[Structural arrangement of chromatin (author's transl)]. / Struktureller Aufbau des Chromatins.

Recent observations and hypotheses on the structure of chromatin are reviewed. Elementary "subunit" for higher structural orders is the nucleosome, consisting of a histone octamer and doublehelical DNA wrapped around it. During the last years details of the nucleosomal structure could be deduced down to a resolution of 2 nm. In the chromatin fiber, built up by (mono-)nucleosomes, the superhelical DNA has a tertiary structure, from which structures of still higher order (quaternary structure of DNA) can be formed. The correlation of these structures to the terms euchromatin and heterochromatin are discussed. Finally, some functional aspects, especially transcription and replication, are discussed in view of the new knowledge of chromatin structure.

Structural investigations on isolated chromatin of higher-order organisation.

Chromatin of a high structural order was prepared by a method avoiding drastic mechanical stress, as well as strong changes of the ionic strength, and was investigated by small-angle X-ray scattering. The results obtained support the idea that the quaternary structure of completely hydrated chromatin in solution is a solenoid with a diameter of about 32-34 nm. Gel chromatography of this chromatin yields solutions containing mainly quaternary structures with differing portions of tertiary structure. Decrease of the ionic strength results in an increase of loosening and unfolding of quaternary structures. Reconstitution by readjustment of the physiological ionic strength indicates slight differences between reconstituted and original higher-order structures.

[Interaction of the three alkylating drugs, cyclophosphamide, ifosfamide and trofosfamide, with DNA and DNA-constituents in vitro (author's transl)]. / In-vitro-Reaktion der drei alkylierenden Pharmaka Cyclophosphamid, Ifosfamid und Trofosfamid mit DNS und DNS-Bausteinen.

In vitro experiments were done for investigating the interactions of the three 3H-labelled alkylating drugs cyclophosphamide, ifosfamide and trofosfamide with DNA and DNA-constituents. Evidence is presented for a preferred reaction with orthophosphate groups, leading to phosphotriesters in the DNA. The results are discussed in view of the chemotherapeutic action of the drugs their mutagenic or carcinogenic effects.

Studies on the structure of isolated chromatin in three different solvents.

Properties of calf thymus chromatin, prepared by mild procedures, have been studied in various solvents. In 0.2 mM EDTA s-values ranged from 20 to 30 S and intrinsic viscosities from 5 to 24 dl/g. Dialysis against 0.15 M NaCl or 0.2 mM MgCl2 changed these values to 80 to 100 S and 0.2 to 5 dl/g, respectively, indicating an essentially more compact structure. In 0.2 mM EDTA X-ray scattering yielded a cross section diameter of 9 nm, which is associated with the tertiary structure of chromatin fiber (M/L = 21200 Dalton/nm). By dialysis against 0.15 M NaCl or 0.2 mM MgCl2 part of the material spontaneously formed quarterny structures (cross section diameters 25-29 nm). The rest of the material with cross section diameters less than 9 nm is supposed to be more strongly sheared tertiary structure which seems to be unable to form quarterny structure due to artificial conformational changes.

[Investigation of alkylation in rat liver chromatin after application of 3H-cyclophosphamid--"fine-distribution" and kinetics]. / Untersuchungen zur Alkylierung von Chromatin in der Rattenleber nach Gabe von 3H-Cyclophosphamid--"Feinverteilung" und Kinetik

The alkylation of chromatin constituents (DNA, histones and non-histones) in liver cell nuclei was investigated at various times after intraperitoneal injection of rats with 3H-cyclophosphamid. The highest alkylation was found in the DNA, the lowest in the histones; the euchromatic portions were alkylated several times higher compared to those of the heterochromatin. The eventual elimination of 3H-activity with time indicates that cyclophosphamid leads to repair processes in the DNA, this conclusion being supported by other experimental observations. In some of the 16 subfractions of the nonhistone proteins, alkylated portions are apparantly eliminated by normal protein turnover; however, in other nonhistones this elimination is inhibited, whereby in one subfraction it seems to be accelerated. The results of analog experiments with 14C-tryptophan as a precursor for nonhistone protein synthesis serve as a reference for this. In supplementary in-vitro model experiments using triaziquon as an alkylating agent indications for DNA-Protein-cross-links in chromatin could be obtained by the technique of X-ray low angle scattering, and according to sedimentation behaviour.

Studies on deoxyribonucleoproteins. IX. Subfractions of deoxyribonucleoproteins from rat and mouse liver.

By carrier-free continuous electrophoresis, deoxyribonucleoprotein from rat and mouse liver could be separated into two subfractions. The more anodic fraction (DNP I), comprising 5 - 8 per cent of the total, contains fewer proteins (two types of histones only). [3H]Cyclophosphamide caused in vivo a 2.5 times higher alkylation of the DNA in in DNP I than of the DNA in DNP II. These and additional results led to the suggestion of a structural model with DNP I as a spacer in the deoxyribonucleoprotein fiber.