Response of respiratory components in x-irradiated Micrococcus radiodurans

Iron-containing cytochromes provide energy to repair radiation damage. The cytochromes of Micrococcus radiodurans are atypical in that they strongly resist reduction by sodium hydrosulfite or oxidation by potassium ferricyanide, but are typically highly sensitive to potassium cyanide inhibition of respiration. Respiration in this organism is highly resistant to radiation damage, being reduced to 60% of the control level 2 hours after an exposure of 600 kR, whereas other bacteria, such as E. coli, are reduced to near zero. This reduction in respiration may be due either to a defect produced in existing cytochromes or to a decrease in synthesis of new cytochromes. Growth in low iron medium yields reductions in growth rate, oxygen consumption per cell and radioresistance, the shoulder of the survival curve being obliterated. Analysis of 59Fe-labeled cell contents on Sephadex G-200 immediately following exposure shows little change in elution patterns of the cytochromes. Only about 7% of 59Fe label is lost from cells after 3 hours of incubation in YHC following 600 kR. No immediate loss of label is detected up to total exposures of 750 kR. The uptake of iron following x-ray exposure shows a depression of 40% of control at 600 kR. Incubation of labeled cells in Triton X-100 (0.25%, v/v) releases 20% of the total activity in 5 minutes. Spectral analysis of this material reveals the presence of both pigmentation and cytochrome. These observations suggest that very high doses of radiation do not produce gross structural changes in existing cytochromes, but their respiratory function is depressed. The delay exhibited in the depression of respiratory function may be the result of initial protein damage followed by some type of proteolytic degradation since there is an immediate decrease in iron uptake post-irradiation with no subsequent recovery even at sublethal exposures