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.