Photoreactivation in the extreme halophilic archaebacterium Halobacterium cutirubrum was studied both in vivo and in v i m . Cells irradiated with ultraviolet (UV)-fluences up to 350 JimZ could be completely photoreactivated, indicating very efficient repair of pyrimidine dimers in UV-irradiated DNA. Dark repair is apparently absent in Halobacterium since liquid holding under non-growth conditions did not influence the survival of UV-irradiated cells, while cells remained completely photoreactivable with no change in the kinetics of photoreactivation. Experiments with Halobacterium isolates of different carotenoid content indicated that carotenoids do not influence either UV-inactivation or photoreactivation. Small differences in the rates of UV-inactivation and photoreactivation could be assigned to the occurrence of gas vesicles. Flash experiments and the temperature dependence of photoreactivation indicated an enzymatical reaction. This was confirmed by in vitro experiments with partially purified photoreactivating enzyme. The in vivo action spectrum of photoreactivation showed a main band in the 400470 nm region with a maximum at 440 nm. Comparison with action spectra of other microorganisms classified the Halobacterium enzyme as a 8-hydroxy-5-deazaflavin type photoreactivating enzyme.

Abstract— Photoreactivation in the extreme halophilic archaebacterium Halobacterium cutirubrum was studied both in vivo and in vitro. Cells irradiated with ultraviolet (UV)‐fluences up to 350 J/m2 could be completely photoreactivated, indicating very efficient repair of pyrimidine dimers in UV‐irradiated DNA. Dark repair is apparently absent in Halobacterium since liquid holding under non‐growth conditions did not influence the survival of UV‐irradiated cells, while cells remained completely photoreactivable with no change in the kinetics of photoreactivation. Experiments with Halobacterium isolates of different carotenoid content indicated that carotenoids do not influence either UV‐inactivation or photoreactivation. Small differences in the rates of UV‐inactivation and photoreactivation could be assigned to the occurrence of gas vesicles. Flash experiments and the temperature dependence of photoreactivation indicated an enzymatical reaction. This was confirmed by in vitro experiments with partially purified photoreactivating enzyme. The in vivo action spectrum of photoreactivation showed a main band in the 400‐470 nm region with a maximum at 440 nm. Comparison with action spectra of other microorganisms classified the Halobacterium enzyme as a 8‐hydroxy‐5‐deazaflavin type photoreactivating enzyme.