The Type I Homodimer Reaction Center in Heliobacterium Modesticaldum View Homepage


Ontology type: schema:MonetaryGrant     


Grant Info

YEARS

2019-2022

FUNDING AMOUNT

2951138.0 USD

ABSTRACT

Photosystems, the protein-pigment complexes that allow photosynthetic organisms to harvest light energy and convert it into a biologically useful form of chemical energy, first appeared on this planet over 3 billion years ago and have since diversified so that they are present in very different forms of life. They are divided into two groups based on their composition and types of electron acceptor: type I photosystems (like Photosystem I in plants, algae and cyanobacteria) contain iron-sulfur clusters and reduce soluble low-potential proteins, such as ferredoxins, while type II photosystems (like Photosystem II) contain pheophytin and reduce membrane-soluble quinones. Photosystem I (PSI) is heterodimeric and has been the exemplar of Type I photosystems, but the other members of this group are all homodimeric. Our understanding of these homodimeric photosystems is rudimentary, but what we have discovered indicates that they function quite differently from PSI, especially in terms of their internal electron transfer cofactors. The heliobacteria are a group of phototrophic anaerobic Gram-positive bacteria that contain a homodimeric Type I photosystem and uniquely use bacteriochlorophyll (BChl) g as their major pigment. Our DOE-funded work has shown that the heliobacterial photosystem (HbRC) lacks a peripheral subunit containing 2 additional iron-sulfur clusters present in PSI. Instead it uses the inter-peptide FXiron-sulfur cluster to reduce low-potential acceptors. We have found that the midpoint potential of the FXcluster (-0.50 V) is slightly more negative than that of the small ferredoxin proteins, allowing it to reduce them directly. We have confirmed previous findings that the embedded quinone is not required for forward electron transfer in the HbRC, in contrast to PSI, and our work indicates that the HbRC can reduce membrane-soluble quinones in the absence of soluble acceptors. These results blur the distinction between Type I and II photosystems. The two key limitations to this project have been the lack of a structural model of the HbRC and the lack of a genetic system for heliobacteria. In the last funding period we reported in Sciencethe structure of the HbRC from Heliobacterium modesticaldum, determined at a resolution of 2.2 Å by X-ray crystallography. During the process, we discovered an additional subunit: PshX, a 31-residue polypeptide consisting of a single transmembrane helix. The HbRC, which exhibits perfect C2 symmetry, is thus a homodimer of two PshA/PshX pairs. On the symmetry axis are the primary electron donor, a pair of BChl g′molecules on the periplasmic side, and the terminal acceptor, the FXcluster on the cytoplasmic side. On each side of the axis are a branch of redox cofactors connecting them composed of an accessory BChl gand a Chl a, the latter of which serves as primary electron acceptor. Unlike all previous RC structures, there is no quinone; instead an unidentified molecule with an isoprenyl tail is located near the Chlaacceptor, although not on the path to FX. As in PSI, the ET domain is formed by the union of the last five helices of the two PshA subunits and is flanked by the two antenna domains formed by the six N-terminal helices of each PshA. There are 54 BChl gmolecules and two C30 carotenoids surrounding the ET domain that presumably serve as antenna pigments. We have also succeeded in creating a genetic system in H. modesticaldum. We can introduce replicating plasmids, allowing us to express genes either of heliobacterial origin or from other species. A major leap forward in the past funding period was development of a working system to delete genes cleanly from the chromosome using its own CRISPR/Cas system. We have now deleted the genes for both subunits of the HbRC; loss of the major subunit (PshA) resulted in no HbRC, while loss of the minor subunit (PshX) had little effect. Expression of mutant versions of PshA on a plasmid in the strain lacking the endogenous gene now allow us to study the effects of modifications of key amino acid residues identified in the structure. A new thrust in the last period has been to work out the pathway of the biosynthesis of BChl g, which was reconstructed in the phototrophic purple bacterium, Rhodobacter sphaeroides. Through a combination of gene introductions and deletions, we were able to convert the bacterium from synthesizing BChl a, the native pigment, to making BChl gnearly exclusively. Our goals for the 3 years of this grant are to: (1) map the light-driven electron transport pathways in heliobacteria by deleting genes encoding components of the pathways; (2) determine the function of the small ferredoxin proteins via biochemical and genetic studies; (3) investigate structural features of the HbRC via X-ray crystallography and targeted mutagenesis; (4) determine the role of the quinone using reconstitution of the HbRC into artificial membranes and biochemical/biophysical work; (5) explore the excited (radical and radical-pair) states of the HbRC via advanced spectroscopic techniques in combination with targeted mutagenesis; and (6) identify factors critical for assembly of the HbRC by deleting genes in H. modesticaldumand expressing the PshA polypeptide in R. sphaeroidescells engineered to synthesize BChl g. Our long-term goal is to bring knowledge of the HbRC to the same level of sophistication as that of the purple bacterial RC and of Photosystems I and II. This fundamental knowledge will provide insight into ways that nature has modified photosynthetic RCs for different purposes over evolutionary time, which should facilitate the engineering of RCs for specific technological purposes. More... »

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Our understanding of these homodimeric photosystems is rudimentary, but what we have discovered indicates that they function quite differently from PSI, especially in terms of their internal electron transfer cofactors. The heliobacteria are a group of phototrophic anaerobic Gram-positive bacteria that contain a homodimeric Type I photosystem and uniquely use bacteriochlorophyll (BChl) g as their major pigment. Our DOE-funded work has shown that the heliobacterial photosystem (HbRC) lacks a peripheral subunit containing 2 additional iron-sulfur clusters present in PSI. Instead it uses the inter-peptide FXiron-sulfur cluster to reduce low-potential acceptors. We have found that the midpoint potential of the FXcluster (-0.50 V) is slightly more negative than that of the small ferredoxin proteins, allowing it to reduce them directly. We have confirmed previous findings that the embedded quinone is not required for forward electron transfer in the HbRC, in contrast to PSI, and our work indicates that the HbRC can reduce membrane-soluble quinones in the absence of soluble acceptors. These results blur the distinction between Type I and II photosystems.  \nThe two key limitations to this project have been the lack of a structural model of the HbRC and the lack of a genetic system for heliobacteria. In the last funding period we reported in Sciencethe structure of the HbRC from Heliobacterium modesticaldum, determined at a resolution of 2.2 Å by X-ray crystallography. During the process, we discovered an additional subunit: PshX, a 31-residue polypeptide consisting of a single transmembrane helix. The HbRC, which exhibits perfect C2 symmetry, is thus a homodimer of two PshA/PshX pairs. 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Our understanding of these homodimeric photosystems is rudimentary, but what we have discovered indicates that they function quite differently from PSI, especially in terms of their internal electron transfer cofactors. The heliobacteria are a group of phototrophic anaerobic Gram-positive bacteria that contain a homodimeric Type I photosystem and uniquely use bacteriochlorophyll (BChl) g as their major pigment. Our DOE-funded work has shown that the heliobacterial photosystem (HbRC) lacks a peripheral subunit containing 2 additional iron-sulfur clusters present in PSI. Instead it uses the inter-peptide FXiron-sulfur cluster to reduce low-potential acceptors. We have found that the midpoint potential of the FXcluster (-0.50 V) is slightly more negative than that of the small ferredoxin proteins, allowing it to reduce them directly. We have confirmed previous findings that the embedded quinone is not required for forward electron transfer in the HbRC, in contrast to PSI, and our work indicates that the HbRC can reduce membrane-soluble quinones in the absence of soluble acceptors. These results blur the distinction between Type I and II photosystems. The two key limitations to this project have been the lack of a structural model of the HbRC and the lack of a genetic system for heliobacteria. In the last funding period we reported in Sciencethe structure of the HbRC from Heliobacterium modesticaldum, determined at a resolution of 2.2 Å by X-ray crystallography. During the process, we discovered an additional subunit: PshX, a 31-residue polypeptide consisting of a single transmembrane helix. The HbRC, which exhibits perfect C2 symmetry, is thus a homodimer of two PshA/PshX pairs. On the symmetry axis are the primary electron donor, a pair of BChl g′molecules on the periplasmic side, and the terminal acceptor, the FXcluster on the cytoplasmic side. On each side of the axis are a branch of redox cofactors connecting them composed of an accessory BChl gand a Chl a, the latter of which serves as primary electron acceptor. Unlike all previous RC structures, there is no quinone; instead an unidentified molecule with an isoprenyl tail is located near the Chlaacceptor, although not on the path to FX. As in PSI, the ET domain is formed by the union of the last five helices of the two PshA subunits and is flanked by the two antenna domains formed by the six N-terminal helices of each PshA. There are 54 BChl gmolecules and two C30 carotenoids surrounding the ET domain that presumably serve as antenna pigments. We have also succeeded in creating a genetic system in H. modesticaldum. We can introduce replicating plasmids, allowing us to express genes either of heliobacterial origin or from other species. A major leap forward in the past funding period was development of a working system to delete genes cleanly from the chromosome using its own CRISPR/Cas system. We have now deleted the genes for both subunits of the HbRC; loss of the major subunit (PshA) resulted in no HbRC, while loss of the minor subunit (PshX) had little effect. Expression of mutant versions of PshA on a plasmid in the strain lacking the endogenous gene now allow us to study the effects of modifications of key amino acid residues identified in the structure. A new thrust in the last period has been to work out the pathway of the biosynthesis of BChl g, which was reconstructed in the phototrophic purple bacterium, Rhodobacter sphaeroides. Through a combination of gene introductions and deletions, we were able to convert the bacterium from synthesizing BChl a, the native pigment, to making BChl gnearly exclusively. Our goals for the 3 years of this grant are to: (1) map the light-driven electron transport pathways in heliobacteria by deleting genes encoding components of the pathways; (2) determine the function of the small ferredoxin proteins via biochemical and genetic studies; (3) investigate structural features of the HbRC via X-ray crystallography and targeted mutagenesis; (4) determine the role of the quinone using reconstitution of the HbRC into artificial membranes and biochemical/biophysical work; (5) explore the excited (radical and radical-pair) states of the HbRC via advanced spectroscopic techniques in combination with targeted mutagenesis; and (6) identify factors critical for assembly of the HbRC by deleting genes in H. modesticaldumand expressing the PshA polypeptide in R. sphaeroidescells engineered to synthesize BChl g. Our long-term goal is to bring knowledge of the HbRC to the same level of sophistication as that of the purple bacterial RC and of Photosystems I and II. 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