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H Leonhardt

Publications and source records attributed to H Leonhardt.

At least 37 records · Page 2Linked to original sources
PubMed1998

A short DNA methyltransferase isoform restores methylation in vivo.

Two murine DNA methyltransferase isoforms (MTases) have been observed, a longer form in somatic and embryonic stem (ES) cells and a shorter form in oocytes and preimplantation embryos. While the longer MTase is associated with maintenance methyltransferase activity in replicating cells, little is known about the shorter form. We present genetic and biochemical evidence that both isoforms are expressed from the same Dnmt1 gene by using different translation initiation sites in exons 1 and 4. We further demonstrate that the shorter isoform can functionally rescue Dnmt1 null ES cells that have a hypomethylated genome. These rescued ES cells differentiate in vivo into a variety of cell types, unlike the Dnmt1 null ES cells that die upon induction of differentiation. These results show that the shorter isoform can substitute for the longer maintenance MTase in ES and differentiated cells. Our data further indicate that the shorter MTase isoform found in oocytes is fully functional in vivo and may play an active role in the regulation of DNA methylation and the establishment of imprinting patterns.

Amino Acid Sequence↗
PubMed1998

Protein targeting to subnuclear higher order structures: a new level of regulation and coordination of nuclear processes.

Though there are no separating membranes within the nucleus, different factors are often concentrated at sites where their respective function is required, a phenomenon referred to as functional organization of the nucleus. How is then this organization achieved and how are the different metabolic processes integrated in the nucleus? One emerging principle was revealed by the identification of protein domains that, though not involved in catalysis, regulate enzyme activity at a higher order level by targeting enzymes to the right place at the right time. These targeting sequences constitute an assembly code for nuclear 'protein factories,' which ensure the extremely high efficiency and accuracy needed in a complex and competitive environment as the living mammalian cell.

Animals↗
PubMed1998

Intranuclear targeting of DNA replication factors.

Mammalian nuclei are highly organized into functional compartments. Major nuclear processes like DNA replication and RNA processing take place in distinct foci. These microscopically visible foci are formed by the assembly of, for example, DNA replication factors and associated proteins into megadalton complexes often referred to as protein machines or factories. Thus far, two proteins, DNA ligase I and DNA methyltransferase (DNA MTase), have been analyzed in greater detail. In both cases, the assembly process appears to be controlled by distinct targeting sequences that were attached to the catalytic protein core in the course of evolution and mediate the association with replication factories in mammalian cells. The dynamics of these nuclear structures throughout the cell cycle are analyzed using green fluorescent protein (GFP). Further studies are needed to elucidate the architecture, regulation, and role of these subnuclear structures.

Animals↗
PubMed1997

Mapping and use of a sequence that targets DNA ligase I to sites of DNA replication in vivo.

The mammalian nucleus is highly organized, and nuclear processes such as DNA replication occur in discrete nuclear foci, a phenomenon often termed "functional organization" of the nucleus. We describe the identification and characterization of a bipartite targeting sequence (amino acids 1-28 and 111-179) that is necessary and sufficient to direct DNA ligase I to nuclear replication foci during S phase. This targeting sequence is located within the regulatory, NH2-terminal domain of the protein and is dispensable for enzyme activity in vitro but is required in vivo. The targeting domain functions position independently at either the NH2 or the COOH termini of heterologous proteins. We used the targeting sequence of DNA ligase I to visualize replication foci in vivo. Chimeric proteins with DNA ligase I and the green fluorescent protein localized at replication foci in living mammalian cells and thus show that these subnuclear functional domains, previously observed in fixed cells, exist in vivo. The characteristic redistribution of these chimeric proteins makes them unique markers for cell cycle studies to directly monitor entry into S phase in living cells.

Amino Acid Sequence↗
PubMed1996

Complementation of methylation deficiency in embryonic stem cells by a DNA methyltransferase minigene.

Previous attempts to express functional DNA cytosine methyltransferase (EC 2.1.1.37) in cells transfected with the available Dnmt cDNAs have met with little or no success. We show that the published Dnmt sequence encodes an amino terminal-truncated protein that is tolerated only at very low levels when stably expressed in embryonic stem cells. Normal expression levels were, however, obtained with constructs containing a continuation of an ORF with a coding capacity of up to 171 amino acids upstream of the previously defined start site. The protein encoded by these constructs comigrated in SDS/PAGE with the endogenous enzyme and restored methylation activity in transfected cells. This was shown by functional rescue of Dnmt mutant embryonic stem cells that contain highly demethylated genomic DNA and fail to differentiate normally. When transfected with the minigene construct, the genomic DNA became remethylated and the cells regained the capacity to form teratomas that displayed a wide variety of differentiated cell types. Our results define an amino-terminal domain of the mammalian MTase that is crucial for stable expression and function in vivo.

Amino Acid Sequence↗
PubMed1995

Targeting and association of proteins with functional domains in the nucleus: the insoluble solution.

The mammalian nucleus is highly organized into distinct functional domains separating different biochemical processes such as transcription, RNA processing, DNA synthesis, and ribosome assembly. A number of proteins known to participate in these processes were found to be specifically localized at their corresponding functional domains. A distinct targeting sequence, necessary and sufficient for the localization to DNA replication foci, was identified in the N-terminal, regulatory domain of DNA methyltransferase and DNA ligase I and might play a role in the coordination of DNA replication and DNA methylation. The fact that the targeting sequence is absent in lower eukaryotic and prokaryotic DNA ligase I homologs suggests that "targeting" is a rather recent development in evolution. Finally, targeting sequences have also been identified in some splicing factors and in viral proteins, which are responsible for their localization to the speckled compartment and to the nucleolus, respectively. These higher levels of organization are likely to contribute to the regulation and coordination of the complex and interdependent biochemical processes in the mammalian nucleus.

Animals↗
PubMed1993

[Eosinophilic fasciitis (Shulman syndrome) as a rare differential diagnosis of sclerodermiform skin changes].

For 3 months, a 70-year-old woman had been affected by painless, symmetrical swellings in the limbs, spreading proximally, causing hardening of the skin and impaired joint movements, but no fever. Her general condition and findings on physical examination were according to age. ESR was 22/42 mm and there was a thrombocytosis (533,000/microliters) and eosinophilia (16%, absolute 2,140/microliters). Radiological and ultrasound examinations were normal. Skin biopsy revealed marked oedema of the connective tissue septa and perivascular lymphohistiocytic infiltrations, which--together with the other findings--suggested eosinophilic fasciitis. She was treated with prednisone, 100 mg daily, reduced gradually to 8 mg daily, as well as cimetidine, 800 mg daily, as immunomodulator, and intensive physiotherapy. The swellings and eosinophilia regressed within 8 weeks, but the hardening and impaired mobility persisted. Maintenance glucocorticoids are being continued.

Aged↗
PubMed1993

Reversal of terminal differentiation and control of DNA replication: cyclin A and Cdk2 specifically localize at subnuclear sites of DNA replication.

DNA replication in mammalian cells occurs in discrete nuclear foci. Here we show that terminally differentiated myotubes can be induced to reenter S phase and show the same pattern of replication foci as cycling cells. We used this cellular system to analyze the interaction of cell cycle proteins with these foci in vivo. Cyclin A and cdk2, but not cyclin B1 and cdc2, were specifically localized at nuclear replication foci, just like the replication protein proliferating cell nuclear antigen. A potential target of cyclin A and cdk2 is the 34 kd subunit of replication protein A (RPA34). In contrast with the 70 kd subunit, which localizes to the foci, RPA34 was not detected at these replication sites, which may reflect a transient interaction. The specific localization of cyclin A and cdk2 at nuclear replication foci provides a direct link between cell cycle regulation and DNA replication.

Animals↗
PubMed1992

A targeting sequence directs DNA methyltransferase to sites of DNA replication in mammalian nuclei.

Tissue-specific patterns of methylated deoxycytidine residues in the mammalian genome are preserved by postreplicative methylation of newly synthesized DNA. DNA methyltransferase (MTase) is here shown to associate with replication foci during S phase but to display a diffuse nucleoplasmic distribution in non-S phase cells. Analysis of DNA MTase-beta-galactosidase fusion proteins has shown that association with replication foci is mediated by a novel targeting sequence located near the N-terminus of DNA MTase. This sequence has the properties expected of a targeting sequence in that it is not required for enzymatic activity, prevents proper targeting when deleted, and, when fused to beta-galactosidase, causes the fusion protein to associate with replication foci in a cell cycle-dependent manner.

3T3 Cells↗
PubMed1991

Expression in mammalian cells of a cloned gene encoding murine DNA methyltransferase.

Mammalian DNA cytosine-5-methyltransferase (MTase, EC 2.1.1.37) is an essential component for establishing and maintaining cell-type specific methylation patterns in the genome. The cDNA for the murine enzyme was previously cloned in segments. We have reconstructed the entire gene, encoding a protein of 1517 amino acids, from a set of overlapping cDNA clones. We report the assembly of two expression constructs in bacterial/mammalian shuttle vectors. Transcription in the first construct (pEMT) is driven by the cytomegalovirus enhancer/promoter and encodes a fusion protein with 15 additional aa at the N terminus, while the second construct (pJMT) is driven by the simian virus 40 early promoter/enhancer upstream from the natural ATG codon. Immunofluorescence microscopy and immunoblot analysis have shown that both constructs direct the synthesis of MTase in COS-1 cells. Enzyme activity in whole-cell lysates of transfected COS-1 cells transfected with pEMT and pJMT are on average tenfold and fivefold higher than in controls, respectively. The specific activities of the recombinant and endogenous mouse-cell enzyme are similar. These expression constructs will be of use in studies of DNA methylation in mammals.

Animals↗
PubMed1991

Parameters affecting plasmid stability in Bacillus subtilis.

For the analysis of parameters affecting plasmid stability in Bacillus subtilis, we used a pUB 110-derived shuttle plasmid containing direct and inverted nucleotide repeats (DRs and IRs). Deletions of up to 6 kb were found to occur between DRs of 7 to 16 bp. IRs as small as 43 or 58 bp were shown to stimulate the formation of these deletions in their neighbourhood. However, these structural features (DRs and IRs) per se were not responsible for plasmid instability. The unstable recombinant plasmids, but not their deletion-carrying (delta) derivatives, were found to impair the growth of the host and to accumulate high amounts of linear plasmid multimers [high mol. wt. (hmw) DNA]. We propose that the accumulation of hmw DNA may be the major reason for the selective pressure against recombinant plasmids, and the enrichment of delta-plasmids. Host mutations and other parameters increasing the stability of recombinant plasmids in B. subtilis are described.

Bacillus subtilis↗
PubMed1991

[Effect of meals on resorption of bismuth from an oral bismuth gallate/bismuth nitrate preparation].

In a cross-over study in ten healthy volunteers the effect of food on the absorption of bismuth following a single oral dose of 1200 mg (= 12 Bismofalk tablets) was evaluated by measuring its serum levels (0 to 24 h) and urinary excretion (for seven days). If this high dose was ingested one hour prior to the breakfast maximal serum concentrations (16.5 +/- 13.7 micrograms/l; mean +/- SD) were rapidly achieved (tmax = 0.7 +/- 0.5 h). These levels and the low urinary recovery of 0.32 +/- 0.25 mg (corresponding to 0.027% of the dose) indicated a minimal absorption. If the tablets were taken one hour after the breakfast absorption was slightly delayed (tmax = 1.9 +/- 2.4 h), however, the extent appeared to be unchanged. In spite of the high dosage as well as the large interindividual variability in the urinary recovery and the serum concentrations potential "toxic" serum levels of 100 micrograms/l were never reached. Thus, it can be concluded that the bismuth preparation used is suitable for topical action.

Administration, Oral↗
PubMed1991

Physical and biochemical characterization of recombination-dependent synthesis of linear plasmid multimers in Bacillus subtilis.

The synthesis and structure of linear multimeric plasmid molecules (hmw DNA) in Bacillus subtilis were investigated. The replication of covalently-closed-circular supercoiled (form I) DNA requires the rate-limiting plasmid-encoded replication initiation protein. Unlike form I, hmw DNA synthesis is partially resistant to inhibition of cellular transcription or translation and requires the host DnaB protein. In addition, hmw DNA synthesis involves host recombination and repair functions (RecE and Poll). Analysis of hmw DNA by electron microscopy displayed linear DNA molecules up to 100 kb in size, which were either single-stranded, double-stranded or double-stranded with single-stranded ends. Structural features of hmw DNA molecules were mapped by means of heteroduplex studies using defined strand-specific probes. The results suggest that a recombination intermediate, but not plasmid-encoded replication, is involved in the initiation of hmw DNA synthesis.

Bacillus subtilis↗
PubMed1990

Identification of a low-copy-number mutation within the pUB110 replicon and its effect on plasmid stability in Bacillus subtilis.

A mutation (cop1) within the minimal replicon of plasmid pUB110, reducing its copy number from 48 +/- 2 to 11 +/- 2 in a Bacillus subtilis host, was isolated. This mutation is a G----T transversion within the translation control region of the replication initiation gene (repU). The cop1 mutation, when present in the recombinant plasmid, increases both its structural and segregational stability.

Bacillus subtilis↗
PubMed1990

Feasibility and effectiveness of a defined-formula diet regimen in treating active Crohn's disease. European Cooperative Crohn's Disease Study III.

In a randomized multicenter trial the efficacy of treatment of active Crohn's disease by means of a liquid defined formula diet (DFD) was tested and compared with a combination of 6-methyl-prednisolone and sulfasalazine. A total of 95 patients participated in the study. By the end of 6 weeks, among 44 patients randomized to drug treatment, 32 showed improvement of the Crohn's disease activity index (CDAI) as compared with 21 of 51 patients receiving oral DFD (p less than 0.05). The proportion of withdrawals in the DFD group (29 of 51) was sevenfold higher than in the drug group (4 of 44). However, most patients (20 of 29) receiving DFD withdrew because of the unpalatability of the liquid diet. Analysis of patients in each group who finished the study showed equal effectiveness of DFD and the drug regimen. In these subsets of patients the CDAI decreased from 280.8 +/- 90.6 to 151.7 +/- 86.5 (DFD) and from 263.7 +/- 86.3 to 129.3 +/- 63.7 (drug), respectively. Improvement of inflammation factors was similar in both groups at the end of the study, although improvement was delayed in the DFD group. In conclusion, our data show a superiority of the drug combination over DFD in the treatment of Crohn's disease under the conditions of this trial. The results do suggest, however, that DFD offers a therapeutic alternative to prednisolone and sulfasalazine in a subgroup of patients, which has to be closer characterized in further studies.

Adult↗
PubMed1988

Functional analysis of the leading strand replication origin of plasmid pUB110 in Bacillus subtilis.

Supercoiled plasmid DNA is the substrate for initiation of pUB110 replication, and - by inference - for binding of its initiator protein (RepU) to the plasmid replication origin (oriU) in vivo. No hairpin structure is required for RepU-oriU recognition. RepH (the pC194 replication initiation protein) failed to initiate replication in trans at oriU. The nucleotides that determine the specificity of the replication initiation process are located within oriU but termination is unefficient. Therefore the segment that forms the full recognition signal for termination is probably located 3' of the oriU recognition sequence. Two overlapping domains, one for initiation and one required for termination, compose the leading strand replication origin of plasmid pUB110.

Bacillus subtilis↗