Role of the PufB and PufA C-Terminal Extensions in the Assembly of Rhodospirillum rubrum Light-Harvesting Antenna

Cryo-EM Structure of the Photosynthetic LH1-RC Complex from Rhodospirillum rubrum

Rhodospirillum (Rsp.) rubrum is one of the most widely used model organisms in bacterial photosynthesis. This purple phototroph is characterized by the presence of both rhodoquinone (RQ) and ubiquinone as electron carriers and bacteriochlorophyll (BChl) a esterified at the propionic acid side chain by geranylgeraniol (BChl aG) instead of phytol. Despite intensive efforts, the structure of the light-harvesting-reaction center (LH1-RC) core complex from Rsp. rubrum remains at low resolutions. Using cryo-EM, here we present a robust new view of the Rsp. rubrum LH1-RC at 2.76 Å resolution. The LH1 complex forms a closed, slightly elliptical ring structure with 16 αβ-polypeptides surrounding the RC. Our biochemical analysis detected RQ molecules in the purified LH1-RC, and the cryo-EM density map specifically positions RQ at the QA site in the RC. The geranylgeraniol side chains of BChl aG coordinated by LH1 β-polypeptides exhibit a highly homologous tail-up conformation that allows for interactions with the bacteriochlorin rings of nearby LH1 α-associated BChls aG. The structure also revealed key protein-protein interactions in both N- and C-terminal regions of the LH1 αβ-polypeptides, mainly within a face-to-face structural subunit. Our high-resolution Rsp. rubrum LH1-RC structure provides new insight for evaluating past experimental and computational results obtained with this old organism over many decades and lays the foundation for more detailed exploration of light-energy conversion, quinone transport, and structure-function relationships in this pigment-protein complex.

Characterisation of the LH2 spectral variants produced by the photosynthetic purple sulphur bacterium Allochromatium vinosum

This study systematically investigated the different types of LH2 produced by Allochromatium (Alc.) vinosum, a photosynthetic purple sulphur bacterium, in response to variations in growth conditions. Three different spectral forms of LH2 were isolated and purified, the B800-820, B800-840 and B800-850 LH2 types, all of which exhibit an unusual split 800 peak in their low temperature absorption spectra. However, it is likely that more forms are also present. Relatively more B800-820 and B800-840 are produced under low light conditions, while relatively more B800-850 is produced under high light conditions. Polypeptide compositions of the three different LH2 types were determined by a combination of HPLC and TOF/MS. The B800-820, B800-840 and B800-850 LH2 types all have a heterogeneous polypeptide composition, containing multiple types of both α and β polypeptides, and differ in their precise polypeptide composition. They all have a mixed carotenoid composition, containing carotenoids of the spirilloxanthin series. In all cases the most abundant carotenoid is rhodopin; however, there is a shift towards carotenoids with a higher conjugation number in LH2 complexes produced under low light conditions. CD spectroscopy, together with the polypeptide analysis, demonstrates that these Alc. vinosum LH2 complexes are more closely related to the LH2 complex from Phs. molischianum than they are to the LH2 complexes from Rps. acidophila.

A new system for heterologous expression of membrane proteins: Rhodospirillum rubrum

Heterologous expression of membrane proteins has met with only limited success. This work presents a new host/vector system for the production of heterologous membrane proteins based on a mutant of the facultatively phototrophic bacterium Rhodospirillum rubrum. Under certain growth conditions, R. rubrum forms an intracytoplasmic membrane (ICM) that houses the photosynthetic apparatus, the structural proteins of which are encoded by puhA and pufBALM. The mutant R. rubrum H2, which was constructed by allelic exchange deleting puhA and pufBALM, does not form ICM. This strain was used as a host for a plasmid expressing the Pseudomonas aeruginosa membrane protein MscL from the Rhodobacter capsulatus puc promoter. ICM was formed in the H2 strain producing MscL but not in the vector control strain. These results suggest that a heterologous membrane protein stimulates ICM formation in R. rubrum and indicate that the capacity to form an ICM that can accommodate heterologous proteins makes R. rubrum a host that will be useful for membrane protein production. P. aeruginosa MscL, which forms inclusion bodies when produced in Escherichia coli, was expressed in R. rubrum H2 and purified from membranes with a yield of 22.8-23.4 mg/L culture (5.53-5.60 mg/g cell paste). Additionally Streptomyces lividans KcsA and P. aeruginosa CycB were produced and purified from R. rubrum H2 with yields of 13.7-14.4 mg/L culture (2.19-2.55 mg/g cell paste) and 6.6-7.4 mg/L culture (1.1-1.2mg/g cell paste), respectively.