John M. Olson

Chlorophyll Organization and Function in Green Photosynthetic Bacteria

The green photosynthetic bacteria are characterized by the presence of chlorosomes appressed to the cytoplasmic side of the cytoplasmic membrane. The chlorosomes are filled with bacteriochlorophyll (BChl)

Photosynthesis in the Archean era.

c, d or e molecules in a highly aggregated state. The truly “green” bacteria contain mainly BChl c or d ; while the others look orange or brown because of a high content of carotenoid. From a phylogenetic point of view the green bacteria are really two separate “phyla” based on 16s rRNA (1-3), reaction center (RC) type and physiology. It is truly remarkable that such different types of bacteria (green filamentous bacteria and green sulfur bacteria) contain such similar light-harvesting entities as chlorosomes. Green filamentous bacteria (Chloroflexaceae) contain a quinone-type RC similar to those found in purple bacteria (Proteobacteria), whereas the green sulfur bacteria (Chlorobiaceae) contain an iron-sulfur-type RC similar to those found in heliobacteria and in photosystem I of cyanobacteria and chloroplasts. The filamentous bacteria live either as facultative photoautotrophs that grow in relatively bright light or as respiring chemoheterotrophs. They are found predominantly in hot springs, often in mixed population with cyanobacteria that provide organic carbon compounds for them. Most of our knowledge about the filamentous bacteria at the molecular level comes from one species, Chlorojlexus aurantiacus. A second species, Chlorojlexus aggregans, has recently been isolated and characterized by Hanada et al.

Thinking about the evolution of photosynthesis

The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0xa0Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8xa0Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3xa0Ga.

The FMO Protein.

Photosynthesis is an ancient process on Earth. Chemical evidence and recent fossil finds indicate that cyanobacteria existed 2.5–2.6 billion years (Ga) ago, and these were certainly preceded by a variety of forms of anoxygenic photosynthetic bacteria. Carbon isotope data suggest autotrophic carbon fixation was taking place at least a billion years earlier. However, the nature of the earliest photosynthetic organisms is not well understood. The major elements of the photosynthetic apparatus are the reaction centers, antenna complexes, electron transfer complexes and carbon fixation machinery. These parts almost certainly have not had the same evolutionary history in all organisms, so that the photosynthetic apparatus is best viewed as a mosaic made up of a number of substructures each with its own unique evolutionary history. There are two schools of thought concerning the origin of reaction centers and photosynthesis. One school pictures the evolution of reaction centers beginning in the prebiotic phase while the other school sees reaction centers evolving later from cytochrome b in bacteria. Two models have been put forth for the subsequent evolution of reaction centers in proteobacteria, green filamentous (non-sulfur) bacteria, cyanobacteria, heliobacteria and green sulfur bacteria. In the selective loss model the most recent common ancestor of all subsequent photosynthetic systems is postulated to have contained both RC1 and RC2. The evolution of reaction centers in proteobacteria and green filamentous bacteria resulted from the loss of RC1, while the evolution of reaction centers in heliobacteria and green sulfur bacteria resulted from the loss of RC2. Both RC1 and RC2 were retained in the cyanobacteria. In the fusion model the most recent common ancestor is postulated to have given rise to two lines, one containing RC1 and the other containing RC2. The RC1 line gave rise to the reaction centers of heliobacteria and green sulfur bacteria, and the RC2 line led to the reaction centers of proteobacteria and green filamentous bacteria. The two reaction centers of cyanobacteria were the result of a genetic fusion of an organism containing RC1 and an organism containing RC2. The evolutionary histories of the various classes of antenna/light-harvesting complexes appear to be completely independent. The transition from anoxygenic to oxygenic photosynthesis took place when the cyanobacteria learned how to use water as an electron donor for carbon dioxide reduction. Before that time hydrogen peroxide may have served as a transitional donor, and before that, ferrous iron may have been the original source of reducing power.

‘Evolution of Photosynthesis’ (1970), re-examined thirty years later

In this article I review the history of research on the Fenna—Matthews—Olson (FMO) protein with emphasis on my contributions. The FMO protein, which transfers energy from the chlorosome to the reaction center in green sulfur bacteria, was discovered in 1962 and shown to contain bacteriochlorophyll a. From the absorption and circular dichroism spectra, it was clear that there was an exciton interaction between the bacteriochlorophyll molecules. Low temperature spectra indicated a seven-fold exciton splitting of the Qy band. The FMO protein was crystallized in 1964, and the X-ray structure determined in 1979 by B.W. Matthews, R.E. Fenna, M.C. Bolognesi, M.F. Schmidt and J.M. Olson. The structure showed that the protein consisted of three subunits, each containing seven bacteriochlorophyll molecules. The optical spectra were satisfactorily simulated in 1997. In living cells the FMO protein is located between the chlorosome and the reaction centers with the C3 symmetry axis perpendicular to the membrane. The FMO protein may be related to PscA in the reaction center.

Orientation and excitonic interactions of the Fenna-Matthews-Olson bacteriochlorophyll a protein in membranes of the green sulfur bacterium Chlorobium tepidum

I have re-examined my 1970 article ‘Evolution of Photosynthesis’ (Olson JM, Science 168: 438–446) to see whether any of my original proposals still survive. My original conviction that the evolution of photosynthesis was intimately connected with the origin of life has been replaced with the realization that photosynthesis may have been invented by the Bacteria after their divergence from the Archea. The common ancestor of all extant photosynthetic bacteria and cyanobacteria probably contained bacteriochlorophyll a, rather than chlorophyll a as originally proposed, and may have carried out CO2 fixation instead of photoassimilation. The first electron donors were probably reduced sulfur compounds and later ferrous iron. The common ancestor of all extant reaction centers was probably similar to the homodimeric RC1 of present-day green sulfur bacteria (Chlorobiaceae) and heliobacteria. In the common ancestor of proteobacteria and cyanobacteria, the gene for the primordial RC1 was apparently duplicated and one copy split into two genes, one for RC2 and the other for a chlorophyll protein similar to CP43 and CP47 in extant cyanobacteria and chloroplasts. Homodimeric RC1 and homodimeric RC2 functioned in series as in the Z-scheme to deliver electrons from Fe(OH)+ to NADP+, while RC1 and/or RC2 separately drove cyclic electron flow for the production of ATP. In the line of evolution leading to proteobacteria, RC1 and the chlorophyll protein were lost, but RC2 was retained and became heterodimeric. In the line leading to cyanobacteria, both RC1 and RC2 replaced bacteriochlorophyll a with chlorophyll a and became heterodimeric. Heterodimeric RC2 further coevolved with a Mn-containing complex to utilize water as the electron donor for CO2 fixation. The chlorophyll–protein was also retained and evolved into CP43 and CP47. Heliobacteria are the nearest photosynthetic relatives of cyanobacteria. The branching order of photosynthetic genes appears to be (1) proteobacteria, (2) green bacteria (Chlorobiaceae plus Chloroflexaceae), and (3) heliobacteria plus cyanobacteria.

The FMO protein is related to PscA in the reaction center of green sulfur bacteria

Linear and circular dichroism spectra of isolated bacteriochlorophyll a proteins (FMO proteins) and membrane vesicles containing FMO protein from the green sulfur bacterium Chlorobium tepidum were measured at room temperature and 77 K. The orientation of membranes and isolated FMO protein was obtained by gel squeezing. Linear dichroism (LD) data indicate that isolated FMO protein and membrane vesicles associated with the FMO protein are oriented in a similar way in a squeezed polyacrylamide gel. Both samples show a characteristic negative LD band around 814 nm with flanking positive bands at 802 and 824 nm ascribed to the Qy excitonic transitions of BChl a of the FMO protein. This confirms that the C3 symmetry axis of the trimer is perpendicular to the membrane plane, which is supported by the model of the disc-like structure of FMO protein trimers of Cb. tepidum [Li Yi-Fen, Zhou W, Blankenship RE, and Allen JP (1997) J Mol Biol 272: 456–471]. The LD data are consistent with either BChl 3 or 6, but not 7 as the principal contributor to the low temperature band at 825 nm. The low temperature linear and circular dichroism spectra of FMO protein trimers from Chlorobium tepidum show significant differences from the low temperature LD and CD spectra of FMO protein trimers from Prosthecochloris aestuarii. The data are interpreted in terms of somewhat different pigment-protein and pigment-pigment interactions in the two complexes.

Iron‐Sulfur‐Type Reaction Centers Introduction

The Fenna–Matthews–Olson protein is a water-soluble protein found only in green sulfur bacteria. Each subunit contains seven bacteriochlorophyll (BChl) a molecules wrapped in a string bag of protein consisting of mostly β sheet. Most other chlorophyll-binding proteins are water-insoluble proteins containing membrane-spanning α helices. We compared an FMO consensus sequence to well-characterized, membrane-bound chlorophyll-binding proteins: L & M (reaction center proteins of proteobacteria), D1 & D2 (reaction center proteins of PS II), CP43 & CP47 (core proteins of PS II), PsaA & PsaB (reaction center proteins of PS I), PscA (reaction center protein of green sulfur bacteria), and PshA (reaction center protein of heliobacteria). We aligned the FMO sequence with the other sequences using the PAM250 matrix modified for His binding-site identities and found a signature sequence (LxHHxxxGxFxxF) common to FMO and PscA. (The two His residues are BChl a. binding sites in FMO.) This signature sequence is part of a 220-residue C-terminal segment with an identity score of 13%. PRSS (Probability of Random Shuffle) analysis showed that the 220-residue alignment is better than 96% of randomized alignments. This evidence supports the hypothesis that FMO protein is related to PscA.

Green Bacteria: The Light-Harvesting Chlorosome

All chlorophyll (Chl)*-containing photosynthetic organisms utilize photochemical reaction centers (RCs) to carry out the initial charge separation reactions. These RCs fall into two groups: the quinone-type and the iron-sulfur (Fe-S) type. The quinone type (represented by the RCs of purple bacteria, filamentous green bacteria and photosystem [PSI-2 of cyanobacteria and chloroplasts) contains a special pair of Chl molecules (bacteriochlorophyll [BChl] a, BChl b or Chl a ) that serve as the primary electron donor. The quinone type also utilizes a pheophytin molecule (bacteriopheophytin [Bpheo] a, Bpheo b or pheophytin [Pheo] a ) as primary electron acceptor and two quinone molecules (ubiqinone, menaqinone or plastoquinone) as secondary electron acceptors. The Fe-S-type of RC is represented by the RCs of green sulfur bacteria, heliobacteria and PS1 of cyanobacteria and chloroplasts. Like the quinone type of RC the Fe-S type contains a special pair of Chl molecules (BChl a, BChl g or Chl a ) that serve as the primary electron donor. Unlike the quinone type the Fe-S type utilizes another Chl molecule (BChl 663, 8I-OH-Chl a, or Chl a ) as the primary electron acceptor and three Fe-S centers as secondary electron acceptors. A quinone molecule may or may not serve as an intermediate carrier between primary electron acceptor (Chl) and secondary acceptors (Fe-S centers). The quinone-type RCs are made up of at least two homologous polypeptides of about 30-40 kDa. These are called L and M in purple and filamentous bacteria and D1 and D2 in cyanobacteria and chloroplasts. These proteins form heterodimers, bind the Chl, Pheo and quinone cofactors and organize the RC in the photosynthetic membrane. The Fe-S-type RC can be made up of several subunits, but only three subunits bind the electron transport cofactors. There are two large polypeptides (65-83 kDa) and one smaller polypeptide (9-22 kDa). In the green sulfur bacteria

Is FMO-protein related to PscA in the reaction center of green sulfur bacteria?

Chlorosomes are the main light-harvesting structure of green filamentous bacteria and green sulfur bacteria and are filled with aggregated bacteriochlorophyll (BChl) c , d , or e . BChl a is associated with a small protein in the envelope of the chlorosome, whereas the other BChls are organized in rod-like structures located in the interior of the chlorosome and contain little or no protein. Chlorosomes are appressed to the cytoplasmic side of the cytoplasmic membrane, and the main path of excitation energy transfer can be written as BChl c , d , or e ( chlorosome ) → BChl a ( chlorosome ) → BChl a ( Fenna – Matthews – Olson , FMO or membrane proteins ) → BChl a ( reaction center , RC )