The global response of nostoc punctiforme ATCC 29133 to UVA stress, assessed in a temporal DNA microarray study

Cyanobacteria in nature are exposed not only to the visible spectrum of sunlight but also to its harmful ultraviolet components (UVA and UVB). We used Nostoc punctiforme ATCC 29133 as a model to study the UVA response by analyzing global gene expression patterns using genomic microarrays. UVA exposure resulted in the statistically detectable differential expression of 573 genes of the 6903 that were probed, compared with that of the control cultures. Of those genes, 473 were up-regulated, while only 100 were down-regulated. Many of the down-regulated genes were involved in photosynthetic pigment biosynthesis, indicating a significant shift in this metabolism. As expected, we detected the up-regulation of genes encoding antioxidant enzymes and the sunscreen, scytonemin. However, a majority of the up-regulated genes, 47%, were unassignable bioinformatically to known functional categories, suggesting that the UVA stress response is not well understood. Interestingly, the most dramatic up-regulation involved several contiguous genes of unassigned metabolism on plasmid A. This is the first global UVA stress response analysis of any phototrophic microorganism and the differential expression of 8% of the genes of the Nostoc genome indicates that adaptation to UVA in Nostoc has been an evolutionary force of significance. Cyanobacteria respond to ultraviolet radiation using several strategies. These can include an increase in the production of antioxidant and DNA damage repair enzymes, UV-absorbing pigments, a decrease in the production of photosynthesis and light-harvesting complexes, as well as physical motility away from the UV source. The sensing of UV in cyanobacteria is not well understood and could involve an unidentified photoreceptor or occur as a secondary response to other effects of UV, such as oxidative stress.