Three-dimensional Model and Characterization of the Iron Stress-induced CP43′-Photosystem I Supercomplex Isolated from the Cyanobacterium Synechocystis PCC 6803

Abstract

The cyanobacterium Synechocystis PCC 6803 has been subjected to growth under iron-deficient conditions. As a consequence, the isiA gene is expressed, and its product, the chlorophyll a-binding protein CP43, accumulates in the cell. Recently, we have shown for the first time that 18 copies of this photosystem II (PSII)-like chlorophyll a-binding protein forms a ring around the trimeric photosystem I (PSI) reaction center (Bibby, T. S., Nield, J., and Barber, J. (2001) Nature, 412, 743-745). Here we further characterize the biochemical and structural properties of this novel CP43-PSI supercomplex confirming that it is a functional unit of approximately 1900 kDa where the antenna size of PSI is increased by 70% or more. Using electron microscopy and single particle analysis, we have constructed a preliminary three-dimensional model of the CP43-PSI supercomplex and used it as a framework to incorporate higher resolution structures of PSI and CP43 recently derived from x-ray crystallography. Not only does this work emphasize the flexibility of cyanobacterial lightharvesting systems in response to the lowering of phycobilisome and PSI levels under iron-deficient conditions, but it also has implications for understanding the organization of the related chlorophyll a/b-binding Pcb proteins of oxychlorobacteria, formerly known as prochlorophytes.Iron is the most abundant transition metal in the crust of the earth and is an absolute requirement for photosynthetic organisms such as cyanobacteria, because it is needed for many of the redox reactions of the photosynthetic electron transport system. However, in most aquatic ecosystems it can be sufficiently low to limit photosynthetic activity (1, 2). This finding is attributed mainly to the low solubility of Fe 3ϩ above neutral pH in oxygenic ecosystems (3). As a result, cyanobacteria and other microorganisms have evolved a number of responses to cope with frequently occurring conditions of iron deficiency (4). One such response is to express two "iron stress-induced" genes, isiA and isiB (5, 6), which are located on the same operon. The isiB gene encodes for flavodoxin, which can functionally replace the iron containing ferredoxin (7). The isiA gene encodes for a protein often called CP43Ј, because it has an amino acid sequence homologous to that of the chlorophyll a-binding protein, CP43 of photosystem II (PSII) 1 (8, 9). Like CP43, CP43Ј is predicted to have six transmembrane helices, and judged by the conservation of histidine residues, it is likely to bind the same number of chlorophyll a molecules. The major difference is that CP43Ј lacks the large hydrophilic loop that joins the luminal ends of helices V and VI of CP43. For this reason, it has 342 amino acids rather than 472 (see Fig. 1).Under iron stress, the isiA gene is transcribed into two messages, a monocistronic message containing only isiA and a dicistronic message that also contains the isiB gene (10). Although the discovery of the CP43-like iron stress-induced protein was made some time ago (11), its precise...

Structure of a photosystem II supercomplex isolated from Prochloron didemni retaining its chlorophyll a/b light-harvesting system

“…3b). By overlaying the outlines for the PSII dimer of cyanobacterial PSII and for CP43 based on x-ray models [the latter as an analog of a Pcb protein (15,30)], the large particles are seen to consist of a core dimer with a f lanking region accommodating five Pcb proteins along each edge, making a total of 10 Pcb subunits per PSII dimer (Fig. 3c).…”

“…In fact, the pcb gene sequence was similar to that of the gene that encodes the Chl a-binding protein CP43 of photosystem II (PSII) and the IsiA protein induced in cyanobacteria under conditions of limiting iron levels (12,13). Recent studies (14)(15)(16) have shown that the CP43-like IsiA protein forms a ring of 18 subunits around the cyanobacterial trimeric photosystem I (PSI) reaction center core complex. In so doing it increases the light-harvesting capacity of PSI by Ϸ80%.…”

“…The procedure was similar to that used previously for isolating PSI supercomplexes from Synechocystis PCC 6803 (14,15) and Prochlorococcus SS120 (17). The green pigment-containing fractions were removed carefully by using a syringe.…”

“…All studies were conducted by using the O software package (26), with structural coordinates derived from the 3.8-Å resolved model of PSII (27) designated under Protein Data Bank ID code 1FE1 (www.rcsb.org͞) elucidated from the x-ray diffraction of crystals of PSII isolated from Synechococcus elongatus. Subunits attributed to Pcb protein were modeled by using the coordinates assigned to the six transmembrane helices of CP43 in 1FE1 (27), given the high structural homology known to exist between these two protein families (15).…”

A Chlorophyll a/b-binding Protein Homolog That Is Induced by Iron Deficiency Is Associated with Enlarged Photosystem I Units in the Eucaryotic Alga Dunaliella salina

“…4) and the accumulation of a major chlorophyll a-binding protein, the isiA gene product CP43Ј that forms an 18-subunit ring around the trimeric PS-I core. Spectroscopic evidence suggests that CP43Ј is functionally coupled to PS-I reaction centers, indicating that it may act as a peripheral light-harvesting antenna (5,6). In the green alga Chlamydomonas reinhardtii, iron deprivation is associated with remodeling of PS-I, resulting in changes in the polypeptide composition of LHC-I and in a decreased efficiency of energy transfer between the light harvesting and reaction center (RC) (7,8).…”