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Biomedical subjects

R T Johnson

Publications and source records attributed to R T Johnson.

At least 217 records · Page 12Linked to original sources
PubMed1979

The quantitative measurement of electrolyte elements in nuclei of maturing erythrocytes of chick embryo using electron-probe X-ray microanalysis.

Na, K and Cl measurements have been made on frozen sections of chick red blood cells throughout embryonic development, using electron-probe microanalysis. There is an apparent fluctuation in the levels of these elements during maturation, although the Na/K ratio remains fairly constant. The nuclear Na concentration resembles that of the cytoplasm, rather than that of the medium, at all stages. Inhibitor studies indicate that when cytoplasmic Na, K and Cl levels are altered, their corresponding nuclear levels are similarly affected. Additionally, the measurements in nuclei isolated in anhydrous media from lyophilized cells have shown artefactual accumulation of high Na, K, Ca and Mg.

Animals↗
PubMed1979

The organization of supercoiled DNA from human chromosomes.

Nucleoids obtained from metaphase-arrested HeLa cells by gentle lysis in non-ionic detergent and 2 M salt were prepared for electron microscopy by Kleinschmidt spreading techniques. From biophysical studies nucleoids have been shown to contain high molecular weight DNA which is supercoiled and topologically constrained. The contents of nucleoids disperse and collapse into 2 dimensions on the water surface as discrete particles (spreads), which are provisionally equated with metaphase chromosomes. In some cases separate spreads are linked. The structure of the spreads is complex. A preliminary description is presented, although we believe that some of the structures reported may be products of preparation and spreading rather than significant features of the organization of intact nucleoids. There is considerable variation in the appearance of different spreads which may be related to the degree of unfolding, spreading or damage of the different preparations. Each spread consists of one or more core areas surrounded by a network of fibres. Cores are composed, at least in part, of compressed fibres and superficially appear to constrain fibres of the network. In the network the inter-twining of pairs of fibres and the occurrence of flat fibre spirals (disks) are interpreted as evidence of DNA supercoiling, but other fibres of similar thickness are not visibly supercoiled. High-order structures including groupings of disks are observed: these may be the structural correlates of the folded domains of supercoiled DNA established biophysically. The relative proportion of disks and other network structures varies between spreads.

Chromosomes, Human↗
PubMed1979

Ultraviolet-induced DNA excision repair in human B and T lymphocytes. III. Repair in lymphocytes from chronic lymphocytic leukaemia.

There have been conflicting reports about the capacity of lymphocytes from individuals with chronic lymphocytic leukaemia (CLL) to undertake u.v.-induced DNA repair. We have examined repair in CLL cells and in control and age-matched purified B and T lymphocytes by a technique independent of incorporation of radioactive precursor, i.e. by the recovery of normal sedimentation behaviour of nucleoid bodies obtained from these cells by lysis in high salt and non-ionic detergent. Recovery of normal sedimentation is associated with restoration of DNA supercoiling. CLL cells were found to be as sensitive to u.v. and to repair at similar rates as age-matched B controls. They are considerably more sensitive than young B cells and repair less efficiently. Reasons for the reported discrepancies in CLL repair are discussed.

Aged↗
PubMed1978

Determinants of measles virus (hamster neurotropic strain) replication in mouse brain.

In newborn mice the hamster neurotropic strain of measles virus produces a severe meningoencephalitis with readily recoverable virus, while in weanling mice a fatal encephalopathy is produced with scant histopathology and no viral infectivity in brain homogenates. In this study various host factors that may change with maturation and determine the restriction of viral expression were investigated, including immune response, interferon production, host temperature, and the possible role of proteases. None of these factors appeared to be responsible for host restriction of viral expression. Recovery of virus from brains of weanling mice was not significantly enhanced by cocultivation with Vero cells, complementation with temperature-sensitive mutants, or phenotypic mixing with the Edmonston strain of measles virus. These data, combined with our previous observations of production of viral proteins including nucleocapsid proteins without development of recognizable nucleocapsids in neurons of weanling mice, suggest that with maturation the neural cells fail to replicate sufficient amounts of encapsidated viral ribonucleic acid.

Aging↗
PubMed1978

Transfer of human chromosomes via human minisegregant cells into mouse cells and the quantitation of the expression of hypoxanthine phosphoribosyltransferase in the hybrids.

The behaviour of human cells arrested in mitosis can be severely perturbed so as to generate numerous small minisegregants containing very few chromosomes. These cells can be separated according to size and DNA content and fused with intact cells. In this paper we describe the production and some properties of proliferating cell hybrids generated by fusion of human minisegregant cells derived from a HeLa strain with mouse A9 cells deficient in hypoxanthine phosphoribosyltransferase (HPRT, EC 2.4.2.8). The hybrids were shown to contain up to 10 human chromosomes including a single X. Independently derived hybrid clones were quantitatively characterized and compared with the parental phenotypes with respect to HPRT. Human isozymes of each of the 3 enzymes HPRT, glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and phosphoglycerate kinase (EC 2,7.2.3) were found. Tests to evaluate both structure and function of HPRT were utilized. The specific activity of HPRT of more than 10 hybrids tested was approximately 10% that of the HeLa parent. Structural characterization of HPRT from hybrid cells as evidenced by heat inactivation and electrophoretic mobility results in a 'human-like' phenotype. Functional characterization of parental HPRT results in kinetic constants for cofactor and substrate which do not permit distinction of human and of human and mouse enzymes; HPRT from the minisegregant hybrids had normal kinetic constants. The reduced specific activity of HPRT in the hybrids is discussed in terms of the inability of the mouse environment to regulate the full expression of the human structural gene.

Animals↗