Oddvar F. Nygaard

Control of DNA replication by protein synthesis at defined times during the S period in Physarum polycephalum

Abstract The initiation and continuation of DNA replication in the myxomycete Physarum polycephalum was found to be dependent on concomitant protein synthesis. Addition of cycloheximide to the medium during the DNA synthetic period (S) resulted in incomplete replication; moreover, the amount of DNA synthesized increased in discrete steps in response to variation in the time of cycloheximide addition. The data suggest that the genome of Physarum consists of at least ten replicative units, the syntheses of which are controlled by proteins synthesized at defined times during the S period. Density shift experiments indicated that the replicative units were unique and were synthesized in the same temporal sequence in two consecutive S periods. Control of DNA replication by short-lived, newly synthesized proteins could explain the confinement of major nuclear DNA replication to the S period as well as the temporal order of the synthesis of this DNA in Physarum.

Electron transfer in ehrlich ascites tumor cells in the presence of nitrofurans.

Abstract Electron-affinic nitrofuran derivatives interfere with normal cellular metabolism by providing an electron shunt, apparently via free radical intermediates, between endogenous cellular reducing species and oxygen, in a manner analogous to that of vitamin K 3 . Pulse radiolysis was used to demonstrate the reactivity of nitrofuran radical anions with oxygen, as well as the NAD free radicals with nitrofurans. The reduction of nitrofurans under anaerobic conditions and the increased oxygen consumption (indicative of free radical formation) are enhanced by the addition of glucose and suppressed by the removal of endogenous reducing species, e.g. by the addition of diamide. Nitrofuran free radical production under aerobic conditions may result in the production of the Superoxide radical anion O 2 − . It is postulated that aerobic production of nitrofuran or oxygen free radicals or the resulting products may be responsible for the previously described cytotoxic effect of nitrofurans.

The use of the oxidant 'diamide' for studying the non-mitochondrial reducing capacity of ehrlich ascites tumor cells.

Abstract Diazenedicarboxylic acid bis(N,N-dimethylamide), (“diamide”) lowered non-mitochondrial NAD(P)H stores in Ehrlich ascites tumor cells in vitro by indirect reactions involving oxidation of glutathione and reduction of GSSG via glutathione reductase. The concentrations of diamide used did not alter the mitochondrial capacity to reduce NAD(P)H under anaerobic conditions. “Endogenous substrates” could be removed by multiple additions of diamide which indirectly inhibited NAD(P)H and GSH regeneration because of a lack of cellular reducing capacity. The regenerative power of the cells was restored by the addition of glucose. We conclude that diamide may prove to be a useful agent for studying the reducing capacity as well as the redox compartmentalization of cells in vitro.

Correlation of cell cycle parameters with radiation sensitivity in a series of murine L5178Y cells.

The X-ray dose--response parameters (Do, n and Dq) of 10 strains of murine L5178Y cells spanning a wide range of radiation sensitivities were characterized. The proportions of cells residing in G1, S and G2 + M phases of the cell cycle in exponentially growing cell populations were estimated using DNA microfluorometric techniques. The radiosensitive strains contained a lower proportion of cells in G1 phase and a somewhat larger proporation of cells in S phase than did the radioresistant strains, reflecting a direct correlation between radioresistance and the duration of G1 phase (and possibly an inverse correlation with the duration of the S phase) in these closely related cell strains. These results are discussed in terms of the putative role of G1 in governing radiosensitivity of cells and tumours.

DNA-histone interactions and priming activities of two DNA fractions from rat liver

Abstract 1. When a nuclear sonicate from rat liver was chromatographed on an organomercurial polysaccharide column the DNA was recovered in two distinct fractions; approx. 20 % of the DNA was not adsorbed to the column material, whereas the remainder could be eluted with 0.01 M EDTA or 0.5 M ammonium sulphate. 2. Melting profiles, and the identification of lysine and arginine in the hydrolysate of an acid extract of each fraction, showed that the DNA present in both fractions occurred as nucleohistone. 3. The priming activity of the DNA in the non-adsorbed fraction and in the “EDTA fraction” was very low. When the adsorbed DNA was eluted with ammonium sulphate rather than EDTA, the priming activity of this fraction was greatly enhanced. The non-adsorbed fraction of the nuclear sonicate was shown to contain some inhibitor which completely suppressed the priming activity of added free calf thymus DNA. 4. The DNA of both isolated fractions readily underwent exchange reactions when added to a complete nuclear sonicate, in a manner which suggests that the interactions between DNA and histones are non-specific.