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 ¹³C chemical shift assignments were obtained from well-resolved homonuclear ¹³C- ¹³C RFDR data. For proton assignments heteronuclear ¹³C- ¹H (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.