title: Nitrogen starvation-induced chlorosis in Synechococcus PCC 7942. Low-level photosynthesis as a mechanism of long-term survival.
abstract: Cells of the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 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-starved cells are investigated in an attempt to elucidate the mechanisms of maintenance of viability. Preservation of energetic homeostasis is based on a low level of residual photosynthesis; activities of photosystem II and photosystem I were approximately 0.1% of activities of vegetatively growing cells. The low levels of photosystem I activity were measured by a novel colorimetric assay developed from the activity staining of ferredoxin:NADP+ oxidoreductase. Photosystem II reaction centers, as determined by chlorophyll fluorescence measurements, exhibited normal properties, although the efficiency of light harvesting was significantly reduced compared with that of control cells. Long-term chlorotic cells carried out protein synthesis at a very low, but detectable level, as revealed by in vivo [35S]methionine labeling and two-dimensional gel electrophoresis. In conjunction with the very low levels of total cellular protein contents, this implies a continuous protein turnover during chlorosis. Synthesis of components of the photosynthetic apparatus could be detected, whereas factors of the translational machinery were stringently down-regulated. Beyond the massive loss of protein during acclimation to nitrogen deprivation, two proteins that were identified as SomA and SomB accumulated due to an induced expression following nitrogen reduction.
authors.affiliation: Institut für Mikrobiologie und Molekularbiologie der Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 26-32, 35392 Giessen, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Sauer
authors.forename: U
authors.initials: U
authors.lastname: Schreiber
authors.forename: R
authors.initials: R
authors.lastname: Schmid
authors.forename: U
authors.initials: U
authors.lastname: Völker
authors.forename: K
authors.initials: K
authors.lastname: Forchhammer
mesh_terms: D000595:Amino Acid Sequence; D000747:Anemia, Hypochromic / Q000209:etiology*; D000458:Cyanobacteria / Q000502:physiology*; D008969:Molecular Sequence Data; D009584:Nitrogen / Q000378:metabolism*; D010788:Photosynthesis
chemical_list: D009584:Nitrogen