Moderate UVB triggers dynamic regulation of cpc, psbA and rbcL transcripts in the cyanobacterium Synechococcus sp. PCC 7942

Abstract

Natural UVB levels are increasing in aquatic habitats. Three potential molecular targets of UVB in cyanobacteria are the abundant, light harvesting pigment-protein phycocyanin encoded by the cpc operon, the rapid-turnover D1 protein of Photosystem II encoded by psbA, and the abundant and stable carbon fixation enzyme RUBISCO encoded by rbcL. We grew Synechococcus sp. PCC 7942 under optimal temperature and nutrient conditions under two light regimes (50 or 300 mmol photons m-2 s-1) and then exposed them to UVB (0.125 or 0.75 mmol photons m-2 s-1 respectively), giving a ratio of PAR:UVB typical of shallow aquatic habitats in temperate latitudes. Short-term exposure to these environmentally appropriate levels of UVB caused little or no net inhibition of Photosystem II or growth, although excess UVB levels did inhibit both Photosystem II and growth. Thus acclimation to light confers pre-acclimation to tolerate short-term exposure to appropriate UVB. Levels of cpc transcripts were highly sensitive to UVB, dropping drastically but then recovering rapidly within 45 min of exposure to appropriate UVB levels. As expected, over the same time psbAII/III transcripts encoding the alternate D1:2 protein accumulated rapidly as part of the cellular response to UVB. rbcL transcript levels did not change significantly in response to moderate short-term UVB, although excess UVB did cause loss of rbcL transcripts. We hypothesize that in these cells UVB exposure triggers a re-allocation of transcriptional resources from cpc expression to generate the psbAII/III transcripts more important for short-term UVB resistance.