Figure 1.

Cytochrome b₆f Complexes and Rubisco Are Selectively Lost during Nitrogen Starvation under Low Light (5 to 10 µE⋅m⁻²⋅s⁻¹).

(A) Whole-protein extracts of cells harvested at indicated time points (0, 4, 10, 24, and 34 h) after the onset of nitrogen starvation, probed with the antibodies indicated between the two panels. To detect all proteins investigated here but also in Figures 3A and 4, the samples were loaded several times on independent gels. WT-S24 (left panel) and strain tmFH8, which expresses tagged versions of MCA1 and TCA1 but is otherwise wild-type (right panel), are shown. Antibodies recognize the major cytochrome b₆f subunits (top) or one major subunit of each other photosynthetic protein complex (bottom), whose abundance reflects that of the whole complex.

(B) Quantification of the loss of cytochrome b₆f complex in strain WT-S24. Abundance of cytochrome f, expressed as a fraction of its initial amount, was quantified from phosphor imager scans of immunoblots revealed with ¹²⁵I-protein A and normalized to the amount of the invariant CF1 subunit β to correct for variations in loading; mean of three independent experiments ± sd.

(C) Kinetics of fluorescence induction of dark-adapted WT-S24 cells at the onset (left panel) and at the end (34 h; right panel) of the nitrogen starvation, recorded in the presence (gray curves) or in the absence (black curves) of DCMU (5 µM final). Relevant parameters (F_m, F_s, and F₀) are shown. Curves were normalized to the maximum fluorescence recorded in the presence of DCMU. a.u., arbitrary units.

(D) Change in photosynthetic efficiency of WT-S24 cells during nitrogen starvation assessed by the quantum yield of PSII [Φ_PSII = (F_m − F_s)/F_m]; mean of four independent experiments ± sd.