Iron deficiency in cyanobacteria causes monomerization of photosystem I trimers and reduces the capacity for state transitions and the effective absorption cross section of photosystem I in vivo

Abstract

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

Figure 1.

Typical modulated Chl fluorescence traces of control (A and B) and iron-stressed (C and D) Synechococcus cells. Nontreated cells, A and C; HgCl₂ (40 μm)-treated cells, B and D. ML, Measuring modulated light (650 nm, 0.12 μmol m⁻² s⁻¹); SP, saturated light pulse (0.8 s, 2,800 μmol m⁻² s⁻¹); AL, actinic light (50 μmol m⁻² s⁻¹).

Figure 2.

A, Fluorescence emission spectra at 77 K of iron-sufficient (solid line) and iron-depleted (250×, dashed line; 500×, dotted line) Synechocystis cells. B, 77 K fluorescence emission spectra of control (iron sufficient) Synechococcus cells (solid line) and after a 48 h shift from control BG-11 medium to iron-deficient conditions (dotted line). C, Time course of the PSII fluorescence at 685 nm (○) and the position of PSI peak in Synechococcus cells (•) after transfer to iron-deficient medium. All fluorescence spectra are averages from four to eight corrected scans. Chl fluorescence was excited at 436 nm. Mean values ± se were calculated from four to eight independent measurements. D, Representative immunoblots of CP43′ polypeptide during the shift of Synechococcus cells from iron-sufficient to iron-deficient medium.

Figure 3.

Effect of HgCl₂ (40 μm) on the 77 K fluorescence emission spectra of dark-adapted control (A) and iron-stressed (B) Synechococcus cells after a shift from control BG-11 medium to iron-deficient conditions. The presented fluorescence spectra are averages from five to six corrected scans.

Figure 4.

A, Representative western blots of polypeptides from Synechococcus cells probed with antibodies raised against PsaL and NDH-H proteins during the shift of control (iron sufficient) cells to iron-deficient medium. B, Densitometric analysis of PsaL and NDH-H polypeptides. The numerical data for the relative protein abundance were normalized to the maximal values in control (iron sufficient) cells. Mean values ± se were calculated from three independent experiments.

Figure 5.

Nondenaturating PAGE profiles (A and D) and densitograms (B and E) of the Chl-protein complexes (1–5) in thylakoid membranes of control (A and B) and iron-stressed (D and E) Synechococcus cells. Band 1 is designated to the PSI trimer (PsaA/PsaB) core complex; band 2 is designated to the PSII (D1 + CP43) reaction center complex; band 3 is a mixture of PSII and PSI; band 4 is designated to the PSII (D1 + CP43) complex; and band 5 is PsaA/PsaB + CP43′. The traces in B and E represent averages from three to five independent experiments. C and F, Immunodetection of PsaB, D1:1, CP43, and CP43′ polypeptides in Chl-protein complexes separated by nondenaturating PAGE in control and iron-deficient Synechococcus cells, respectively.

Figure 6.

Postillumination transients from steady-state fluorescence to F_o′ after the AL (50 μmol photons m⁻² s⁻¹, 8 min) was turned off in Synechococcus cells grown under control (+Fe; A and B) and iron-deficient (C and D) conditions. The intensity of far-red (FR) light applied after turning off the AL was 10 W m⁻² (B and D).

Figure 7.

Flash-saturation curves of the relative extent of PSI absorbance changes at 820 nm (ΔA_820–860 nm) in control (A) and iron-stressed (B) whole cells of Synechococcus sp PCC 7942. The experimental data points were acquired by flashing the samples with single-turnover flashes of varying intensity and measuring the resulting P700 photooxidation. Mean values ± se were calculated from six to nine measurements in three independent experiments. The solid curves represent the fit of the experimental data.