1. The effect of Ni 2+ on respiration, volume changes and Ca 2+ movements was investigated in rat liver mitochondria. 2. Ni 2+ inhibited Ca 2+ uptake into respiring mitochondria, Ca 2+-stimulated respiration and swelling in Ca 2+ salts, whereas it did not inhibit either state 4 and NDP-stimulated respiration, or swelling in K + salt in the presence of valinomycin. 3. The inhibitory concentration of Ni 2+ depended strongly on the applied Ca 2+ concentration. As revealed by direct methods, 50% inhibition of Ca 2+ influx was achieved by approx. 2-fold excess of Ni 2+. 4. If added to Ca 2+-loaded mitochondria, Ni 2+ gave rise to slow Ca 2+ release and inhibited uncoupler-induced efflux slightly. 5. It is concluded that Ni 2+ is a potent inhibitor of mitochondrial Ca 2+ transport. Ca 2+ influx is far more sensitive to inhibition than Ca 2+ efflux.

Chinese hamster V-79 cells were treated with metabolic inhibitors of DNA or protein synthesis for various intervals of time after exposure of 3.0 or 5.0 J · m −2. After removal of the metabolic block(s) the rate of DNA synthesis was followed by measuring the incorporation of [ 14C]thymidine into acid-insoluble material. A 2.5 or 5.0 h incubation with cycloheximide or hydroxyurea was effective in delaying the onset of the recovery in the rate of DNA synthesis that normally becomes evident several hours after exposure to ultraviolet light. By using concentrations of cycloheximide or hydroxyurea that inhibit DNA synthesis by a similar amount (70%), but protein synthesis by vastly different amounts (95% for cycloheximide; 0% for hydroxyurea), it was apparent that the delay in recovery caused by the treatment of the cells with cycloheximide could be accounted for entirely by its inhibitory effect on DNA synthesis. This suggests that the recovery in DNA synthetic rates following exposure of V-79 cells to ultraviolet light does not appear to require de novo protein synthesis, and therefore does not appear to require the involement of an inducible DNA repair process.

Repair of ultraviolet-light-induced DNA damage in a highly pathogenic Gram-negative bacterium, Vibrio cholerae, has been examined. All three strains of V. cholerae belonging to two serotypes, Inaba and Ogawa, are very sensitive to ultraviolet irradiation, having inactivation cross-sections ranging from 0.18 to 0.24 m 2/J. Although these cells are proficient in repairing the DNA damage by a photoreactivation mechanism, they do not possess efficient dark repair systems. The mild toxinogenic strain 154 of classical Vibrios presumably lacks any excision repair mechanism and studies of irradiated cell DNA indicate that the ultraviolet-induced pyrimidine dimers may not be excised. Ultraviolet-irradiated cells after saturation of dark repair can be further photoreactivated.

The DNA methylase activity present in embryos and cultured cells of Xenopus laevis resembles DNA methylase from mammalian tissues. Little or no activity is found in mature germinal vesicles, though nuclear activity rises rapidly after fertilization. This rise may result in part from a relocation of cytoplasmic enzyme.

The disappearance of tRNA during the maturation of rabbit reticulocytes produced under the stress of phenylhydrazine-induced hemolysis was studied. The tRNA content of reticulocytes and of erythrocytes derived from them was compared. The results show that tRNA persists longer after reticulocyte maturation than ribosomes and than the ability to incorporate amino acids into protein. Considerable uniformity of tRNA degradation was noted with about 15% of the tRNA for most amino acids remaining after reticulocyte maturation. The half-life of tRNA in the maturing cells is estimated to be 50–60 h. There is little tRNA lacking the 3′-terminal pCpCpA moiety in cells derived from reticulocytes.

Gamma-irradiation or introduction of hydrocortisone bring about degradation of nuclear DNA in rat thymocytes. The chromatin degradation products were extracted from purified nuclei by 0.7 mM EDTA. The quantity of low molecular weight chromatin fragments formed 6 h after irradiation increases up to the doses of 3 Gy, then remains constant up to 30 Gy and decreases at doses 100 to 300 Gy. Whatever the irradiation dose, DNA degradation starts after a 2-h lag, reaches a maximum by the 6th hour and remains constant between the 6th and 10th hours. The quantity of chromatin fragments formed coincides with the number of cells with pycnotic nuclei. The chromatin fragments present nucleosomes and their oligomers with a normal histone content and an intact structure, as judged from how they are split by DNAase I. The number of intranucleosomal breaks in DNA is negligible. DNA fragmentation is not accompanied by degradation of histones and nonhistone proteins of chromatin. Hence, DNAase I and proteases are not involved in degradation of chromatin. The ratio between mononucleosomes and oligomeres of different lengths does not depend on the dose and the time after irradiation. The quantity of DNA degraded is determined by the number of dying cells in which all DNA is fragmented rather than the degree of chromatin degradation over the whole thymocyte population. Hydrocortisone-induced degradation of chromatin in rat thymocytes occurs similarly. A possible role of chromatin degradation in cell death is discussed.

The mechanism by which brusatol inhibits protein synthesis in rabbit reticulocytes has been investigated. When added to reticulocyte lysates, brusatol inhibits endogenous protein synthesis only after a lag of 2–4 min at 30°C. During this period 80 S ribosomes accumulate. Brusatol is equally effective in inhibiting endogenous protein synthesis in lysates and poly(U)-directed polyphenylalanine synthesis with runoff ribosomes. In fractionated reticulocyte systems, brusatol does not inhibit formation of the ternary, 40 S, and 80 S initiation complexes, but does inhibit the reaction of puromycin with initiation complexes containing [ 35S]Met-tRNA f. These data suggest that brusatol inhibits the peptidyl transferase elongation reaction of protein synthesis, but can do so only after one round of protein synthesis has been completed. Thus, the mechanism of action of brusatol in the rabbit reticulocyte system is very similar to the effects previously reported for bruceantin in a yeast system.

The androgen receptor from mouse kidney cytosol has been studied for its nucleic acid binding properties by DNA-cellulose centrifugation assay. The receptor appears to bind to RNA (mRNA, tRNA, rRNA) as well as to DNA. Salt and heat activation of the androgen receptor enhances both DNA and RNA binding. The receptor binds slightly better to denatured DNA than to native DNA. The androgen receptor binds about 2-fold tighter to poly(dG-dC) than to poly(dA-dT). The interaction of the receptor with DNA is not greatly affected by the BrdUrd substitution. The observation that androgen receptor shows a significant affinity to RNA may imply that androgen receptor-RNA interaction could play a role in gene regulation.

The site of interaction of phage MS2 A protein on MS2 RNA was determined by analysing the base sequence of the RNA fragment which was released from the RNAase-resistant complex prepared by RNAase digestion of RNA-A protein complex. The result showed that there were two types of RNA fragment: one had a sequence which corresponded to the sequence of the 5′-side maturation region of MS2 RNA and the other corresponded to the sequence of the 3′-side untranslated region. These results were confirmed by a competition experiment in in vitro reconstitution system using f2 defective RNA, lacking about 30% of the 5′-side, and MS2 5FUra (5-fluorouracil) RNA, lacking about 35% of the 3′-side, as competitors. These results seem to indicate that the A protein is bound to at least two sites on MS2 RNA.

A pyrimidine nucleoside phosphorylase was partially purified from human blood platelets. The purified enzyme, as well as crude enzyme preparations, catalyses the phosphorolysis of thymidine and deoxyuridine, but not of uridine, and is able to catalyse direct pentosyl transfer from these deoxyribonucleosides to uracil or thymine; this enzyme has the properties of a thymidine phosphorylase. It has a molecular weight of about 110 000 and is composed of two identical subunits; it is phosphate dependent, has a maximal activity at a pH value of 5.7, and an isoelectric point of 4.4. This enzyme was mainly of cytoplasmic origin. Although platelet thymidine phosphorylase could promote the degradation or synthesis of thymidine, intact platelets degraded thymidine but were not able to synthesize thymidine from thymine. Blood platelets may play an important role in the degradation of plasma thymidine.