Photosystem I reaction centers from maize bundle-sheath and mesophyll chloroplasts lack subunit III

Photosystem I reaction centers were isolated from mesophyll and bundle-sheath chloroplasts of the C4 maize plant. Both preparations were found to be free of chlorophyll b and to have the same spectral properties and chlorophyll/P700 ratio as photosystem I reaction centers isolated from C3 plants. Photosystem I reaction centers from both mesophyll and bundle sheath were found to consist of six subunits with apparent molecular masses of about 70 kDa, 20 kDa, 17 kDa, 16 kDa, 10 kDa and 8 kDa, corresponding to photosystem I reaction center subunits I, II, IV, V, VI and VII of spinach, as tested by their immunological cross-reactivity with antibody raised against the respective spinach subunits. No cross-reactivity was found with antibodies raised against subunit III of spinach, either in whole thylakoids or purified reaction centers of both bundle-sheath and mesophyll chloroplasts. It is concluded that photosystem I reaction centers of bundle-sheath and mesophyll thylakoids of maize are identical and lack the polypeptide corresponding to subunit III present in all C3 plants so far tested.     

Fibronectin is a component of the sodium dodecyl sulfate-insoluble transglutaminase substrate

Liver plasma membranes contain a morphologically distinct protein complex which serves as a substrate for the plasma membrane-associated transglutaminase. The complex, which appears as a two-dimensional sheet, is insoluble in sodium dodecyl sulfate and reducing agents and has been named SITS for sodium dodecyl sulfate-insoluble transglutaminase substrate (Tyrrell, D. J., Sale, W. S., and Slife, C. W. (1988) J. Biol. Chem. 263, 1946-1951). Polyclonal antibodies raised against SITS were used to probe for soluble constituents of the matrix. Immunoblots showed that proteins of 230, 35, and 32 kDa reacted with the anti-SITS antiserum when the soluble fraction from a liver homogenate was examined. The 230-kDa protein was identified as fibronectin after observing cross-reactivity of anti-SITS antiserum with authentic fibronectin and cross-reactivity of anti-fibronectin antiserum with the 230-kDa cytosolic protein and purified SITS. Preincubating anti-SITS antiserum with purified fibronectin decreased immunostaining of the 230-kDa cytosolic protein and authentic fibronectin. Immunoblots of the plasma membrane fraction using anti-SITS and anti-fibronectin antisera showed that both antisera reacted with proteins at the top of the stacking gel (SITS) and of 230 kDa. In addition, the anti-SITS antiserum reacted with proteins of 85, 35, and 32 kDa. Immunofluorescence microscopy revealed that the anti-SITS and anti-fibronectin antisera both react with isolated SITS and with the same filamentous structures associated with intact plasma membranes. These studies show that fibronectin is a component of the plasma membrane matrix, SITS. This finding is consistent with the proposed role of this matrix which is to mediate cell-cell adhesion between hepatocytes in the tissue.     

Evidence from Crosslinking for a Close Association of the Extrinsic 33 kDa Protein with the 9.4 kDa Subunit of Cytochrome b 559 and the 4.8 kDa Product of the psb I Gene in Oxygen-Evolving Photosystem II Complexes from Spinach

Oxygen-evolving photosystem II complexes from spinach, whichlack light-harvesting chlorophyll a/b proteins, were treatedwith a bifunctional crosslinking reagent, hexamethylene-diisocyanate.Identification of crosslinked proteins with antisera raisedagainst various constituent proteins of the oxygen-evolvingPS II complex showed that the extrinsic 33 kDa protein is locatedless than 11 Å from the 9.4 kDa subunit of cytochromeb 559 and the 4.8 kDa product of psb I gene. (Received October 14, 1991; Accepted February 6, 1992)     

EVIDENCE FOR A LACK OF PHOTOSYSTEM SEGREGATION IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE)

The distribution of photosystems I and II (PSI and PSII) in cells of Chlamydomonas reinhardtii Dangeard was studied by immunogold electron microscopy using cultures grown autotrophically at moderate irradiance and harvested in the middle of the light period. Sections of Lowicryl-embedded cells were labeled with monospecific heterologous antisera raised against the reaction center proteins of PSI (CP1-e) or the core antenna proteins of PSII (CP40 and CP47). All three antisera labeled both the appressed and the nonappressed thylakoid membranes at essentially similar densities. Labeling with both PSI and PSII antisera was slightly more concentrated over the outer nonappressed membranes of the thylakoid bands (1.7- to 2.4-fold with anti-CP1- e and 1.5- to 1.8-fold with anti-CP47 and anti-CP40). However, since appressed membranes comprised 73% of the total thylakoid membranes, 50%–62% of the PSI and 58%–65% of the PSII labeling were localized on appressed membranes. We conclude that photosystem distribution in C. reinhardtii is similar to that reported for other algae and different from the lateral heterogeneity observed in higher plants.     

Association of fucoxanthin chlorophyll a/c-binding polypeptides with photosystems and phosphorylation in the centric diatom Cyclotella cryptica

Solubilization of thylakoid membranes of Cyclotella cryptica with dodecyl-beta maltoside followed by sucrose density gradient centrifugation or deriphate polyacrylamide gel electrophoresis resulted in the isolation of pigment protein complexes. These complexes were characterized by absorption and fluorescence spectroscopy, sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western immunoblotting using antisera against fucoxanthin chlorophyll a/c-binding proteins and the reaction center protein D2 of photosystem II. Sucrose density gradient centrifugation yielded four bands. Band 1 consisted of free pigments with minor amounts of fucoxanthin chlorophyll a/c-binding proteins. Bands 2, 3, and 4 represented a major fucoxanthin chlorophyll a/c-binding protein fraction, photosystem II, and photosystem I, respectively. Deriphate polyacrylamide gel electrophoresis gave rise to five bands, representing photosystem I, photosystem II, two fucoxanthin chlorophyll a/c-binding protein complexes, and a band mostly consisting of free pigments. In the Western immunoblotting experiments, the specific association of two fucoxanthin chlorophyll a/c-binding proteins, Fcp2 and Fcp4, to the photosystems could be demonstrated. In vivo experiments using antibodies against phosphothreonine residues and in vitro studies using [gamma-32P]ATP showed that fucoxanthin chlorophyll a/c binding-proteins of 22 kDa became phosphorylated.     

Immunological Quantification of Proteins Related to Light-Harvesting Chlorophyll a/b Protein Complexes of the Two Photosystems in Rice Mutants Totally and Partially Deficient in Chlorophyll b

Ten rice chlorina mutants of Type I, which totally lack chlorophyllb and hence are unable to synthesize light-harvesting chlorophylla/b protein complexes of photosystem II (LHC-II), containedmRNA for proteins related to LHC-II. Immunoblotting with anantiserum, which had been raised against the 24 and 25 kDa apoproteinsof LHC-II and found to cross-react with the 26 kDa protein ofLHC-II and the 20 and 21 kDa apoproteins of light-harvestingchlorophyll a/b protein complexes of photosystem I (LHC-I),revealed that all the five proteins related to LHC-Iand LHC-IIwere present in reduced amounts in the Type I mutants. ThreeType HA mutants, which have a chlorophyll a/b ratio of 10, weremore abundant in the apoproteins, while three Type IIB mutantswith the ratio of 15 were heterogeneous in terms of the apoproteincontent. All the chlorina mutants contained less P700 comparedwith the wild type rice, but were relatively more abundant inthe LHC-I proteins than the LHC-II proteins. The results showthat all the rice chlorina strains are mutants of chlorophyllb synthesis and the deficiency of chlorophyll b differentlyaffects accumulation of the apoproteins of LHC-I and LHC-II.To balance light absorption between the two photosystem, lossof LHC-II is partly counter-balanced by a decrease in the numberof PSI complexes in the mutants. (Received January 21, 1988; Accepted April 28, 1988)     

Photosynthetic pigment localization and thylakoid membrane morphology are altered in Synechocystis 6803 phycobilisome mutants

Cyanobacteria are oxygenic photosynthetic prokaryotes that are the progenitors of the chloroplasts of algae and plants. These organisms harvest light using large membrane-extrinsic phycobilisome antenna in addition to membrane-bound chlorophyll-containing proteins. Similar to eukaryotic photosynthetic organisms, cyanobacteria possess thylakoid membranes that house photosystem (PS) I and PSII, which drive the oxidation of water and the reduction of NADP+, respectively. While thylakoid morphology has been studied in some strains of cyanobacteria, the global distribution of PSI and PSII within the thylakoid membrane and the corresponding location of the light-harvesting phycobilisomes are not known in detail, and such information is required to understand the functioning of cyanobacterial photosynthesis on a larger scale. Here, we have addressed this question using a combination of electron microscopy and hyperspectral confocal fluorescence microscopy in wild-type Synechocystis species PCC 6803 and a series of mutants in which phycobilisomes are progressively truncated. We show that as the phycobilisome antenna is diminished, large-scale changes in thylakoid morphology are observed, accompanied by increased physical segregation of the two photosystems. Finally, we quantified the emission intensities originating from the two photosystems in vivo on a per cell basis to show that the PSI:PSII ratio is progressively decreased in the mutants. This results from both an increase in the amount of photosystem II and a decrease in the photosystem I concentration. We propose that these changes are an adaptive strategy that allows cells to balance the light absorption capabilities of photosystems I and II under light-limiting conditions.     

Antibodies to the photosystem I chlorophyll a + b antenna cross-react with polypeptides of CP29 and LHCII

A chlorophyll (a + b)--protein complex associated with photosystem I (PSI) was isolated from a larger PSI complex (CPIa) produced by electrophoresis of barley thylakoids solubilized with 300 mM octyl glucoside. It had an apparent Mr of 35,000-43,000 on 7.5% and 10% acrylamide gels respectively, and a chlorophyll a/b ratio of 2.5 +/- 1.5. Denaturation released four polypeptides migrating between 21-24 kDa. They were well separated from the polypeptides of the two photosystem II chlorophyll a + b antenna complexes: LHCII (25-27 kDa) and CP29 (28-29 kDa). In order to study the PSI antenna complex, antibodies were raised against highly purified CPIa. The antigen appeared to be pure when electrophoresed, blotted and reacted with its antiserum, i.e. anti-CPIa detected only the 64-66-kDa CPI apoprotein and the four 21-24 kDa antenna polypeptides. However, when blotted against the whole spectrum of thylakoid proteins, it cross-reacted with both LHCII and CP29 apoproteins. Removal of anti-CPI activity from the anti-CPIa did not affect these cross-reactions, showing that they were not due to antibodies directed against CPI. To show that the same antibody population was reacting with both the photosystem I and photosystem II antenna polypeptides, anti-CPIa was adsorbed onto highly purified CPIa on nitrocellulose. The bound antibody was eluted and used again in a Western blot against whole thylakoid proteins. This selected antibody population showed the same relative strength of reaction with photosystem I and photosystem II antenna polypeptides as the original antibody population had. Similar observations have been made with antibodies to the two photosystem II antenna complexes. We therefore conclude that there are antigenic determinants in common among the chlorophyll a + b binding polypeptides, and predict that there could be amino acid sequence similarities.     

Photosynthetic acclimation: structural reorganisation of light harvesting antenna--role of redox-dependent phosphorylation of major and minor chlorophyll a/b binding proteins

In order to carry out photosynthesis, plants and algae rely on the co-operative interaction of two photosystems: photosystem I and photosystem II. For maximum efficiency, each photosystem should absorb the same amount of light. To achieve this, plants and green algae have a mobile pool of chlorophyll a/b-binding proteins that can switch between being light-harvesting antenna for photosystem I or photosystem II, in order to maintain an optimal excitation balance. This switch, termed state transitions, involves the reversible phosphorylation of the mobile chlorophyll a/b-binding proteins, which is regulated by the redox state of the plastoquinone-mediating electron transfer between photosystem I and photosystem II. In this review, we will present the data supporting the function of redox-dependent phosphorylation of the major and minor chlorophyll a/b-binding proteins by the specific thylakoid-bound kinases (Stt7, STN7, TAKs) providing a molecular switch for the structural remodelling of the light-harvesting complexes during state transitions. We will also overview the latest X-ray crystallographic and electron microscopy-derived models for structural re-arrangement of the light-harvesting antenna during State 1-to-State 2 transition, in which the minor chlorophyll a/b-binding protein, CP29, and the mobile light-harvesting complex II trimer detach from the light-harvesting complex II-photosystem II supercomplex and associate with the photosystem I core in the vicinity of the PsaH/L/O/P domain.     

Molecular analysis of the Chlamydomonas nuclear gene encoding PsbW and demonstration that PsbW is a subunit of photosystem II,but not photosystem I

PsbW is a nuclear-encoded protein located in the thylakoid membrane of the chloroplast. Studies in higher plants have provided substantial evidence that PsbW is a core component of photosystem II. However, recent data have been presented to suggest that PsbW is also a subunit of photosystem I. Such a sharing of subunits between the two photosystems would represent a novel phenomenon. To investigate this, we have cloned and characterized the psbW gene from the green alga Chlamydomonas reinhardtii. The gene is split by five introns and encodes a polypeptide of 115 residues comprising the 6.1 kDa mature PsbW protein preceded by a 59 amino acid bipartite transit sequence. Using antibodies raised to PsbW we have examined: (1) C. reinhardtii mutants lacking either photosystem and (2) purified photosystem preparations. We find that PsbW is a subunit of photosystem II, but not photosystem I.     

Some aspects of the immune response of koalas (Phascolarctos cinereus) and in vitro neutralization of chlamydia psittaci (koala strains)

Abstract Western-blot analysis was used to study the reaction of koala antisera, two specific polyclonal antibodies and one monoclonal antibody, with chlamydial antigens in koalas infected with Chlamydia psittaci . The koala sera recognized four C. psittaci surface antigens, corresponding to the major outer membrane protein (39.5 kDa), 31 kDa protein, 18 kDa protein and lipopolysaccharide. The S25-23 LPS specific monoclonal antibody inhibited chlamydial infection (55–67%) with both koala strains (type I and type II). Both koala antiserum and rabbit polyclonal antibodies against either type of chlamydia significantly reduced the number of infected cells resulting from type II infections at a dilution of 1 in 20. Rabbit antiserum against type II was effective in neutralizing infection by type II elementary bodies, but was less effective against type I infection. In addition, no koala antiserum was effective in neutralizing type I infection.     

Purification of collagen-binding proteins ofLactobacillus reuteri NCIB 11951

Collagen type-I-binding proteins ofLactobacillus reuteri NCIB 11951 were purified. The cell surface proteins were affinity purified on collagen Sepharose and eluted with an NaCl gradient. Two protein bands were eluted from the column (29 kDa and 31 kDa), and both bound radio-labeled collagen type I. Rabbit antisera raised against the 29 kDa and 31 kDa protein reacted with the affinity-purified proteins in a Western blot with whole-cell extract used as antigen. The N-terminal sequence of the 29-kDa and 31-kDa proteins demonstrated the closest homologies with internal sequences from anEscherichia coli trigger factor protein (TIG.ECOLI). Out of nine other lactobacilli, the antisera reacted only with theL. reuteri and not with the other species tested.     

Antisera specific for rap 1 proteins distinguish between processed and nonprocessed rap 1b

Polyclonal antisera were generated against synthetic peptides corresponding to distinct regions of the rap 1 protein sequences. A "rap 1-common" antiserum, prepared against an 18-amino acid segment of the rap 1a protein near the proposed GTP-binding region, reacted with both rap 1a and rap 1b recombinant proteins expressed in Escherichia coli and with two low molecular weight GTP-binding proteins of 22 and 24 kDa in unstimulated human platelets. An antiserum raised against a carboxyl-terminal peptide of rap 1b containing the putative site of post-translational processing reacted strongly with bacterial-expressed recombinant rap 1b and with a 24-kDa GTP-binding protein in platelets, but not with recombinant rap 1a or a 22-kDa GTP-binding protein. The mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this rap 1b immunoreactive protein coincided with that of bacterial-expressed rap 1b and not with the faster migrating form of rap 1b that incorporates radioactivity from [3H]mevalonic acid in the insect/baculovirus system. This suggests that our rap 1b-specific antiserum recognizes only one form of rap 1b, that which has not undergone carboxyl-terminal post-translational processing.     

A red-shifted antenna protein associated with photosystem II in Physcomitrella patens

Antenna systems of plants and green algae are made up of pigment-protein complexes belonging to the light-harvesting complex (LHC) multigene family. LHCs increase the light-harvesting cross-section of photosystems I and II and catalyze photoprotective reactions that prevent light-induced damage in an oxygenic environment. The genome of the moss Physcomitrella patens contains two genes encoding LHCb9, a new antenna protein that bears an overall sequence similarity to photosystem II antenna proteins but carries a specific motif typical of photosystem I antenna proteins. This consists of the presence of an asparagine residue as a ligand for Chl 603 (A5) chromophore rather than a histidine, the common ligand in all other LHCbs. Asparagine as a Chl 603 (A5) ligand generates red-shifted spectral forms associated with photosystem I rather than with photosystem II, suggesting that in P. patens, the energy landscape of photosystem II might be different with respect to that of most green algae and plants. In this work, we show that the in vitro refolded LHCb9-pigment complexes carry a red-shifted fluorescence emission peak, different from all other known photosystem II antenna proteins. By using a specific antibody, we localized LHCb9 within PSII supercomplexes in the thylakoid membranes. This is the first report of red-shifted spectral forms in a PSII antenna system, suggesting that this biophysical feature might have a special role either in optimization of light use efficiency or in photoprotection in the specific environmental conditions experienced by this moss.     

Identification of two minor subunits in the pilus of Haemophilus influenzae.

Haemophilus influenzae type b (Hib) organisms produce pili, which mediate attachment to human cells and are multimeric structures composed of a 24-kDa subunit called pilin or HifA. Although pili from other organisms contain additional proteins accessory to pilin, no structural components other than pilin have been identified in Hib pili. Previous analysis of a Hib pilus gene cluster, however, suggested that two genes, hifD and hifE, may encode additional pilus subunits. To determine if hifD and hifE encode pilus components, the genes were overexpressed in Escherichia coli and the resulting proteins were purified and used to raise polyclonal antisera. Antisera raised against C-terminal HifD and HifE fragments reacted with H. influenzae HifD and HifE proteins, respectively, on Western immunoblots. Western immunoblot analysis of immunoprecipitated Hib pili demonstrated that HifD and HifE copurified with pili. In enzyme-linked immunosorbent assays, antisera raised against a recombinant HifE protein that contained most of the mature protein reacted more to piliated Hib than to nonpiliated Hib or to a mutant containing a hifE gene insertion. Immunoelectron microscopy confirmed that the HifE antiserum bound to pili and demonstrated that the antiserum bound predominantly to the pilus tips. These data indicate that HifD and HifE are pilus subunits. Adherence inhibition studies demonstrated that the HifE antiserum completely blocked pilus-mediated hemagglutination, suggesting that HifE mediates pilus adherence.     

Extrinsic proteins of photosystem II: an intermediate member of PsbQ protein family in red algal PS II.

The oxygen-evolving photosystem II (PS II) complex of red algae contains four extrinsic proteins of 12 kDa, 20 kDa, 33 kDa and cyt c-550, among which the 20 kDa protein is unique in that it is not found in other organisms. We cloned the gene for the 20-kDa protein from a red alga Cyanidium caldarium. The gene consists of a leader sequence which can be divided into two parts: one for transfer across the plastid envelope and the other for transfer into thylakoid lumen, indicating that the gene is encoded by the nuclear genome. The sequence of the mature 20-kDa protein has low but significant homology with the extrinsic 17-kDa (PsbQ) protein of PS II from green algae Volvox Carteri and Chlamydomonas reinhardtii, as well as the PsbQ protein of higher plants and PsbQ-like protein from cyanobacteria. Cross-reconstitution experiments with combinations of the extrinsic proteins and PS IIs from the red alga Cy. caldarium and green alga Ch. reinhardtii showed that the extrinsic 20-kDa protein was functional in place of the green algal 17-kDa protein on binding to the green algal PS II and restoration of oxygen evolution. From these results, we conclude that the 20-kDa protein is the ancestral form of the extrinsic 17-kDa protein in green algal and higher plant PS IIs. This provides an important clue to the evolution of the oxygen-evolving complex from prokaryotic cyanobacteria to eukaryotic higher plants. The gene coding for the extrinsic 20-kDa protein was named psbQ' (prime).     

A collagen binding protein from Lactobacillus reuteri is part of an ABC transporter system?

Abstract The gene coding for a collagen binding protein from Lactobacillus reuteri NCIB 11951 was cloned and sequenced. A genomic lambda library was constructed and recombinant plaques were screened using antisera raised against purified collagen binding proteins from the same L. reuteri strain. The positive plaques were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis, which revealed the expression of a 29 kDa protein, which reacted with the antisera and bound ¹²⁵I-labelled type I collagen. The sequence of the corresponding gene, cnb showed that the collagen binding protein has sequence similarities to the solute binding component of bacterial ABC transporters.     

Isolation of the D1 Protein and a 28-kDa Fragment of the D2 Protein from Spinach Photosystem II Membranes

The 32-kDa D1 protein, which contained no lysine in spinachchloroplasts, as deduced from its DNA code, was isolated byhigh-performance gel permeation chromatography in the presenceof 0.1% SDS and 4 M urea. Three proteins of the photosystemII reaction center complex have a molecular mass of 30–35kDa, and two, the D2 protein and the peripheral 33-kDa protein,were severed into peptide fragments by Achromobacter lysyl endopeptidase(EC 3.4.14.50 [EC] ) before the chromatography. The isolated D1 proteindid not contain chlorophylls and pheophytins but had an absorptionmaximum at 265 nm probably due to bound plastoquinone. A peptidefragment of 28 kDa from the D2 protein was also isolated fromspinach photosystem II membranes and the wheat photosystem IIreaction center. Antibodies raised against the 28-kDa peptidefrom wheat bound to the 34-kDa D2 protein, which suggested thatthis peptide was the largest sequence of Aspl4-Lys265. The fragmentof wheat D2 protein showed absorption maxima at 413 and 682nm attributable to bound pheophytin that probably had been convertedfrom chlorophyll a during the isolation process. (Received June 29, 1987; Accepted October 21, 1987)     

Light-dependent accumulation and localization of photosystem II proteins in maize

We have raised antibodies against several major components of photosystem II. These antisera, which are directed against the apoproteins of two chlorophyll-binding proteins (CPa-1 and CPa-2), the apoprotein of light-harvesting complex II and the 33-kDa extrinsic protein of the oxygen-evolving complex, were used to examine the light regulation of photosystem II assembly in maize. The principal findings of this study are as follows. The 33-kDa protein is present in dark-grown maize and the content increases 5-10-fold upon illumination. The level of the protein is mediated at least in part by phytochrome and is independent of the accumulation of chlorophyll. In contrast, none of the three chlorophyll-binding proteins examined was detectable in leaves of maize grown in darkness or under other light regimes where chlorophyll does not accumulate. Even in the absence of photosystem II assembly, the 33-kDa protein is properly transported across the thylakoid into the lumen. However, the protein does not attach in the normal way to the inner surface of the membrane under these conditions.     

Comparison of binding and functional properties of two extrinsic components,Cyt c550 and a 12 kDa protein,in cyanobacterial PSII with those in red algal PSII

Cyt c550 and 12 kDa protein are two extrinsic proteins of photosystem II (PSII) found in cyanobacteria and some eukaryotic algae. The binding patterns of these two extrinsic proteins are different between cyanobacterial (Thermosynechococcus vulcanus) and red algal (Cyanidium caldarium) PSIIs [Shen and Inoue (1993) Biochemistry 32: 1825; Enami et al. (1998) Biochemistry 39: 2787]. In order to elucidate the possible causes responsible for these differences, we first cloned the psbV gene encoding Cyt c550 from a red alga, Cyanidium caldarium, which was compared with the homologous sequences from other organisms. Cross-reconstitution experiments were then performed with different combinations of the extrinsic proteins and the cyanobacterial or red algal PSII. (1). Both the cyanobacterial and red algal Cyt c550 bound directly to the cyanobacterial PSII, whereas none of them bound directly to the red algal PSII, indicating that direct binding of Cyt c550 to PSII principally depends on the structure of PSII intrinsic proteins but not that of Cyt c550 itself. (2). Cyt c550 was functionally exchangeable between the red algal and the cyanobacterial PSII, and the red algal 12 kDa protein functionally bound to the cyanobacterial PSII, whereas the cyanobacterial 12 kDa protein did not bind to the red algal PSII. (3). The antibody against the cyanobacterial or red algal 12 kDa protein reacted with its original one but not with the homologous protein from the other organism, whereas the antibody against the red algal Cyt c550 reacted with both cyanobacterial and red algal Cyt c550. These results imply that the structure and function of Cyt c550 have been largely conserved, whereas those of the 12 kDa protein have been changed, in the two organisms studied here.