Alternating syn-anti bacteriochlorophylls form concentric helical nanotubes in chlorosomes.

Chlorosomes are the largest and most efficient light-harvesting antennae found in nature, and they are constructed from hundreds of thousands of self-assembled bacteriochlorophyll (BChl) c, d, or e pigments. Because they form very large and compositionally heterogeneous organelles, they had been the only photosynthetic antenna system for which no detailed structural information was available. In our approach, the structure of a member of the chlorosome class was determined and compared with the wild type (WT) to resolve how the biological light-harvesting function of the chlorosome is established. By constructing a triple mutant, the heterogeneous BChl c pigment composition of chlorosomes of the green sulfur bacteria Chlorobaculum tepidum was simplified to nearly homogeneous BChl d. Computational integration of two different bioimaging techniques, solid-state NMR and cryoEM, revealed an undescribed syn-anti stacking mode and showed how ligated BChl c and d self-assemble into coaxial cylinders to form tubular-shaped elements. A close packing of BChls via pi-pi stacking and helical H-bonding networks present in both the mutant and in the WT forms the basis for ultrafast, long-distance transmission of excitation energy. The structural framework is robust and can accommodate extensive chemical heterogeneity in the BChl side chains for adaptive optimization of the light-harvesting functionality in low-light environments. In addition, syn-anti BChl stacks form sheets that allow for strong exciton overlap in two dimensions enabling triplet exciton formation for efficient photoprotection.

Specific inhibition of antenna bacteriochlorophyll synthesis in Chlorobium vibrioforme by anesthetic gases.

The green sulfur bacterium Chlorobium vibrioforme contains two types of bacteriochlorophyll (Bchl). The minor pigment, Bchl a, is associated primarily with the cell membrane and its reaction centers; and the major light-harvesting antenna pigment, Bchl d, is found primarily in the chlorosomes, which are attached to the inner surface of the cell membrane. Anesthetic gases, such as N2O, ethylene, and acetylene, were found to inhibit the synthesis of Bchl d, but not of Bchl a, thus allowing the cells to grow at high light intensities with a greatly diminished content of antenna pigment. Chlorosomes were absent or sparse in inhibited cells. Porphyrins accumulated in the inhibited cells. The major one was identified as the Bchl precursor magnesium-protoporphyrin IX monomethyl ester (Mg-PPME) by comparative absorption and fluorescence spectroscopy and thin-layer chromatography of the porphyrin and its derivatives with those of authentic protoporphyrin IX. Small amounts of Mg-PPME were present in control cells, but the addition of inhibitor caused a rapid increase in the Mg-PPME concentration, accompanying the inhibition of Bchl d synthesis. Cells grown in the presence of ethephon (as a source of ethylene) and allowed to stand in dim light for long periods accumulated large amounts of PPME and other porphyrins and excreted or released porphyrins, which accumulated as a brown precipitate in the culture. Inhibition of Bchl d synthesis was relieved upon removal of the inhibitor. These results suggest that the gases act at a step in pigment biosynthesis that affects the utilization of Mg-PPME for isocyclic ring formation. Synthesis of Bchl d and Bchl a may be differentially affected by the gases because of compartmentation of their biosynthetic apparatus or because competition for precursors favors Bchl a synthesis. An ethephon-resistant mutant strain was isolated by selection for growth in dim, long-wavelength light. The mutant cells were also resistant to acetylene, but not to N2O. The ability to reversibly generate viable Chlorobium cells that lack antenna pigments may be useful in photosynthesis research. The ethephon- and acetylene-resistant strain may be useful in the study of the enzymes and genes that are involved in the biosynthetic step that the gases affect.

Role of the core region of the PufX protein in inhibition of reconstitution of the core light-harvesting complexes of Rhodobacter sphaeroides and Rhodobacter capsulatus.

PufX, the protein encoded by the pufX gene of Rhodobacter capsulatus and Rhodobacter sphaeroides, has been further characterized. The mature forms of these proteins contain 9 and 12 fewer amino acids, respectively, at the C-terminal end of the protein than are encoded by their pufX genes. To identify the portion of PufX responsible for inhibition of LH1 formation in reconstitution experiments, different regions (N-terminus and several core regions containing different lengths of the C-terminus) of Rb. sphaeroides and Rb. capsulatus PufX were chemically synthesized. Neither the N- nor C-terminal polypeptides of Rb. sphaeroides were inhibitory to LH1 reconstitution. However, all core segments were active, causing 50% inhibition at a concentration ratio of between 3:1 and 6:1 relative to the LH1 alpha-polypeptides whose concentrations were 3-4 microM. CD measurements indicated that the core segment containing 39 amino acids of Rb. sphaeroides PufX exhibited 47% alpha-helix in trifluoroethanol while the core segment containing 43 amino acids of Rb. capsulatus PufX exhibited 59 and 55% alpha-helix in trifluoroethanol and in 0.80% octylglucoside in water, respectively. Approximately 50% alpha-helix was also indicated by a PHD (Burkhard-Rost) structure prediction. Binding of bacteriochlorophyll to these PufX core segments is implicated.