Biomedical subjects

W O Weischet

Publications and source records attributed to W O Weischet.

12 recordsLinked to original sources

PubMed1983

Protection of expressed immunoglobulin genes against nuclease cleavage.

Fragmentation of the actively transcribed kappa immunoglobulin gene in mouse myeloma nuclei with micrococcal nuclease and the restriction nuclease Bsp RI reveals a chromatin structure without the regularity of repeating nucleosomes found in bulk chromatin. Such regularity is restored about 2.2 kb 3' of the coding region. An only moderately increased micrococcal nuclease sensitivity and a 65% average protection of the Bsp RI sites indicates a DNA-protein interaction in the transcribed region which is not very different from that of an inactive gene. As determined by indirect endlabeling the frequency of Bsp RI cleavage both, after very mild and exhaustive digestion, varied moderately from site to site along the gene. In addition, it was not in each case the same at analogous sites on both alleles which are both transcribed. Thus, the experiments demonstrate differences between the chromatin structures of the genes which may be related to regulatory phenomena and thereby corroborate earlier findings made with DNAase I.

Animals↗

PubMed1983

Nucleosome distribution on the Jk and Ck immunoglobulin gene segments of mouse liver chromatin.

The chromatin structure of the transcriptionally inactive kappa immunoglobulin gene in mouse liver was investigated by mainly employing indirect endlabeling on Bsp RI restriction nuclease digestions of intact nuclei. The disclosed strong (about 85%) but not uniform protection of the Bsp RI sites by nucleosomes is inconsistent with both a uniquely sequence-oriented localization and a completely random distribution of nucleosomes in this region of the genome. Several possibly applicable models are discussed. A model with multiple phases and non-uniform linker lengths cannot be excluded; however a largely random localization with a weak superimposed organization in two confined areas was tested and found sufficient for explaining the data.

Animals↗

PubMed1982

Differences in the nuclease sensitivity between the two alleles of the immunoglobulin kappa light chain genes in mouse liver and myeloma nuclei.

In mouse myeloma T the productive kappa light chain gene differs from its aberrantly rearranged allele in the patterns of DNAase I hypersensitive sites. In the region of the alleles where they are identical in sequence they have one site in common which lies 0.8 kb downstream of the coding region; but two sites upstream of and within the C gene segment (2) are found only on the non-productive allele. Within the region of different sequences both alleles have analogously located DNAase I hypersensitive sites; they lie 0.15 kb upstream of the respective leader segments and cover putative promoter sequences. Only one of the six DNAase I hypersensitive sites is also very sensitive towards micrococcal nuclease due to its particular DNA sequence. The non-rearranged gene studied in liver nuclei has no DNAase I hypersensitive sites but is preferentially cleaved in A/T rich regions.

Alleles↗

PubMed1980

Nuclease digestion promotes structural rearrangements in H1-depleted chromatin.

Digestion of H1-depleted chromatin with micrococcal nuclease at an ionic strength of 0.35M gives rise to structural rearrangements indicating nucleosomal sliding. The ionic strength necessary to reveal this effect is significantly lower than that required in the absence of an accompanying digestion. As an explanation, a model is presented in which the progressing terminal degradation of oligomeric nucleosomes is made responsible for promoting structural rearrangements.

Animals↗

PubMed1980

DNA-histone interactions in nucleosomes.

We have utilized micrococcal nuclease digestion and thermal denaturation studies to investigate the binding of DNA to the histone core of the nucleosome. We conclude that a total of approximately 168 base pairs (bp) of DNA can interact with the histone core under appropriate solution conditions, even in the absence of lysine-rich histones. The interactions in this total length of DNA can be divided into three classes: (a) approximately 22 bp at the ends is bound only at moderate ionic strength. It is easily displaced, and its removal yields the 146 bp core particle. (b) approximately 46 bp near the ends of the core DNA are quite weakly bound to the core, and are displaced at quite moderate temperatures. (c) The remaining central 100 bp are strongly bound, and interact with all of the sites on the histones which strongly protect DNA against DNAse I digestion. A theoretical analysis of the cleavage of nucleosomal DNA by DNAse I has been used to develop evidence that the pattern of protection offered by the histone core is very similar in nuclei to that in isolated core particles.

Animals↗

PubMed1979

On the de novo formation of compact oligonucleosomes at high ionic strength. Evidence for nucleosomal sliding in high salt.

Histone H 1-depleted chromatin made from acid extracted, intact nuclei was exposed to various ionic strengths. NaCl concentrations above 0.3 M sufficed to generate novel oligonucleosomes formerly characterized as "compact oligomers" and "spacerless dinucleosomes". Such particles could not be identified within H 1-depleted nuclei or chromatin at low ionic strengths. Their formation, proceeding within days at 0 degrees C, was accelerated by increasing ionic strengths. The data was discussed in terms of a salt-induced motion of nucleosomal core particles along the DNA to form compact oligomers.

Animals↗

PubMed1979

The effects of salt concentration and H-1 depletion on the digestion of calf thymus chromatin by micrococcal nuclease.

We have removed histone H1 specifically from calf thymus nuclei by low pH treatment, and studied the digestion of such nuclei in comparison with undepleted nuclei. By a number of criteria the nuclei do not appear damaged. The DNA repeat-length in nuclear chromatin is found to be the same (192 +/- 4 bp) in the presence or absence of H1. These experiments demonstrate that the core histone complex of H2A, H2B, H3, and H4 can itself protect DNA sequences as long as 168 bp from nuclease. Our interpretation is that this represents an important structural element in chromatin, carrying two full turns of superhelical DNA. Depending on conditions of digestion this 168 bp fragment may be metastable and is normally rapidly converted by exonucleolytic trimming to the well-known "core-particle" containing 145 bp. Larger stable DNA fragments observed indigestion of H-1 depleted nuclei appear to arise from oligomers assembled from 168 bp cores in close contact exhibiting trimming of 0-20 bp at the ends. Electrophorograms of undepleted nuclear digests reveal oligomer bands in several size classes, each corresponding to one or more combinations of 168 bp particles, H1-protected spacers of about 20 bp length, and particles with ends trimmed to varying degrees.

Animals↗

PubMed1978

Thermal denaturation of nucleosomal core particles.

Thermal denaturation of very homogeneous preparations of core particles from chicken erythrocyte chromatin is studied by several techniques. The change in absorbance, which is very closely paralleled by changes in heat capacity, which is very closely paralleled by changes in heat capacity, is a biphasic process with inflexions at 60 degrees C and 74 degrees C. In contrast, isolated DNA of the same length denatures in a single transition around 44 degrees C. Monitoring the circular dichroism of the cores during thermal denaturation reveals biphasic changes in the secondary structure of the DNA, preceding the base unstacking by 10 degrees C in the first and 3 degrees C in the second phase. However, measurable alterations in the secondary structure of the histones are confined to the second phase with a melting temperature at 71 degrees C. Increase in the ionic strength of the buffer from 1 mM to 10 mM leads to almost monophasic melting curves as measured by absorbance and CD, while not causing any measurable conformational changes at room temperature. The melting of core particles is interpreted as a denaturation of about 40 base pairs in the first phase, followed by a massive breakdown of the native structure of a tight histone-DNA complex, which frees the remaining 100 base pairs for unstacking.

Animals↗

PubMed1976

The mechanism of the synthesis of indoleglycerol phosphate catalyzed by tryptophan synthase from Escherichia coli. Steady-state kinetic studies.

The mechanism of indoleglycerol phosphate synthesis from indole and D-glyceraldehyde 3-phosphate catalyzed by tryptophan synthase has been investigated by steady-state kinetic techniques. The equilibrium constant and the progress curves were measured by use of the difference in absorbance between indole and indoleglycerol phosphate. Stopped-flow measurements show that only the non-hydrated form of D-glyceraldehyde 3-phosphate serves as substrate. The product analogue indolepropanol phosphate was used as an inhibitor to discriminate between possible mechanisms. The data agree well with an ordered addition mechanism with D-glyceraldehyde 3-phosphate adding first. Mechanisms involving random addition of substrates or ordered addition with indole adding first can be excluded because indolepropanol phosphate is a competitive inhibitor only towards glyceraldehyde 3-phosphate. The high affinity of tryptophan synthase for indoleglycerol phosphate leads to product inhibition even at small extents of reaction. Glyceraldehyde 3-phosphate combines with the enzyme with an apparent second-order rate constant, which is not diffusion controlled and generates a site with high affinity for indole.

Chemical Phenomena↗

PubMed1976

Steady-state kinetic studies of the synthesis of indoleglycerol phosphate catalyzed by the alpha subunit of tryptophan synthase from Escherichia coli. Comparison with the alpha2 beta2-complex.

For the alpha subunit of tryptophan synthase and at constant concentration of D-glyceraldehyde 3-phosphate the saturation curves with respect to indole concentration are weakly sigmoidal. This phenomenon can be explained by interaction between indole bound to the effector site established previously and the active center of the monomeric alpha subunit. Kinetic studies of the inhibition of indoleglycerol phosphate synthesis by the analogue indolepropanol phosphate show that the inhibition is competitive with respect to D-glyceraldehyde 3-phosphate and non-competitive with respect to indole. Mechanisms with random addition of substrates or ordered addition with indole binding first can therefore be excluded. A quantitative fit of the data has been obtained to an ordered addition mechanism with D-glyceraldehyde 3-phosphate binding first and with a distribution of the enzyme between two states differing in V, governed by the binding of indole to the effector site. The kinetic constants obtained for the alpha subunit have been compared with those of the alpha 2 beta 2 complex of tryptophan synthase. Protein-protein interaction of the alpha subunit with the beta 2 subunit (a) does not alter the catalytic of the indoleglycerol phosphate synthesis, (b) suppresses the substrate activation by indole, and (c) changes the various equilibrium, rate and steady-state constants in the sense of conveying higher substrate specificity and catalytic efficiency to the alpha-subunit. The occurrence of local and gross conformational changes in the tryptophan synthase system is discussed.

Binding Sites↗

PubMed1976

The binding of indole to the alpha-subunit and beta2-subunit and to the alpha2beta2-complex of tryptophan synthase from Escherichia coli. Identification of a second indole-binding site on the alpha-subunit.

The binding of indole and indolepropanol phosphate, an analogue of the substrate indoleglycerol phosphate, to the individual alpha and beta2-subunits and to the alpha2beta2-complex of tryptophan synthase was studied by equilibrium dialysis. The use of [14C]indole and indolepropanol [32P]phosphate permitted simultaneous binding studies to be carried out. Competition between indole and indolepropanol phosphate in binding to a particular site was taken as evidence for that site being part of the active site of the alpha-subunit. The binding of indole to the active site of the alpha-subunit is weak (Kd = 18mM). A second distinct site binds indole more strongly (Kd = 1.5 mM) and interacts with the active site indirectly. It is therefore designated an effector site. Furthermore, the binding of indole and/or indolepropanol phosphate appears to stabilize different conformations of the alpha-subunit. The beta2-subunit binds indole only weakly (Kd = 12 mM) to many (n = 10) sites per polypeptide chain. The alpha2beta2-complex retains one or two sites per alphabeta-equivalent of relatively high affinity (Kd = 1.2 mM). The active sites of the component alpha and beta-subunits probably belong to the second class of many (n = 40) sites of low (Kd = 30 mM) affinity for indole. These findings support conclusions from the literature that both bi-substrate reactions involving indole catalyzed by tryptophan synthase and its subunits must follow strictly ordered addition mechanisms with the respective other substrate adding first.

Binding Sites↗

PubMed1975

Circular dichroism and fluorescence studies on the binding of ligands to the alpha subunit of tryptophan synthase.

Binding to the alpha subunit of tryptophan synthase induces extrinsic Cotton effects in the substrates indole (IND), indoleglycerol phosphate (IGP), and D-glyceraldehyde-3-P (D-GAP) and in the inhibitor indolepropanol phosphate (IPP). These effects disappear when the enzyme is denatured in guanidinium chloride. The induced circular dichroism (CD) was used to determine the dissociation constant and the number of binding sites for IPP. The dissociation constant so determined is equal to 48 muM and is in good agreement with the value of 48 muM obtained by equilibrium dialysis. From the temperature dependence of the dissociation constant, a value of -2.8 kcal/mol for the binding enthalpy was obtained. The determination of dissociation constants by means of extrinsic Cotton effects is shown to be quite feasible. CD competition experiments with glycerol phosphate (GP) suggest that IPP binds bifunctionally to the enzyme: via its indole part and its phosphate group. Indolepropanol, which lacks the phosphate group, does not show an extrinsic Cotton effect. Since the induced CD is strongly dependent on the binding geometry, the close similarity between the induced spectra in IPP and IGP is additional evidence that IPP is a good substrate analog. Binding to the enzyme results in a blue shift of the IPP fluorescence emission maximum. The dissociation constant determined by fluorescence titration equals 46 muM and agrees well with the values determined by the other two methods. Previous biochemical and fast kinetic studies suggested the existence of multiple conformational states for the enzyme and of ligand-induced conformational changes. No evidence was found in the far-uv CD spectra for conformational changes upon binding of IND and D-GAP. For IPP a very small effect was observed.

Binding, Competitive↗