[Fragmentation of the coding region chromatin of trp-dioxygenase (to) and tyr-aminotransferase (tat) genes in their active and repressed state by micrococcal nuclease].

The blot-hybridization technique-assisted we have studied the pattern of fragmentation by mirococcal nuclease (MNase) of DNA tyr-aminotransferase (tat) and trp-dioxygenase (to) genes in active (in rat cell liver nuclei) and repressed (in brain nuclei) states. It was provided, over a wide range of enzyme concentration two types of fragments are mainly produced: near full-size to- and tat-transcription unit (19,000 and 11,000 bp, respectively) and their large (from 1500 bp) heterogeneous in length. To-and tat-fragments of both kinds are preserved in hydrolyzates at limit of MNase digestion of total chromatin DNA when nuclease breaks occur in nearly all accessible sites of chromatin. This means that these fragments originate from two distinct subsets of transcription units coexistent in liver nuclei. The first of them do not contain MNase accessible sites over their entire length, whereas in other resistant regions alternate with rare irregular located MNase-sensitive segments. We presume that resistance to MNase within transcription units is peculiar to the competent genes. As a result of transcription MNase-sensitive areas arise which possibly flank elongating RNA polymerases.

[Contractile proteins in the fractions of soluble and insoluble chromatin of liver and thymus]. / Sokratitel'nye belki v sostave rastvorimogo i ostatochnogo khromatinA pecheni i timusa.

The proteins corresponding in molecular weight and solubility in salt solutions to skeletal muscle actin and myosin were revealed in liver and thymus chromatin fragments. When the ionic strength reached 0.3, about 60% of the myosin-like protein identified by electrophoretic mobility of high chains and the K+-EDTA-ATPase activity was cosedimented with nucleohistones. In the presence of ATP or PPi and Mg2+ the solubility of myosin in such salt solutions increased up to 90%, which was paralleled with significant stimulation of RNA release from the nucleohistones. The conformity in the degree of extraction and sedimentation of RNA and intranuclear myosin was also observed in other solutions used during myosin purification. The supposition that the nuclear system of contractile proteins causes labile, ATP-dependent binding of RNA to chromatin is discussed. No essential differences in the actin or myosin contents in the fractions of soluble and non-soluble chromatin were detected.

[Planar model of nucleosome and chromatin structure of high orders]. / Proskostnaia model' i struktury khromatina bysshikh poriadkov.

A model of nucleosome is discussed, which consists of two nucleohistone strand folds, located at the same level, similarly directed and having a rhomboid form. The folds are symmetric to each other. Four histones (H3, H2a, H2b and H4) take part in the formation of each fold. Nucleosome begins with a DNA region, bound with H1 histone and terminates with free DNA. Total sequence of histones along DNA is H1-H3-H2a-H2b-H4-H4-H2b-H2a-H3. Polypeptide chains of neighboring histones are oppositely directed and are located at opposite DNA strands. The model explains regularities of chromatin splitting under combined effect of ds-nucleases and trypsin, and of ss-nucleases. It is also in a good agreement with other properties of chromatin. Nucleosomes join to each other "side-to-side" under coincidence of terminal elements with faces of the rhomboid nucleosome structures. The model permits to explain the formation of a highest order structure--a helix of six nucleosomes, forming a fibril of 250--300 A in diameter. The degree of DNA compactness in it reaches 80--100.

[Comparative characteristics of the "ultrasonic" fractions of rabbit leukocyte (neutrophil) and brain chromatin]. / Sravnitel'naia kharakteristika "ul'trazvukovykh" fraktsii khromatina leikotsitov (neitrofilov) i mozga krolikov

Ratios and composition of "ultrasonic" fractions of chromatin in cells with different transcription intensity were studied. The hardly extracted residual fraction of chromatin makes up to about 80% DNA in leukocytes (neutrophils) and about 20% in brain tissue. Comparative characterization of fractions showed that residual DNP of brain and liver include, in addition to fragments of inactive condensed chromatin, the sites actively involved in the synthesis of RNA. The residual DNP of leukocytes, on the contrary, possesses a number of characteristics of inactive chromatin. A hypothesis is discussed of the necessity of close contacts between the most intensively transcribed RNAs and nuclear membrane lipid components, resulting in the incorporation of transcribed sites into the residual fraction.