Effect of Nitrogen Starvation on Polypeptide Composition, Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase, and Thylakoid Carotenoprotein Content of Synechocystis sp. Strain PCC6308

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Abstract
Synechocystis sp. strain PCC6308 cells were starved for nitrogen for 5 days. The polypeptide compositions of whole cell extracts and washed membranes of nitrogen-replete and nitrogen-starved cells were compared by one- and two-dimensional electrophoresis. Immunoblotting of one-dimensional gels indicated that pelletable ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was depleted in cells starved for nitrogen, while levels of soluble Rubisco were comparable in nitrogen-starved and nitrogen-replete cells. This is consistent with the hypothesis that pelletable Rubisco may serve as a nitrogen reserve in Synechocystis 6308. Other polypeptides were differentially enriched in the membrane or soluble fractions of nitrogen-replete cells or nitrogen-starved cells, suggesting nitrogen starvation may alter partitioning of polypeptides into soluble and membrane fractions. Degradation of abundant polypeptides during nitrogen starvation appeared to cause an effective magnification of less abundant polypeptides in the molecular mass range of 20 to 40 kilodaltons, as shown by two-dimensional electrophoresis. A 42-kilodalton thylakoid carotenoid protein identified by immunoblotting was conserved in membranes from nitrogen-starved cells. This may be functional for cells depleted of pigment and thus exposed to higher light levels because of decreased self-shading.