Unstable protein mediated ultraviolet light resistance in Anacystis nidulans

Abstract

CYANOBACTERIA are believed to have been precursors to eukaryotes during Precambrian evolution¹. It is also suggested that the oxygen evolved as a result of photosynthetic activity in these organisms, several of which are obligate photoautotrophs, was the major contributor towards the transition from anaerobic to aerobic atmosphere^1,2. It is, therefore, reasonable to expect cyanobacteria to have evolved in an environment with a relatively high flux of solar ultraviolet light in the absence of ozone shield. This, presumably, could be possible with the evolution of efficient protective mechanisms or repair systems effective against damage by ultraviolet light. It is therefore not surprising that these organisms possess an extremely efficient photoreactivating repair system³. The presence of dark-repair in Anacystis nidulans has also been suggested by the isolation of mutants sensitive to ultraviolet light⁴. We present here physiological evidence for a dark-repair (or protective) system in this organism. A protein, unstable at least in the light, seems to be responsible for the resistance against lethal damage by ultraviolet light. This is observable in conditions of reduced photoreactivation achieved by a 24 h post-irradiation dark incubation, referred to here as ‘dark-survival’ following Asato, who showed that well within this period photoreactivability of damage by ultraviolet light is almost completely lost⁴.