG fusion proteins (human HMG-14 and -17 and rat HMG-1) which bind to reporter molecules containing the beta-galactosidase gene downstream from a lexA operator. Fusion constructs expressing deletion mutants of HMG-14, -17, and -1 were also tested. Analysis of binding to the lexA operator with in vitro-synthesized fusion proteins shows that there are more sites for HMG-14, -17, and -1 binding than for LexA binding and that only the fusion constructs which contain the C-terminal, acidic domains of HMG-17 bind the lexA operator specifically. None of the LexA-HMG fusion protein constructs elevate the level of beta-galactosidase activity in transfected yeast cells. Thus, although HMG-14, -17, and -1 are structurally similar to acidic transcriptional activators, these chromosomal proteins do not function as activators in this test system.

Assessment of the transcriptional activation potential of the HMG chromosomal proteins

Chromosomal proteins HMG-14, HMG-17, and HMG-1 are among the most abundant, ubiquitous, and evolutionarily conserved nonhistone proteins. Analysis of their structure reveals features which are similar to those of certain transcription factors. The distribution of charged amino acid residues along the polypeptide chains is asymmetric: positive charges are clustered toward the N-terminal region, while negative charges are clustered toward the C-terminal region. The residues in the C-terminal region have the potential to form alpha helices with negatively charged surfaces. The abilities of HMG-14, -17, and -1 to function as transcriptional activators were studied in Saccharomyces cerevisiae cells expressing LexA-HMG fusion proteins (human HMG-14 and -17 and rat HMG-1) which bind to reporter molecules containing the beta-galactosidase gene downstream from a lexA operator. Fusion constructs expressing deletion mutants of HMG-14, -17, and -1 were also tested. Analysis of binding to the lexA operator with in vitro-synthesized fusion proteins shows that there are more sites for HMG-14, -17, and -1 binding than for LexA binding and that only the fusion constructs which contain the C-terminal, acidic domains of HMG-17 bind the lexA operator specifically. None of the LexA-HMG fusion protein constructs elevate the level of beta-galactosidase activity in transfected yeast cells. Thus, although HMG-14, -17, and -1 are structurally similar to acidic transcriptional activators, these chromosomal proteins do not function as activators in this test system.

The DNA intercalator, ethidium bromide, alters the pattern of DNAse I hypersensitive sites of the beta A-globin gene in chicken erythrocytes

We have analysed the effects of a DNA intercalator, ethidium bromide (EB), on chromatin structure in nuclei from both chicken mature erythrocytes (RBC) and reticulocytes (Ret). A differential release of nuclear proteins was obtained from both types of nuclei exposed to EB. Among these proteins, a species of 45 kDa is the major component. Furthermore, in the 10 mM EB-treated nuclei, the pattern of DNAse I hypersensitive sites (DHS) around the chicken beta A-globin gene were significantly altered, i.e., the original set was replaced by a new set of DHS. We have discussed the implications of our observations, in the light of current concepts of functional aspects of the conformational heterogeneity of DNA in both protein-DNA interactions and chromatin structure, as well as the effects of DNA intercalators on DNA conformation.

Chromosomal protein HMG-14 is overexpressed in Down syndrome

The physical phenotype of Down syndrome, one of the most prevalent genetic disorders, results from an extra copy of regions q22.1 to q22.3 of chromosome 21 in cells of affected individuals. The gene coding for chromosomal protein HMG-14 is among the limited number of genes, coding for known functions, which has been mapped to this region of chromosome 21. Here we report a gene dosage effect on the expression of HMG-14 in both cultured cells and brain tissue samples obtained from Down syndrome patients. The putative role of HMG-14 in the structure of active chromatin raises the possibility that elevated levels of this protein may be a contributing factor in the etiology of Down syndrome.

Distribution of high mobility group proteins 1/2, E and 14/17 and linker histones H1 and H5 on transcribed and non-transcribed regions of chicken erythrocyte chromatin

Quantitative analysis of distribution of chromosomal proteins on single copy DNA sequences has been further developed. Our approach consists of DNA-protein crosslinking within whole cells or isolated nuclei, specific immunoaffinity isolation of crosslinked complexes via protein and identification of crosslinked DNA by hybridisation with single-stranded DNA probes. The present study shows that transcribed chromatin of chicken embryonic erythrocyte beta globin gene is characterized by about 1.5-2.5-fold higher density of HMG 14/17 and 2-fold lower density of H1 and H5 as compared with non-transcribed chromatin of ovalbumin and lysozyme genes, whereas HMG 1/2, E proteins were equally distributed between DNA of both transcribed and non-transcribed genes. The depletion of H1/H5 in beta globin sequences was verified by the 'protein image' hybridisation technique (1). The DNase I hypersensitive site located 5' upstream from beta globin gene is deficient in all the proteins assayed, what implies a drastic disruption in the nucleosomal array. Minor quantitative changes of protein pattern suggest transient local perturbation of the chromatin on transcription.

Cell-cell interactions trigger the rapid induction of a specific high mobility group-like protein during early stages of conjugation in Tetrahymena

Conjugation in Tetrahymena represents an ordered developmental pathway which represents the sexual phase of the ciliate life cycle. This pathway is initiated when starved cells of opposite mating types are mixed and are allowed to make a series of cell-cell contacts (a period termed costimulation) which lead to the formation of mating pairs. Here, we demonstrate that two previously described abundant high mobility group (HMG)-like proteins, HMG B and HMG C, whose synthesis appeared to be coordinately regulated in vegetative cells, are not required during the same stages of conjugation. The level of mRNA for both HMG B and HMG C is high during vegetative growth and during the development of new macronuclei. However, specific induction of HMG B mRNA is observed soon after cells of opposite mating types are mixed. Thus, the genes which encode HMG B and HMG C in Tetrahymena can be controlled independently or coordinately. Nuclear run-on experiments show that a significant factor underlying the rapid induction of HMG B message early in the sexual cycle is an increase in the transcriptional activity of the HMG B gene. Experiments are presented which show that this induction of HMG B message requires protein synthesis and is dependent upon the cell-cell contacts made during costimulation. Essentially all of the HMG B protein, which is newly synthesized during this period, is targeted to parental macronuclei where it serves an as yet undetermined function(s).

Sequence and genetic analysis of NHP2: a moderately abundant high mobility group-like nuclear protein with an essential function in Saccharomyces cerevisiae

In order to determine the biological functions of moderately abundant, high mobility group (HMG)-like nuclear proteins, a genetic approach has been taken. The gene for one such protein, NHP2, has been cloned and characterized from Saccharomyces cerevisiae. NHP2 has been called 'HMG-like' because of the physical/chemical properties it shares with the HMG proteins from higher eukaryotic cells. However, nucleotide sequence analysis revealed that NHP2 could encode a 17.1 kilodalton basic protein which was not significantly homologous to any previously sequenced HMG proteins. Thus NHP2 defines a new member of the HMG class of proteins. A search of protein databases showed that the amino acid sequence of NHP2 shared significant identities with two ribosomal proteins; the acidic ribosomal protein S6 from Halobacterium marismorium and protein L7a from mammals. The biological relevance of these homologies is unclear since previous biochemical results indicated that NHP2 was not a ribosomal protein. S1 nuclease analysis indicated that the gene contained no introns but had multiple transcription initiation sites 20 to 40 bases before the ATG codon. Finally, NHP2 has been shown to have a critical role in the cell; when a diploid yeast strain deleted of one copy of the NHP2 gene was sporulated and dissected, only half of the spores grew into normal colonies. The rest of the spores germinated, but only formed microcolonies containing 12 to 40 cells. None of the spores which grew into normal-sized colonies contained the mutant NHP2 gene, thus demonstrating that the NHP2 protein has an essential physiological function.

Change in activity of nuclear poly(ADP-ribose) glycohydrolase during the HeLa S3 cell cycle

The change in activity of nuclear poly(ADP-ribose) glycohydrolase during the cell cycle of HeLa S3 cells was investigated. The poly(ADP-ribose) glycohydrolase activity was solubilized from HeLa S3 cell nuclei and chromosomes only by sonication at high ionic strength. The enzyme hydrolyzed poly(ADP-ribose) exoglycosidically, producing ADP-ribose. After release from mitosis, the activity of the solubilized nuclear poly(ADP-ribose) glycohydrolase per nucleus or per unit protein, assayed with [3H]poly(ADP-ribose) (average chain length, n = 15) as substrate, was lowest in the early G1 phase and highest in the late G1 phase. The specific activity in the late G1 phase was about two times that in the early G1 phase. The high activity remained constant during the S-G2-M phase. A similar change during the cell cycle was observed after release from hydroxyurea block. These results suggest that the activity of poly(ADP-ribose) glycohydrolase doubled during the G1 phase of the cell cycle of HeLa S3 cells.

Differentiation-dependent alteration in the chromatin structure of chromosomal protein HMG-17 gene during erythropoiesis

The expression of the gene coding for chromosomal protein HMG-17 is down regulated during chicken erythrocyte maturation. The transcriptional down regulation is associated with major alterations in the chromatin structure of this gene. The 5' region of the gene contains both constitutive and developmental stage-specific deoxyribonuclease I (DNase I) hypersensitive sites. The constitutive sites bracket the "CpG island" present in the gene, which remains hypomethylated throughout the various developmental stages. During erythropoiesis, the gene acquires a distinct structure that, upon digestion with micrococcal nuclease (MNase) yields an unusual repeat. Two nucleosomes, with a 200 base-pair repeat, are positioned immediately downstream from the start of transcription. Immediately downstream and upstream from these nucleosomes, the boundaries between MNase sites change to a 75 base-pair repeat, which indicates an unusual chromatin structure. The differentiation related changes in the DNase I and MNase digestion pattern in the 5' region of the gene suggest that sequences present in the first intron may be involved in gene regulation. The results may be relevant to the regulation of the entire HMG-14/-17 gene family.

Loosened nucleosome linker folding in transcriptionally active chromatin of chicken embryo erythrocyte nuclei

We have investigated the mechanism of the electrophoresis-driven chromatin aggregation which had been described by Weintraub (1984, Cell 38, 17-27) as a putative mean for propagation of genetic repression in eukaryotes. We show that the oligonucleosome aggregates are assembled de novo at the starting zone of DNP electrophoresis. A new system of native two-dimensional DNP electrophoresis has been worked out to separate the oligonucleosome aggregates ('A' particles) and the freely-migrating oligonucleosomes ('B' particles). The 'B' particle fraction which is derived from transcriptionally-active chromatin regions undergoes an extensive nuclease degradation of its DNA termini during the nuclease digestion. This fraction is partially depleted of histones H1 and H5 and is enriched in HMG nonhistone proteins. 'A' particles comprise the repressed chromatin DNA fragments which are about 60 b.p. longer than the corresponding DNA oligomers of 'B' particles. An oligonucleosome preparation containing the elongated DNA oligomers has been also isolated by means of sucrose gradient ultracentrifugation. Exonuclease III mapping reveals that the two chromatin fractions differ by an extent of terminal linker DNA trimming during the Micrococcal nuclease digestion rather than by the nucleosome repeat length. The complex character of nuclease digestion is not observed when the chromatin is digested in solution after the nuclear lysis. We argue that the protection of terminal oligonucleosome linkers is due to selective condensation of inactive chromatin in chicken erythrocyte nuclei and that the terminal DNA tails together with linker histones bound to them mediate the aggregation of repressed chromatin fragments.

Expression of human chromosomal proteins HMG-14 and HMG-17 in Saccharomyces cerevisiae

The cDNAs coding for human chromosomal proteins HMG-14 and HMG-17 were cloned into yeast expression vector pBM150, under the control of the Gal10 promoter. Northern analysis of transformed yeast cells revealed that both cDNAs were efficiently transcribed. Western analysis indicated that the mRNAs were translated into authentic proteins. Expression of human HMG proteins in yeast cell did not produce detectable phenotypic changes, as measured by the growth rate of the yeast cells under a variety of conditions. The antibiotic resistance of the transfected cells was similar to that of control cells, suggesting that the presence of HMG did not affect the expression of actively transcribed genes. However, examination of the protein profile on two-dimensional polyacrylamide gel electrophoresis revealed differences between control and HMG-transfected cells.

Protective methylation of immunoglobulin and T cell receptor (TcR) gene loci prior to induction of class switch and TcR recombination

Methylation of the S gamma 1 switch region and C gamma 1 constant region gene from the immunoglobulin heavy chain locus and of the J beta 2 and C beta regions from the T cell receptor beta chain (TcR beta) locus is compared here in murine germ-line cells, nonlymphoid cells and lymphocytes. In germ-line cells and in lymphocytes prior to recombination all four regions show strong methylation, i.e. most Msp I sites are methylated. After activation of lymphocytes, demethylation is observed for those regions which are activated for recombination, at specific sites 5' of S gamma 1 in B cells activated with bacterial lipopolysaccharide and interleukin 4, and for J beta 2 in thymocytes. In nonlymphoid cells, where these regions cannot be used for recombination, considerable demethylation is observed for all four regions analyzed as compared to lymphocytes. The result implies an important role for methylation of recombinatorial regions. Methylation may be involved in protecting them from uninduced recombination, thus allowing regulated expression of distinct genes in lymphocyte ontogeny.

Protein HMG-17 is hyper-expressed in rat glucagonoma. Single-step isolation and sequencing

High-mobility-group protein 17 (HMG-17) was identified by reversed-phase high-performance liquid chromatography analysis as a major component in acidic extracts of transplantable rat glucagonoma tissue but not in insulinoma tissue of similar origin. The peptide was purified in a single step and the entire sequence of 89 amino acids was determined. Rat HMG-17 has a molecular mass of 9238 Da and shows strong similarity to human, bovine (94.4%) and chicken (88.8%) HMG-17. Six of the seven residues which vary among the mammalian sequences are located within a short segment (positions 64-83) present in the acidic, non-DNA-binding C-terminal part of HMG-17. This region shows least similarity to the otherwise related proteins HMG-14 and H6 (a trout HMG protein). Interestingly, four of the six variable positions are Asp in rat HMG-17 which results in an overall net increase in the negative charge of the C-terminal region. The nature of selective hyper-expression of HMG-17 in glucagon but not in insulin-producing tumor tissue remains to be clarified.

Interaction of HMG14 with chromatin

Neutron scattering has been used to study the interaction of HMG14 with chromatin. Chromatin depleted of H1/H5 was reconstituted separately with histones H1 and H5, and complexed with HMG14. We have also studied the conformation of complexes formed by the binding of HMG14 to nucleosome dimers without linker DNA. Our data on the binding of HMG14 to linkerless nucleosome dimers argue against a significant change in the exit and entry angles of nucleosomal core DNA. Data on the condensation of chromatin into a higher-order structure suggest that there is no dramatic difference between the roles of H1 and H5 in their influence on HMG14 complex formation. However, there is a decrease of about 25% in the mass per unit length of chromatin fibers on HMG14 binding, which is not accompanied by a change in the fiber repeat distance. This is evidence that there are fewer nucleosomes per repeat in HMG14 containing chromatin fibers than in normal chromatin. Alteration of chromatin structure in this manner may be part of the role of HMG14 in actively transcribed chromatin.

High mobility group proteins 1 and 2 function as general class II transcription factors

High mobility group (HMG) proteins 1 and 2 are thought to be associated with chromatin enriched in active gene sequences, to stimulate endogenous transcription of class II and III genes using HMG-depleted nuclei, and to bind specific DNA sequences upstream of the coding regions of trout HMG-T and human beta-globin genes. In testing the possibility that these proteins may act as general transcription factors, the run-off transcription of trout protamine, human beta-globin, adenovirus 2 major late promoter, and herpes simplex virus (HSV) thymidine kinase genes was found to be inhibited by affinity-purified HMG-1 and -2 antibodies. The inhibition was partially relieved by exogenously added HMG-1 or -2. A complementation assay showed that the 0.15 M KCl flowthrough of HeLa nuclear extract fractionated by anion-exchange chromatography (DE-52) could be replaced by purified HMG-1 and/or -2 to complement transcription of the trout protamine gene by the 0.5 M KCl eluate fraction. Inhibition studies with heparin showed that HMG-1 and -2 were required for initiation of transcription. These results indicate an absolute requirement of HMG-1 and -2 for class II gene transcription. Western blotting and transcription reconstituted with purified factors show a copurification of HMG-1 and -2 with factor II B, described earlier by Reinberg and Roeder [(1987) J. Biol. Chem. 262, 3310-3321].

Chromosomal protein HMG-14 gene maps to the Down syndrome region of human chromosome 21 and is overexpressed in mouse trisomy 16

The gene for human high-mobility-group (HMG) chromosomal protein HMG-14 is located in region 21q22.3, a region associated with the pathogenesis of Down syndrome, one of the most prevalent human birth defects. The expression of this gene is analyzed in mouse embryos that are trisomic in chromosome 16 and are considered to be an animal model for Down syndrome. RNA blot-hybridization analysis and detailed analysis of HMG-14 protein levels indicate that mouse trisomy 16 embryos have approximately 1.5 times more HMG-14 mRNA and protein than their normal littermates, suggesting a direct gene dosage effect. The HMG-14 gene may be an additional marker for the Down syndrome. Chromosomal protein HMG-14 is a nucleosomal binding protein that may confer distinct properties to the chromatin structure of transcriptionally active genes and therefore may be a contributing factor in the etiology of the syndrome.