An integrated approach towards extracting structural characteristics of chlorosomes from a bchQ mutant of Chlorobaculum tepidum .

Chlorosomes, the photosynthetic antenna complexes of green sulfur bacteria, are paradigms for light-harvesting elements in artificial designs, owing to their efficient energy transfer without protein participation. We combined magic angle spinning (MAS) NMR, optical spectroscopy and cryogenic electron microscopy (cryo-EM) to characterize the structure of chlorosomes from a bchQ mutant of Chlorobaculum tepidum . The chlorosomes of this mutant have a more uniform composition of bacteriochlorophyll (BChl) with a predominant homolog, [8Ethyl, 12Ethyl] BChl c , compared to the wild type (WT). Nearly complete 13 C chemical shift assignments were obtained from well-resolved homonuclear 13 C- 13 C RFDR data. For proton assignments heteronuclear 13 C- 1 H (hCH) data sets were collected at 1.2 GHz spinning at 60 kHz. The CHHC experiments revealed intermolecular correlations between 13 2 /3 1 , 13 2 /3 2 , and 12 1 /3 1 , with distance constraints of less than 5 Å. These constraints indicate the syn - anti parallel stacking motif for the aggregates. Fourier transform cryo-EM data reveal an axial repeat of 1.49 nm for the helical tubular aggregates, perpendicular to the inter-tube separation of 2.1 nm. This axial repeat is different from WT and is in line with BChl syn - anti stacks running essentially parallel to the tube axis. Such a packing mode is in agreement with the signature of the Q y band in circular dichroism (CD). Combining the experimental data with computational insight suggests that the packing for the light-harvesting function is similar between WT and bchQ , while the chirality within the chlorosomes is modestly but detectably affected by the reduced compositional heterogeneity in bchQ .