If the depletion of the stratospheric ozone shield continues, the influx of solar UV-B radiation (280–320 nm) will increase in the near
future. In photosynthetic organisms there are several targets for the deleterious UV-B radiation, such as proteins, DNA and membranes.
Cyanobacteria use three different types of strategies to counteract UV damage: (i) stress avoidance by gliding mechanisms, (ii) stress
defence by synthesis of UV-absorbing compounds, antioxidants and extracellular polysaccharides, and (iii) repair mechanisms including
DNA repair and resynthesis of UV-sensitive proteins. In the past, most studies concentrated on the physiological aspects of UV tolerance
whereas the molecular basis of UV tolerance in cyanobacteria is poorly understood. We will summarize the effects of and responses to
UV-B at the physiological and molecular levels and present data on the influence of UV-B irradiation on the proteome of the terrestrial
cyanobacterium Nostoc commune. SDS-PAGE and high-resolution two-dimensional gel electrophoresis were used to analyse the influence
of UV-B on the proteome of N. commune. The resolution of SDS-PAGE turned out to be far too low to monitor the UV acclimation
process. In contrast to the latter technique, two-dimensional electrophoresis showed that the UV-B response is extremely complex,
involving the induction and the repression of a large number of proteins.