Structural basis for the assembly and electron transport mechanisms of the dimeric photosynthetic RC-LH1 supercomplex

The reaction center (RC) and light-harvesting complex 1 (LH1) form a RC-LH1 core supercomplex that is vital for the primary reactions of photosynthesis in purple photosynthetic bacteria. Some species possess the dimeric RC-LH1 complex with an additional polypeptide PufX, representing the largest photosynthetic complex in anoxygenic phototrophs. However, the details of the architecture and assembly mechanism of the RC-LH1 dimer are unclear. Here we report seven cryo-electron microscopy (cryo-EM) structures of RC-LH1 supercomplexes from Rhodobacter sphaeroides. Our structures reveal that two PufX polypeptides are positioned in the center of the S-shaped RC-LH1 dimer, interlocking association between the components and mediating RC-LH1 dimerization. Moreover, we identify a new transmembrane peptide, designated PufY, which is located between the RC and LH1 subunits near the LH1 opening. PufY binds a quinone molecule and prevents LH1 subunits from completely encircling the RC, creating a channel for quinone/quinol exchange. Genetic mutagenesis, cryo-EM structures, and computational simulations enable a mechanistic understanding of the assembly and electron transport pathways of the RC-LH1 dimer and elucidate the roles of individual components in ensuring the structural and functional integrity of the photosynthetic supercomplex.

Genome Sequence of the Unusual Purple Photosynthetic Bacterium Phaeovibrio sulfidiphilus, Only Distantly Related to Rhodospirillaceae, Reveals Unique Genes for Respiratory Nitrate Reduction and Glycerol Metabolism

ABSTRACT Phaeovibrio sulfidiphilus was reported to be a divergent member of the purple photosynthetic bacteria with limited ability to metabolize organic compounds. Whole-genome-based analysis shows that it is indeed only distantly related to freshwater species of Rhodospirillaceae. Unexpectedly, the genome contains unique gene clusters for potential respiratory nitrate reduction and anaerobic glycerol metabolism.

Peptide-Mediated Gene Transfer into Marine Purple Photosynthetic Bacteria

Use of photosynthetic organisms is one of the sustainable ways to produce high-value products. Marine purple photosynthetic bacteria are one of the research focuses as microbial production hosts. Genetic transformation is indispensable as a biotechnology technique. However, only conjugation has been determined to be an applicable method for the transformation of marine purple photosynthetic bacteria so far. In this study, for the first time, a dual peptide-based transformation method combining cell penetrating peptide (CPP), cationic peptide and Tat-derived peptide (dTat-Sar-EED) (containing D-amino acids of Tat and endosomal escape domain (EED) connected by sarcosine linkers) successfully delivered plasmid DNA into Rhodovulum sulfidophilum, a marine purple photosynthetic bacterium. The plasmid delivery efficiency was greatly improved by dTat-Sar-EED. The concentrations of dTat-Sar-EED, cell growth stage and recovery duration affected the efficiency of plasmid DNA delivery. The delivery was inhibited at 4 °C and by A22, which is an inhibitor of the actin homolog MreB. This suggests that the plasmid DNA delivery occurred via MreB-mediated energy dependent process. Additionally, this peptide-mediated delivery method was also applicable for E. coli cells. Thus, a wide range of bacteria could be genetically transformed by using this novel peptide-based transformation method.

Tuning the Photophysical Features of Self-Assembling Photoactive Polypeptides for Light-Harvesting

The LH1 complex is the major light-harvesting antenna of purple photosynthetic bacteria. Its role is to capture photons, and then store them and transfer the excitation energy to the photosynthetic reaction center. The structure of LH1 is modular and it cooperatively self-assembles from the subunits composed of short transmembrane polypeptides that reversibly bind the photoactive cofactors: bacteriochlorophyll and carotenoid. LH1 assembly, the intra-complex interactions and the light-harvesting features of LH1 can be controlled in micellar media by varying the surfactant concentration and by adding carotenoid and/or a co-solvent. By exploiting this approach, we can manipulate the size of the assembly, the intensity of light absorption, and the energy and lifetime of its first excited singlet state. For instance, via the introduction of Ni-substituted bacteriochlorophyll into LH1, the lifetime of this electronic state of the antenna can be shortened by almost three orders of magnitude. On the other hand, via the exchange of carotenoid, light absorption in the visible range can be tuned. These results show how in a relatively simple self-assembling pigment-polypeptide system a sophisticated functional tuning can be achieved and thus they provide guidelines for the construction of bio-inspired photoactive nanodevices.