In Anacystis nidulans, upon infection with cyanophage AS-1, after a lag period of 1 h the level of deoxyribonuclease (DNase) activity increaded rapidly up to 15- to 20-fold in 4 to 5 h in the light. In contrast, the ribonuclease and phosphomonoesterase activities increased significantly only 4 to 5 h after infection, i.e. as late as 1 h prior to lysis. In complete darkness, the nuclease levels remained unaltered. However, when the infected cells were exposed to light for 1 or 2 h after infection, the DNase level increased essentially to the same extent in the dark as in continuous light, although the complete replication cycle of the virus was impaired in the dark and cells lysed only in the continuously illuminated cultures. Inhibition of photosystem II with 3-(3,4-dichlorophenyl)-1-dimethylurea during the early illumination period strongly decreased the subsequent, infection-dependent increase in DNase activity in the dark. The virus-induced increase in DNase activity was also inhibited by chloramphenicol. The data suggest that, in spite of the obligate photoautotrophic nature of A. nidulans, dark metabolism is able to support fully the formation of some specific proteins if the triggering of their synthesis takes place in light.
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