Ultraviolet killing and photoreactivation of Phycomyces spores

Abstract

Vegetative spores of the zygomycete fungus Phycomyces blakesleeanus were used to investigate UV-killing and photoreactivation. Irradiation with far-UV light generated shouldered exponential survival curves. Postirradiation with near-UV, blue or white light caused photoreactivation, i.e. increase of the survival rate. Photon fluenceresponse curves for photoreactivation, which were generated for selected wavelengths between 350 and 610 nm of light, were generally monophasic, while they were biphasic when 385-nm light was used. At the latter wavelength one could distinguish a low-irradiance response and a high-irradiance response. A 7-point action spectrum for photoreactivation displayed a maximum at 385 nm indicating that the DNA-photolyase of Phycomyces contains as chromophores 5,10-methenyltetrahydrofolate (MTHF) and FADH 2. Spores which were obtained from dark-grown Phycomyces stocks showed no photoreactivation in response to low-irradiance light, i.e. exposures shorter than 4 min. For exposures longer than 4 min (high-irradiance response) photoreactivation did occur. The latter response was apparently due to newly induced DNA-photolyase, which is presumably absent in spores of dark-grown stocks. Two light-insensitive mutants, C47 madA and C111 madB, which are defective for phototropism and photodifferentiation, lacked photoreactivation after low and high-irradiance conditions. This finding, too, indicated that the DNA-photolyase is under stringent near-UV/blue-light control and is not expressed in darkness.