Nitrogen starvation-induced chlorosis in Synechococcus PCC 7942. Low-levelphotosynthesis as a mechanism of long-term survival

Cells of the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7 942 acclimate to nitrogen deprivation by differentiating into non-pigmented resting cells, which are able to survive prolonged periods of starvation. In this study, the physiological properties of the long-term nitrogen-starv ed cells are investigated in an attempt to elucidate the mechanisms of main tenance of viability. Preservation of energetic homeostasis is based on a l ow level of residual photosynthesis; activities of photosystem II and photo system I were approximately 0.1% of activities of vegetatively growing cell s. The low levels of photosystem I activity were measured by a novel colori metric assay developed from the activity staining of ferredoxin:NADP(+) oxi doreductase. Photosystem II reaction centers, as determined by chlorophyll fluorescence measurements, exhibited normal properties, although the effici ency of light harvesting was significantly reduced compared with that of co ntrol cells. Long-term chlorotic cells carried out protein synthesis at a v ery low, but detectable level, as revealed by in vivo [S-35]methionine labe ling and two-dimensional gel electrophoresis. Ln conjunction with the very low levels of total cellular protein contents, this implies a continuous pr otein turnover during chlorosis. Synthesis of components of the photosynthe tic apparatus could be detected, whereas factors of the translational machi nery were stringently downregulated. Beyond the massive loss of protein dur ing acclimation to nitrogen deprivation, two proteins that were identified as SomA and SomB accumulated due to an induced expression following nitroge n reduction.