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Review
. 2006 Jul;141(3):805-10.
doi: 10.1104/pp.106.079251.

Metal homeostasis in cyanobacteria and chloroplasts. Balancing benefits and risks to the photosynthetic apparatus

Sigal Shcolnick  1 , Nir Keren
Affiliations
Review

Metal homeostasis in cyanobacteria and chloroplasts. Balancing benefits and risks to the photosynthetic apparatus

Sigal Shcolnick et al. Plant Physiol. 2006 Jul.
No abstract available

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Figures

Figure 1.
Figure 1.
Photosynthetic electron transport, ROS generation, and metal requirements. The photosynthetic electron transport chain is composed of the two photosystems (PSI and PSII) and cytochrome b6f. Extraction of electrons from water is catalyzed on the donor side of PSII by a cluster of four Mn ions. In addition, PSII contains two Fe atoms, a nonheme Fe on the acceptor side, and cytochrome b559. Cyanobacteria contain an extra cytochrome (cytochrome c550) attached to the donor side of the complex. The reaction center contains 36 chl molecules. Additional LHCII chls can increase the effective absorption cross section of the photosystem. Electrons are transferred from PSII to cytochrome b6f via plastoquinol molecules. Cytochrome b6f contains four b-type cytochromes (bH, bL, b6, and heme x), one c-type cytochrome f, and one 2Fe-2S cluster. The function of a single chl in the cytochrome structure is still unknown. Cytochrome b6f is the major proton pump that creates the potential gradient that is utilized by ATP synthase to produce ATP. Plastocyanin (PC), a Cu protein, transfers electrons from cytochrome b6f to PSI. In PSI, three 4Fe-4S clusters (FX, FA, and FB) participate in the electron transfer chain. Electrons from PSI are transferred to ferredoxin (Fd), a 4Fe-4S soluble protein. The reaction center core of PSI contains 96 chls with additional LHCI chls that contribute to the absorption cross section. Electron transfer reactions are indicated by blue arrows and ROS-generating reactions are indicated by red arrows.
Figure 2.
Figure 2.
Metal transport pathway in chloroplasts and cyanobacteria. Schematic representation of transport pathways for Mn (pink), Fe (red), Mg (green), and Cu (blue) through the three membrane systems of chloroplasts and cyanobacteria (inner, outer, and thylakoid membranes in plants; envelope, plasma, and thylakoid membranes in cyanobacteria). Known transporters are colored blue green for cyanobacteria and green for chloroplasts. Transport steps for which the carriers are still unknown are marked with a question mark. Mn is transported by MntABC in cyanobacteria (Bartsevich and Pakrasi, 1995). Additional components of the Mn transport pathway are still unknown. Fe is transported by FutABC in cyanobacteria (Katoh et al., 2001), stored in ferritin family complexes (Curie and Briat, 2003; Keren et al., 2004), or utilized for Fe cofactor biosynthesis. The pathways for heme and Fe-S cluster biosynthesis are not represented in this scheme. Mg transport pathways have not been elucidated yet. Cu transport is carried out by the CtaA and PacS P-type ATPases in cyanobacteria (Tottey et al., 2005) and by PAA1, HMA1, and PAA2 in plants (Abdel-Ghany et al., 2005; Seigneurin-Berny et al., 2005). Intracellular Cu transport is carried out by the Atx1 in cyanobacteria and by the Cu chaperone for SOD (CCS) in plants (Chu et al., 2005; Tottey et al., 2005).
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References

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