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S Dimitrov

Publications and source records attributed to S Dimitrov.

At least 37 records · Page 2Linked to original sources
PubMed2000

Somatic linker histone H1 is present throughout mouse embryogenesis and is not replaced by variant H1 degrees.

A striking feature of early embryogenesis in a number of organisms is the use of embryonic linker histones or high mobility group proteins in place of somatic histone H1. The transition in chromatin composition towards somatic H1 appears to be correlated with a major increase in transcription at the activation of the zygotic genome. Previous studies have supported the idea that the mouse embryo essentially follows this pattern, with the significant difference that the substitute linker histone might be the differentiation variant H1 degrees, rather than an embryonic variant. We show that histone H1 degrees is not a major linker histone during early mouse development. Instead, somatic H1 was present throughout this period. Though present in mature oocytes, somatic H1 was not found on maternal metaphase II chromatin. Upon formation of pronuclear envelopes, somatic H1 was rapidly incorporated onto maternal and paternal chromatin, and the amount of somatic H1 steadily increased on embryonic chromatin through to the 8-cell stage. Microinjection of somatic H1 into oocytes, and nuclear transfer experiments, demonstrated that factors in the oocyte cytoplasm and the nuclear envelope, played central roles in regulating the loading of H1 onto chromatin. Exchange of H1 from transferred nuclei onto maternal chromatin required breakdown of the nuclear envelope and the extent of exchange was inversely correlated with the developmental advancement of the donor nucleus.

Animals↗
PubMed2000

Developmentally regulated activity of CRM1/XPO1 during early Xenopus embryogenesis.

In this work, we have investigated the role of CRM1/XPO1, a protein involved in specific export of proteins and RNA from the nucleus, in early Xenopus embryogenesis. The cloning of the Xenopus laevis CRM1, XCRM1, revealed remarkable conservation of the protein during evolution (96.7% amino acid identity between Xenopus and human). The protein and mRNA are maternally expressed and are present during early embryogenesis. However, our data show that the activity of the protein is developmentally regulated. Embryonic development is insensitive to leptomycin B, a specific inhibitor of CRM1, until the neurula stage. Moreover, the nuclear localization of CRM1 changes concomitantly with the appearance of the leptomycin B sensitivity. These data suggest that CRM1, present initially in an inactive form, becomes functional before the initiation of the neurula stage during gastrula-neurula transition, a period known to correspond to a critical transition in the pattern of gene expression. Finally, we confirmed the gastrula-neurula transition-dependent activation of CRM1 by pull-down experiments as well as by the study of the intracellular localization of a green fluorescent protein tagged with a nuclear export signal motif during early development. This work showed that the regulated activity of CRM1 controls specific transitions during normal development and thus might be a key regulator of early embryogenesis.

Amino Acid Sequence↗
PubMed1999

Ultraviolet laser footprinting of histone H1(0)-four-way junction DNA complexes.

We have used a new light footprinting technique to study the interaction of histone H1(0) and a deletion mutant delta CH1(0) (lacking H1(0) COOH-terminal domain) with a synthetic four-way junction DNA. This technique is based on a single 5-ns UV laser pulse and has the ability to map protein-DNA interactions within unperturbed complexes at time scales far faster than molecular rearrangements. We found both H1(0) and delta CH1(0) to affect the photoreactivity of specific guanine residues located on the central part of four-way junction DNA. These observations demonstrate specific recognition of H1(0) for the central domain of four-way junction DNA. In addition, histone H1(0) decreases the photoreactivity of selected guanines located some distance from the crossover, indicating specific involvement of the H1(0) COOH-terminal tail with this region. Immunofractionation of delta CH1(0)-four-way DNA junction complexes with monoclonal anti-H1 antibody combined with the UV laser footprinting method demonstrated the existence of two types of delta CH1(0)-four-way DNA junction complexes.

Animals↗
PubMed1999

Control of the histone-acetyltransferase activity of Tip60 by the HIV-1 transactivator protein, Tat.

Tip60, a cellular histone-acetyltransferase, is known to interact with the HIV-1-encoded transactivator protein, Tat. In this work, we show that the interaction of Tat with Tip60 efficiently inhibits the Tip60 histone-acetyltransferase activity. Besides its histone-acetyltransferase activity, Tip60 can undergo an autoacetylation which is not affected by Tat interaction. Our data show that Tip60 does not significantly influence Tat-dependent transcriptional activation of the 5'-LTR of HIV, suggesting that its interaction with Tat affects some intrinsic cellular process. We were then able to identify a cellular gene, Mn-dependent superoxide dismutase (Mn-SOD), that has a Tip60-dependent transcriptional activity. Interestingly, the simultaneous expression of Tat and Tip60 abolishes the effect of Tip60 on the activity of the Mn-SOD promoter. We postulate that the HIV-1 transactivator, Tat, in targeting Tip60 hinders the expression of cellular genes (such as Mn-SOD) which normally interfere with the efficient replication and propagation of the virus.

Acetyltransferases↗
PubMed1999

Elasticity measurements show the existence of thin rigid cores inside mitotic chromosomes.

Chromosome condensation is one of the most critical steps during cell division. However, the structure of condensed mitotic chromosomes is poorly understood. In this paper we describe a new approach based on elasticity measurements for studying the structure of in vitro assembled mitotic chromosomes in Xenopus egg extract. The approach is based on a unique combination of measurements of both longitudinal deformability and bending rigidity of whole chromosomes. By using specially designed micropipettes, the chromosome force-extension curve was determined. Analysis of the curvature fluctuation spectrum allowed for the measurement of chromosome bending ridigity. The relationship between the values of these two parameters is very specific: the measured chromosome flexibility was found to be 2,000 times lower than the flexibility calculated from the experimentally determined Young modulus. This requires the chromosome structure to be formed of one or a few thin rigid elastic axes surrounded by a soft envelope. The properties of these axes are well-described by models developed for the elasticity of titin-like molecules. Additionally, the deformability of in vitro assembled chromosomes was found to be very similar to that of native somatic chromosomes, thus demonstrating the existence of an essentially identical structure.

Animals↗
PubMed1999

Placental teratoma--a case report.

A rare case of a mature placental teratoma in a 32-year-old woman is reported. We discuss the mechanism responsible for the development of this rare tumor in the placenta as well as the differential diagnostic difficulties encountered in fetus acardius amorphus, underlining the clinical significance of the exact diagnosis.

Adult↗
PubMed1998

The globular domain of histone H1 is sufficient to direct specific gene repression in early Xenopus embryos.

One molecule of a linker histone such as histone H1 is incorporated into every metazoan nucleosome [1]. Histone H1 has three distinct structural domains: the positively charged amino-terminal and carboxy-terminal tails are separated by a globular domain that is similar to the winged-helix motif found in sequence-specific DNA-binding proteins [2]. The globular domain interacts with DNA immediately contiguous to that wrapped around the core histones [3,4], whereas the tail domains are important for the compaction of nucleosomal arrays [5]. Experiments in vivo indicate that histone H1 does not function as a global transcriptional repressor, but instead has more specific regulatory roles [6-9]. In Xenopus, maternal stores of the B4 linker histone that are assembled into chromatin during the early cleavage divisions are replaced by somatic histone H1 during gastrulation [10]. This transition in chromatin composition causes the repression of genes encoding oocyte-type 5S rRNAs, and restricts the competence of ectodermal cells to differentiate into mesoderm [6,9-11]. Here, we demonstrate that the globular domain of histone H1 is sufficient for directing gene-specific transcriptional repression and for restricting the mesodermal competence of embryonic ectoderm. We discuss our results in the context of specific structural roles for this domain in the nucleosome.

Animals↗
PubMed1998

Persistent interactions of core histone tails with nucleosomal DNA following acetylation and transcription factor binding.

In this study, we examined the effect of acetylation of the NH2 tails of core histones on their binding to nucleosomal DNA in the absence or presence of bound transcription factors. To do this, we used a novel UV laser-induced protein-DNA cross-linking technique, combined with immunochemical and molecular biology approaches. Nucleosomes containing one or five GAL4 binding sites were reconstituted with hypoacetylated or hyperacetylated core histones. Within these reconstituted particles, UV laser-induced histone-DNA cross-linking was found to occur only via the nonstructured histone tails and thus presented a unique tool for studying histone tail interactions with nucleosomal DNA. Importantly, these studies demonstrated that the NH2 tails were not released from nucleosomal DNA upon histone acetylation, although some weakening of their interactions was observed at elevated ionic strengths. Moreover, the binding of up to five GAL4-AH dimers to nucleosomes occupying the central 90 bp occurred without displacement of the histone NH2 tails from DNA. GAL4-AH binding perturbed the interaction of each histone tail with nucleosomal DNA to different degrees. However, in all cases, greater than 50% of the interactions between the histone tails and DNA was retained upon GAL4-AH binding, even if the tails were highly acetylated. These data illustrate an interaction of acetylated or nonacetylated histone tails with DNA that persists in the presence of simultaneously bound transcription factors.

Acetylation↗
PubMed1997

Histone acetylation: influence on transcription, nucleosome mobility and positioning, and linker histone-dependent transcriptional repression.

We demonstrate using a dinucleosome template that acetylation of the core histones enhances transcription by RNA polymerase III. This effect is not dependent on an increased mobility of the core histone octamer with respect to DNA sequence. When linker histone is subsequently bound, we find both a reduction in nucleosome mobility and a repression of transcription. These effects of linker histone binding are independent of core histone acetylation, indicating that core histone acetylation does not prevent linker histone binding and the concomitant transcriptional repression. These studies are complemented by the use of a Xenopus egg extract competent both for chromatin assembly on replicating DNA and for RNA polymerase III transcription. Incorporation of acetylated histones and lack of linker histones together facilitate transcription by >10-fold in this system; however, they have little independent effect on transcription. Thus core histone acetylation significantly facilitates transcription, but this effect is inhibited by the assembly of linker histones into chromatin.

Acetylation↗
PubMed1997

What do linker histones do in chromatin?

Knockout experiments in Tetrahymena show that linker histone H1 is not essential for nuclear assembly or cell viability. These results, together with a series of biochemical and cell biological observations, challenge the existing paradigm that requires linker histones to be a key organizing component of higher-order chromatin structure. The H1 knockouts also reveal a much more subtle role for H1. Instead of acting as a general transcriptional repressor, H1 is found to regulate a limited number of specific genes. Surprisingly, H1 can both activate and repress transcription. We discuss how this architectural protein might accomplish this important regulatory role.

Amino Acid Sequence↗
PubMed1997

[Problems in the diagnosis and treatment of solitary and multiple kidney cysts].

Over the period 1989-1994, thirty-three patients presenting solitary cysts, and 19--polycystosis undergo clinical study. A variety of examination and differential diagnostic methods are used, namely: echography, venous urography, gamma chamber scintigraphy, CAT and biochemical analysis of the cystic fluid. The listed methods contribute greatly to make exact diagnosis and undertake adequate treatment of the patients.

Biopsy, Needle↗
PubMed1996

Remodeling somatic nuclei in Xenopus laevis egg extracts: molecular mechanisms for the selective release of histones H1 and H1(0) from chromatin and the acquisition of transcriptional competence.

The molecular mechanisms responsible for the remodeling of entire somatic erythrocyte nuclei in Xenopus laevis egg cytoplasm have been examined. These transitions in chromosomal composition are associated with the capacity to activate new patterns of gene expression and the re-acquisition of replication competence. Somatic linker histone variants H1 and H1 (0) are released from chromatin in egg cytoplasm, whereas the oocyte-specific linker histone B4 and HMG1 are efficiently incorporated into remodeled chromatin. Histone H1 (0) is released from chromatin preferentially in comparison with histone H1. Core histones H2A and H4 in the somatic nucleus are phosphorylated during this remodeling process. These transitions recapitulate the chromosomal environment found within the nuclei of the early Xenopus embryo. Phosphorylation of somatic linker histone variants is demonstrated not to direct their release from chromatin, nor does direct competition with cytoplasmic stores of linker histone B4 determine their release. However, the molecular chaperone nucleoplasmin does have an important role in the selective removal of linker histones from somatic nuclei. For Xenopus erythrocyte nuclei, this disruption of chromatin structure leads to activation of the 5S rRNA genes. These results provide a molecular explanation for the remodeling of chromatin in Xenopus egg cytoplasm and indicate the capacity of molecular chaperones to disrupt a natural chromosomal environment, thereby facilitating transcription.

Animals↗
PubMed1996

Evidence for a shared structural role for HMG1 and linker histones B4 and H1 in organizing chromatin.

The high mobility group proteins 1 and 2 (HMG1/2) and histone B4 are major components of chromatin within the nuclei assembled during the incubation of Xenopus sperm chromatin in Xenopus egg extract. To investigate their potential structural and functional roles, we have cloned and expressed Xenopus HMG1 and histone B4. Purified histone B4 and HMG1 form stable complexes with nucleosomes including Xenopus 5S DNA. Both proteins associate with linker DNA and stabilize it against digestion with micrococcal nuclease, in a similar manner to histone H1. However, neither histone B4 nor HMG1 influence the DNase I or hydroxyl radical digestion of DNA within the nucleosome core. We suggest that HMG1/2 and histone B4 have a shared structural role in organizing linker DNA in the nucleosome.

Amino Acid Sequence↗
PubMed1994

Nuclear assembly is independent of linker histones.

The role of linker histones in the assembly of functional nuclei was examined with the use of a cell-free extract of Xenopus eggs that transforms condensed sperm chromatin into DNA-replication-competent pronuclei. When linker histones were removed from the extract, the resultant pronuclei were indistinguishable from those formed in the complete extract. The assembly of functional nuclear membrane, nuclear lamina, and prereplication centers allowed identical DNA replication efficiencies. Thus, linker histones are not required for the assembly of morphologically normal nuclei capable of DNA replication.

Animals↗