Protein synthesis and viability in x-irradiated Rhodopseudomonas spheroides

IONIZING RADIATION: EFFECT ON GENETIC TRANSCRIPTION

Cells of Escherichia coli grown on maltose can be induced by the addition of thiomethyl galactoside to produce beta-galactosidase. If cells are irradiated shortly after induction, the transcription of the DNA ceases, and the enzyme produced by the messenger RNA is observed to reach a maximum. From these data the calculated half-life of unstable messenger RNA is given over a temperature range from 8.1 minutes at 10 degrees C to 0.7 minute at 45 degrees C. The kinetics of cessation of transcription give information on both meassenger RNA decay and rate of transcription. Arrhenius graphs for both these rates are given, and the activation energies mtieasured are 11,000 calories per mole for decay and 22,000 calories per mole for transcription. This relation to temperature is characteristic of enzymatic behavior.

Radiosensitivity of mammalian cells. II. Radiation effects on macromolecular synthesis

Radiation effects on macromolecular synthesis essential for the Chinese hamster cell to traverse the life cycle and to divide have been investigated. Life-cycle analysis techniques employing inhibitors of macromolecular synthesis were used in determining the kinetics of cell growth for specific segments of the population following spontaneous recovery from radiation-induced division delay. The results indicated that recovery does not occur in the absence of functional protein synthesis. Under conditions which inhibit normal RNA and DNA synthesis, irradiated cells can recover the capacity to traverse the life cycle and to divide. The stability of mRNA species coding for proteins essential for division in irradiated cells was also measured. The mean functional lifetime of these mRNA species was 1 hr. The data demonstrate the existence of a specific segment of the population consisting of cells which have completed transcription related to division but not concomitant translation and which can recover from the radiation injury without synthesis of additional RNA. Thus, initial recovery of the ability to divide has an obligate requirement for protein synthesis but no corresponding requirement for nucleic acid synthesis during the period when original messenger remains intact.

Time-dependence of the effect of X-irradiation on the formation of glycerol phosphate dehydrogenase and other dehydrogenases in the developing rat brain

X-irradiation (100-1500 r) administered to the heads of rats 8-30 days of age inhibited the development of glycerol phosphate dehydrogenase (l-glycerol 3-phosphate-NAD oxidoreductase, EC 1.1.1.8) in the brain stem and cerebral hemispheres. At 40 days of age and older no effect was observed. This inhibition was a delayed phenomenon, dose-dependent and with no recovery. It is proposed that the inhibition of enzyme formation is related to radiation damage caused to DNA. Actinomycin D inhibited the development of glycerol phosphate dehydrogenase in a manner similar to ionizing radiation. Four other dehydrogenases also showed age-dependent radiosensitivities. ;Malic enzyme' (EC 1.1.1.40), lactate dehydrogenase (EC 1.1.1.27) and malate dehydrogenase (EC 1.1.1.37) ceased to be radiosensitive at about 8 days of age and isocitrate dehydrogenase (NADP) (EC 1.1.1.42) at 16 days. The correlation between developmental increase in enzyme activity and radiosensitivity held closely for glycerol phosphate dehydrogenase and isocitrate dehydrogenase and to a smaller extent for the others.

Inhibition of lipolytic action of growth hormone and glucocorticoid by ultraviolet and x-radiation

Addition of catecholamines or growth hormone plus glucocorticoid activates lipolysis in isolated white-fat cells of the rat. If the fat cells were irradiated just before the addition of growth hormone and glucocorticoid, their lipolytic action was abolished. However, irradiation did not affect the activation of lipolysis by catecholamines.