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1.
In an accompanying publication by Duckwitz-Peterlein, Eilenberger and Overath ((1977) Biochim. Biophys. Acta 469, 311–325) it is shown that the exchange of lipid molecules between negatively charged vesicles consisting of total phospholipid extracts from Escherichia coli occurs by the transfer of single lipid monomers or small micelles through the water. Here a kinetic interpretation is presented in terms of a rate constant, k?, for the escape of lipid molecules from the vesicle bilayer into the water. The evaluated rate constants are k?P = (0.86 ± 0.05) · 10?5s?1 and k?E = (1.09 ± 0.13) · 10?6s?1 for phospholipid molecules with trans-Δ9-hexadecenoate and trans-Δ9-octadecenoate, respectively, as the predominant acyl chain component. The rate constants are discussed in terms of the acyl chain and polar head group composition of the lipids.  相似文献   

2.
The immobilization of Rhodopseudomonas capsulata chromatophores by entrapment in an alginate gel is described. Alginate beads were prepared with Ba2+, Sr2+ and Ca2+ as gel-forming agents and compared for their mechanical strength, chemical resistance against disruption by phosphate-induced swelling, and yield of photophosphorylation activity. Barium alginate beads proved to have better physico-chemical properties than the more commonly used calcium alginate beads. After embedding in barium alginate gel, R. capsulata chromatophores retained a high yield (up to 70%) of their photophosphorylation capacity. Alginate entrapment did not cause a large increase in the Michaelis constant for ADP and phosphate, the substrates of adenosinetriphosphatase (ATPase). These constants were KADPm = 1.4 × 10?5m and KPim = 2.2 × 10?4m for free chromatophores and KADPm = 2.3 × 10?4m and KPim = 5.6 × 10?4m for chromatophores entrapped in barium alginate gel. However, embedding gave no additional protection against rapid inactivation of chromatophores upon storage at 3°C. Preliminary results with a batch reactor for continuous ATP regeneration are presented. The barium alginate method has two features which are not generally encountered at the same time, extremely mild conditions for entrapment and excellent physical properties of the gels beads, which make this method a suitable tool for the construction of bioreactors with immobilized cells or organelles.  相似文献   

3.
The apparent Arrhenius energy of activation (Ea) of the water osmotic permeability (Posc) of the basolateral plasma cell membrane of isolated rabbit proximal straight tubules has been measured under control conditions and after addition of 2.5 mM of the sulfhydryl reagent, para-chloromercuribenzenesulfonic acid (pCMBS), of mersalyl and of dithiothreitol. Ea (kcal/mol) was 3.2 ± 1.4 (controls) and 9.2 ± 2.2 (pCMBS), while Posc decreased with pCMBS to 0.26 ± 0.17 of its control value. Mersalyl also decreased Posc both in vitro and in vivo (using therapeutical doses). These actions of pCMBS and mersalyl were quickly reverted with 5 mM dithiothreitol and prevented by 0.1 M thiourea. Ea for free viscous flow is 4.2 and greater than 10 for non-pore-containing lipid membranes. By analogy with these membranes and with red blood cells, where similar effects of pCMBS on Pos are observed, it is concluded that cell membranes of the proximal tubule are pierced by aqueous pores which are reversibly shut by pCMBS. Part of the action of mercurial diuretics can be explained by their action on Posc.  相似文献   

4.
Premeiotic DNA synthesis in fission yeast   总被引:57,自引:0,他引:57  
Sporulating and various non-sporulating strains of S. pombe, especially several mutants deficient in conjugation or meiosis, were compared with respect to DNA synthesis under sporulation conditions. Meiosis and sporulation were induced by a transfer to nitrogen-free medium. As synchronized mitotic division was observed in all the strains as a first response to the shift, reducing the DNA amount per cell from the replicated state in G2 to the unreplicated state in the G1 phase of the cell cycle. Cells of the heterothallic wild-type strains (h+h+ or h?h?) accumulated in G1 with respect to DNA synthesis when they were incubated separately. In a mixed culture of these strains a period of enhanced DNA synthesis was observed after the start of zygote formation. This period of synthesis was absent in mutant fus1, where only prezygotes accumulated. Hence we conclude that in zygotic meiosis the premeiotic DNA synthesis is confined to zygotes after conjugation has been completed. In the diploid sporulating wild-type strain (h+h?), capable of azygotic meiosis without prior conjugation, premeiotic DNA synthesis occurred between 212 and 5 h after the shift to the sporulation medium. There was no significant premeiotic DNA synthesis observed in diploid cells of the meiosis-deficient mutants mei1 or mei3, whereas premeiotic DNA synthesis proceeded normally in mutant mei4, which is blocked at a stage after commitment to meiosis in opposition to both the other mutants.  相似文献   

5.
A large proportion of the heterotrophic bacteria isolated from sea water sampled from the Pacific Ocean and the neritic Sea of Japan were able to hydrolyze deoxyribonucleic acid (DNA). Their numbers ranged from 102 to 104100 ml in the upper layers of the water profile and were less than 102100 ml in deeper water in the open sea. In neritic regions, such as Aburatsubo Inlet, their numbers were higher, ranging between 105 and 107100 ml. These organisms formed a constant proportion of the total heterotrophic bacterial populations and showed few seasonal fluctuations.In incubation experiments it was shown that the DNA occurring in sea water was extensively degraded in situ by the indigenous bacterial flora. More detailed examination of the degradation, using an isolated marine Vibrio sp., has demonstrated that purine and pyrimidine bases and inorganic orthophosphate are released into the medium. The bacterium totally assimilated the cytosine released and appeared to convert adenine to hypoxanthine. In contrast guanine and thymine attained constant concentrations in the medium.  相似文献   

6.
Showdomycin inhibited pig brain (Na+ + K+)-ATPase with pseudo first-order kinetics. The rate of inhibition by showdomycin was examined in the presence of 16 combinations of four ligands, i.e., Na+, K+, Mg2+ and ATP, and was found to depend on the ligands added. Combinations of ligands were divided into five groups in terms of the magnitude of the rate constant; in the order of decreasing rate constants these were: (1)Na+ + Mg2+ + ATP, (2) Mg2+, Mg2+ + K+, K+ and none, (3) Na+ + Mg2+, Na+, K+ + Na+ and Na+ + K+ + Mg2+, (4) Mg2+ + K+ + ATP, K+ + ATP and Mg2+ + ATP, (5)K+ + Na+ + ATP, Na+ + ATP, Na+ + ATP, Na+ + K+ + Mg2+ + ATP and ATP. The highest rate was obtained in the presence of Na+, Mg2+ and ATP. The apparent concentrations of Na+, Mg2+ and ATP for half-maximum stimulation of inhibition (K0.5s) were 3 mM, 0.13 mM and 4μM, respectively. The rate was unchanged upon further increase in Na+ concentration from 140 to 1000 mM. The rates of inhibition could be explained on the basis of the enzyme forms present, including E1, E2, ES, E1-P and E2-P, i.e., E2 has higher reactivity with showdomycin than E1, while E2-P has almost the same reactivity as E1-P. We conclude that the reaction of (Na+ + K+)-ATPase proceeds via at least four kinds of enzyme form (E1, E2, E1 · nucleotide and EP), which all have different conformations.  相似文献   

7.
Human embryonic, fetal, and adult globin chains (ζ, ε, Aγ, Gγ, β, α) can be separated by electrophoresis on Triton Acid urea gels. K562, a human leukemia cell line, was induced with hemin, labelled with [3H]-leucine, and globin synthesis analyzed. All globins except β were produced. ε > ζ; Gγ:Aγ=70:30; non-α:α=>2:1. Thus, hemin-induced K562 synthesized embryonic and fetal globin chains, and had globin synthetic imbalance, with “α-thalassemia.”  相似文献   

8.
Soluble (Na++K+)-ATPase consisting predominantly of αβ-units with Mr below 170 000 was prepared by incubating pure membrane-bound (Na++K+)-ATPase (35–48 μmol Pi/min per mg protein) from the outer renal medulla with the non-ionic detergent dodecyloctaethyleneglycol monoether (C12E8). (Na++K+)-ATPase and potassium phosphatase remained fully active in the detergent solution at C12E8/protein ratios of 2.5–3, at which 50–70% of the membrane protein was solubilized. The soluble protomeric (Na++K+)-ATPase was reconstituted to Na+, K+ pumps in phospholipid vesicles by the freeze-thaw sonication procedure. Protein solubilization was complete at C12E8/protein ratios of 5–6, at the expense of partial inactivation, but (Na++K+)-ATPase and potassium phosphatase could be reactivated after binding of C12E8 to Bio-Beads SM2. At C12E8/protein ratios higher than 6 the activities were irreversibly lost. Inactivation could be explained by delipidation. It was not due to subunit dissociation since only small changes in sedimentation velocities were seen when the C12E8/protein ratio was increased from 2.9 to 46. As determined immediately after solubilization, S20,w was 7.4 S for the fully active (Na++K+)-ATPase, 7.3 S for the partially active particle, and 6.5 S for the inactive particle at high C12E8/protein ratios. The maximum molecular masses determined by analytical ultracentrifugation were 141 000–170 000 dalton for these protein particles. Secondary aggregation occurred during column chromatography, with formation of enzymatically active (αβ)2-dimers or (αβ)3-trimers with S20,w=10–12 S and apparent molecular masses in the range 273 000–386 000 daltons. This may reflect non-specific time-dependent aggregation of the detergent micelles.  相似文献   

9.
The molecular weight of Na- and K-hyaluronate has been determined by low angle laser light scattering (LALLS) technique. Two preparations of hyaluronate from rooster comb (Mw= 0.9 × 106 and 4 × 106) were investigated. The LALLS was carried out both in a static mode and on the effluent from a column filled with porous gel. In contrast to Sheehan et al.1, no significant difference was found in the molecular weight of viscosity of Na- and K-hyaluronate in 2.0 M salt solutions  相似文献   

10.
Dispersed acini from dog pancreas were used to examine the ability of dopamine to increase cyclic AMP cellular content and the binding of [3H]dopamine. Cyclic AMP accumulation caused by dopamine was detected at 1·10?8 M and was half-maximal at 7.9±3.4·10?7M. The increase at 1·10?5 M, (7.5-fold) was equal to the half-maximal increase caused by secretin at 1·10?9 M. Haloperidol, a dopaminergic receptor antagonist inhibited cyclic AMP accumulation caused by dopamine. The IC50 value for haloperidol, calculated from the inhibition of cyclic AMP increase caused by 1·10?5 M dopamine was 2.3±0.9·10?6M. Haloperidol did not alter basal or secretin-stimulated cyclic AMP content. [3H]Dopamine binding was studied on the same batch of cells as cyclic AMP accumulation. At 37°C, it was rapid, reversible, saturable and stereospecific. The Kd value for high affinity binding sites was 0.43±0.1·10?7M and 4.7±1.6·10?7M for low affinity binding sites. The concentration of drugs necessary to inhibit specific binding of dopamine by 50% was 1.2±0.4·10/t-7M noradrenaline, 2·10/t-7 M epinine, 4.1±1.8·10/t-6M fluphenazine, 8.0±1.6·10/t-6M haloperidol, 4.2±1.2·10?6Mcis-flupenthixol, 2.7±0.4·10?5Mtrans-flupenthixol, >1·10?5M apomorphine, sulpiride, naloxone and isoproterenol.  相似文献   

11.
Seedlings carrying mutations in regulatory genes for protochlorophyll(ide) synthesis accumulate protochlorophyll(ide) in darkness in amounts exceeding the wildtype level. Thus, +/tig-d12 and tig-b24tig-b24accumulate 2-fold, tig-o34tig-o34 5- to 6-fold, and tig-d12tig-d12 15-fold more protochlorophyll(ide) than the wild type.The amount of photoconvertible protochlorophyll(ide) accumulated in darkness is the same in all genotypes, despite the large differences in total protochlorophyll(ide) content, indicating a constant number of photoconversion sites.When briefly illuminated leaves are returned to darkness, regeneration of active protochlorophyll(ide) from the pool of inactive protochlorophyll(ide) takes place in wild-type and mutant leaves. Compared to the wild type, the rate of protochlorophyll(ide) activation during 4- and 10-min dark periods is higher in +/tig-d12, tig-b24tig-b24, and tig-o34tig-o34, but lower in tig-d12tig-d12.There was no indication that the accumulation of protochlorophyll(ide) influences the conversion sites of the protochlorophyll(ide) holochrome, as the kinetics of photoconversion of initially active protochlorophyll(ide) in leaves with the genotypes +/+, +/tig-o34, and tig-o34tig-o34 are similar or identical.  相似文献   

12.
(1) Treatment of (Na+ + K+)-ATPase from rabbit kidney outer medulla with the γ-35S labeled thio-analogue of ATP in the presence of Na+ + Mg2+ and the absence of K+ leads to thiophosphorylation of the enzyme. The Km value for [γ-S]ATP is 2.2 μM and for Na+ 4.2 mM at 22°C. Thiophosphorylation is a sigmoidal function of the Na+ concentration, yielding a Hill coefficient nH = 2.6. (2) The thio-analogue (Km = 35 μM) can also support overall (Na+ + K+)-ATPase activity, but Vmax at 37°C is only 1.3 γmol · (mg protein)? · h?1 or 0.09% of the specific activity for ATP (Km = 0.43 mM). (3) The thiophosphoenzyme intermediate, like the natural phosphoenzyme, is sensitive to hydroxylamine, indicating that it also is an acylphosphate. However, the thiophosphoenzyme, unlike the phosphoenzyme, is acid labile at temperatures as low as 0°C. The acid-denatured thiophosphoenzyme has optimal stability at pH 5–6. (4) The thiophosphorylation capacity of the enzyme is equal to its phosphorylation capacity, indicating the same number of sites. Phosphorylation by ATP excludes thiophosphorylation, suggesting that the two substrates compete for the same phosphorylation site. (5) The (apparent) rate constants of thiophosphorylation (0.4 s?1 vs. 180 s?1), spontaneous dethiophosphorylation (0.04 s?1 vs. 0.5 s?1) and K+-stimulated dethiophosphorylation (0.54 s?1 vs. 230 s?1) are much lower than those for the corresponding reactions based on ATP. (6) In contrast to the phosphoenzyme, the thiophosphoenzyme is ADP-sensitive (with an apparent rate constant in ADP-induced dethiophosphorylation of 0.35 s?1, KmADP = 48 μM at 0.1 mM ATP) and is relatively K+-insensitve. The Km for K+ in dethiophosphorylation is 0.9 mM and in dephosphorylation 0.09 mM. The thiophosphoenzyme appears to be for 75–90% in the ADP-sensitive E1-conformation.  相似文献   

13.
The changes in polymer-solvent interactions that occur when native calf thymus DNA is dialyzed against Na2SO4 solutions of a given ionic strength and buffer concentration but of varying concentrations in methylmercuric hydroxide have been investigated with the help of solution density measurements at 25 °C and pH 6.8–7.0. From measurements executed under equilibrium dialysis conditions at the three salt levels 5 mm, 0.05 m, and 0.5 m Na2SO4 (m refers to molality) and in the presence of 5 mm cacodylic acid buffer, the density increments (???c2)μ0 for native calf thymus DNA were determined as a function of CH3HgOH concentration. (???c2)μ0 was found not to vary with organomercurial concentration, irrespective of the concentration of supporting electrolyte, until a certain CH3HgOH concentration level has been reached, viz., pM1 ? 3.5 (pM1 = ?log mCH3HgOH), beyond which (???c2)μ0 increases strongly with increasing concentration of CH3HgOH. As is shown by optical melting, (???c2)μ0 becomes a function of organomercurial concentration the moment DNA undergoes denaturation brought about by the complexing of CH3HgOH with the various N-binding sites of the base residues in the DNA double helix.Polymer-solvent interactions, expressed in terms of preferential water interactions (“net hydration”) and preferential salt interactions (“salt solvation”), were derived from the (???c2)μ0 data in combination with data obtained on the preferential interaction of CH3HgOH with denatured DNA and data on the partial specific volumes of all major solution components, gathered from density measurements on solutions with fixed concentrations of diffusible components. Evidence is presented which shows that denaturation in general decreases the net hydration while salt becomes preferentially associated with the polyelectrolyte. This process is further amplified by the interaction of CH3HgOH with denatured DNA: Methylmercurated DNA alters the redistribution of diffusible components at dialysis equilibrium to such an extent that in a formal sense large amounts of water are rejected from the immediate vicinity of the polymer. The molecular implications of these findings are explored. The results are further discussed in the light of previous findings where the methylmercury-induced denaturation of DNA had been studied with the help of buoyant density measurements in a Cs2SO4 density gradient and by velocity-sedimentation in a variety of sulfate media.  相似文献   

14.
Mutations at the bithoraxoid (bxd) and postbithorax (pbx) loci cause a transformation of posterior haltere to posterior wing. It has previously been shown that pbx and pbxUbx101 cause this transformation by affecting the maintenance (or cell heredity function) of determination so that the transformed cells are indistinguishable from normal wing cells, and have no “memory” of having been part of a haltere disk (Adler, 1978a). I report here that Tp(3) bxd100Ubx101 and bxd1, pbx, ew both cause the transformation of posterior haltere to posterior wing in the same way as pbx. On the other hand, bxd1, bxd1Ubx101, bxd51j, bxd51jUbx101, and bxd51jred pbx cause this same transformation of posterior haltere to posterior wing by interfering with the expression of the determined state so that the developmental information of posterior haltere is “misread” as posterior wing. The transformed cells in these disks retain the memory of having been part of a haltere disk; that is, these posterior cells that would secrete wing cuticle during metamorphosis regenerate anterior haltere structures. Thus it appears clear that it is possible to uncouple the expression and cell heredity functions of determination in the haltere disk of Drosophila.  相似文献   

15.
The catalytic component of adenylate cyclase and [3H]dopamine binding protein were solubilized with 2% Lubrol PX in the presence of NaF from the synaptic membranes of canine caudate nucleus and were separated into distinct fractions by gel exclusion chromatography on a Sephadex G-200 column. The dissociated adenylate cyclase was no longer responsive to dopamine but was considerably stimulated by 10 mm NaF. Dissociated [3H]-dopamine binding protein possessed the apparent dissociation constant of 3.2 μm for dopamine, almost identical to that of the particulate preparations. The affinities of [3H]-dopamine binding protein to catecholamines and neuroleptics were also very similar to those of particulate preparations. After the adenylate cyclase and [3H]dopamine binding protein were preincubated together at 4 °C for 30 min, the cyclase activity displayed a dose-dependent increase by dopamine with the Ka of 1.6 μm, the concentration of dopamine to stimulate half-maximally. Stimulation of the reconstituted adenylate cyclase by dopamine was maximally 2.7-fold and was strongly inhibited by neuroleptics such as chlorpromazine and haloperidol. These results suggest that [3H]dopamine binding protein is identical to the regulatory subunit of dopamine-sensitive adenylate cyclase in the synaptic membranes of canine caudate nucleus.  相似文献   

16.
Mature human embryonic erythrocytes (hemoglobin is ≥ 90% of the cellular protein) contained at least 20 times as much ferritin as human adult erythrocytes, suggesting the possibility that the embryonic red cells participate in iron storage as they do in other embryonic or larval vertebrates. The ferritin content of mature red cells in the circulation declined when fetal red cells replaced embryonic red cells; the cell replacement was monitored by the disappearance of embryonic ε-chains and the appearance of the fetal globin chains, γA and γG. A constant ratio of 0.67 was obtained for γGγA + γG from the first detectable appearance (4 weeks after conception) until 13 weeks, a value which is similar to the value previously obtained at 20 weeks gestation and birth but higher than that observable in adults; thus, both γG and γA chains are produced in similar amounts throughout gestation. The high levels of ferritin in normal human embryonic erythrocytes emphasize the similarity of erythropoiesis in human embryos and other vertebrates. In addition, the results show that red cell ferritin can be used as a marker for studying the mechanism of induction of embryonic erythropoiesis in cultured cell lines, such as K562 from human chronic myelocytic leukemia, and that ferritin content may also serve as a marker for cellular transformations involving reversions to embryonic erythropoiesis.  相似文献   

17.
Gray matter and white matter membranes catalyze the transfer of label from UDP-N-acetyl-[14C] glucosamine into N-acetyl[14C]glucosaminyl-pyrophosphoryl-dolichol, N,N′-diacetyl [14C]chitobiosyl-pyrophosphoryl-dolichol, and N-acetyl[14C]glucosamine-labeled glycoprotein. Gel filtration of the Pronase digests of gray matter N-acetyl[14C]glucosamine-labeled glycoprotein reveals two N-acetyl[14C]glucosamine-labeled glycopeptide fractions. One fraction (A) contains approximately eight glycose units. All of the radioactivity is at nonreducing termini and can be released by treatment with an exo-β-N-acetylglucosaminidase. A smaller N-acetyl[14C]glucosamine-labeled glycopeptide (B) is recovered in the elution volume expected for an asparaginyl disaccharide. Structural studies show that the labeled saccharide unit in glycopeptide B is N,N′-diacetyl[14C]chitobiose. The linkage between the 14C-labeled disaccharide and the polypeptide has the properties of an N-glycosidic attachment to asparagine. Only the larger N-acetyl[14C]glucosamine-labeled glycopeptide (A) is found in Pronase digests of white matter membrane N-acetyl[14C]glucosamine-labeled glycoprotein after incubation with UDP-N-acetyl[14C]glucosamine. When gray matter membranes are incubated with UDP-N-acetyl[14C]glucosamine in the presence of tunicamycin or UMP, the labeling of glycolipid and the asparaginyl disaccharide is inhibited. UMP and tunicamycin have no effect on the transfer of N-acetyl[14C]glucosamine to external acceptor sites of the larger glycopeptide (A). The transfer of N,N′-diacetyl[14C]-chitobiose from carrier lipid to protein is observed when extensively washed membranes containing endogenous, prelabeled 14C-labeled glycolipids are incubated in the presence or absence of unlabeled GDP-mannose. UMP treatment of the prelabeled membranes selectively discharged over 80% of the label from N-acetyl[14C]glucosaminyl-pyrophosphoryl-dolichol, but had no effect on the transfer of the 14C-labeled disaccharide to protein. All of these results are concordant with transfer of N,N′-diacetylchitobiose from dolichyl diphosphate to gray matter glycoprotein. The major membrane glycoprotein labeled by the lipid-mediated [14C]disaccharide transfer reaction has an apparent molecular weight of 24,000. Tunicamycin prevents the enzymatic labeling of the gray matter glycoprotein having an apparent molecular weight of 24,000.  相似文献   

18.
The stability of deoxycytidine photohydrates was determined for deoxycytidylic acid and deoxycytidine residues in oligodeoxynucleotides by optical measurements and in native and denatured DNA by a chemical assay. The half lives at 20° in 10?2M tris buffer, pH 7.7, were 102 min. for the mononucleotide, 128 min. for dpApGpG, 152 min. for MeOdpTpCpA, 51 min. for denatured E. coli DNA and 58 min. for native E. coli DNA (at pH 8.1). It is concluded that the stability of deoxycytidine photohydrates is sufficient that they cannot at present be dismissed as lesions of possible biological importance in ultraviolet irradiated cells.  相似文献   

19.
(1) Analysis of the data from steady-state kinetic studies shows that two reactions between cytochrome c and cytochrome c oxidase sufficed to describe the concave Eadie-Hofstee plots (Km ? 1 · 10?8M and Km ? 2 · 10?5M). It is not necessary to postulate a third reaction of Km ? 10?6M. (2) Change of temperature, type of detergent and type of cytochrome c affected both reactions to the same extent. The presence of only a single catalytic cytochrome c interaction site on the oxidase could explain the kinetic data. (3) Our experiments support the notion that, at least under our conditions (pH 7.8, low-ionic strength), the dissociation of ferricytochrome c from cytochrome c oxidase is the rate-limiting step in the steady-state kinetics. (4) A series of models, proposed to describe the observed steady-state kinetics, is discussed.  相似文献   

20.
An analysis of the repeat structure of the highly repetitive sequence, component α DNA of the African green monkey, shows that the DNA contains restriction sites for EcoRI, EcoRI1, HindIII and HaeIII. All four restriction enzyme activities indicate a basic repeat length of 176 ± 4 base-pairs. In addition to primary EcoRI1 and HindIII sites, about 59% of the repeat sequences contain secondary EcoRI1 sites and about 36% of the repeat sequences contain secondary HindIII sites. The secondary sites are located less than 176 base-pairs from the primary sites and their cleavage yields several complex series of minor, intermediate segments in gels of the partial EcoRI1 or HindIII digests. Cleavage at the secondary sites yields segments shorter than the unit monomer in the limit digests. The sites for EcoRI, EcoRI1, HindIII and HaeIII have been mapped within the repeat unit.Treatment of the monkey nuclei with micrococcal nuclease at 2 °C and in the presence of 80 mm-NaCl reveals two distinct populations of nucleosomes. One population contains bulk DNA sequences, and after cleavage with micrococcal nuclease this population yields heterogeneous segments of DNA spanning 180 to 200 base-pairs in length. The other population contains component α sequences and after cleavage with micrococcal nuclease yields homogeneous segments of component α DNA that are exact multiples of the basic sequence repeat unit of 176 base-pairs. Thus, the cleavage by micrococcal nuclease of nucleosomal arrays containing component α sequences is as regular and precise as the cleavage of the purified DNA by the restriction enzymes. The resolution of the two distinct subsets of nucleosomes in the monkey nuclei is dependent upon the conditions of ionic strength and temperature employed during the nuclear isolation and the micrococcal nuclease digestion.These observations are consistent with a phase relation between the component α repeat sequences and the associated nucleosomal proteins (Musich et al., 1977b). They are also in accord with the hypothesis that the subunit structure of constitutive heterochromatin modulates or determines the repeat sequence structure and hence, the evolution of many highly repetitive mammalian DNAs (Maio et al., 1977).  相似文献   

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