The light-harvesting-reaction center complex (LH1-RC) from the purple phototrophic bacterium Thiorhodovibrio strain 970 exhibits an LH1 absorption maximum at 960 nm, the most red-shifted absorption for any bacteriochlorophyll (BChl) a-containing species. Here we present a cryo-EM structure of the strain 970 LH1-RC complex at 2.82 Å resolution. The LH1 forms a closed ring structure composed of sixteen pairs of the αβ-polypeptides. Sixteen Ca ions are present in the LH1 C-terminal domain and are coordinated by residues from the αβ-polypeptides that are hydrogen-bonded to BChl a. The Ca2+-facilitated hydrogen-bonding network forms the structural basis of the unusual LH1 redshift. The structure also revealed the arrangement of multiple forms of α- and β-polypeptides in an individual LH1 ring. Such organization indicates a mechanism of interplay between the expression and assembly of the LH1 complex that is regulated through interactions with the RC subunits inside.

Fermentation processes using taxol-producing fungi other than Taxus spp. may be an alternative way to produce taxol, which is an important antitumor agent used widely in the clinic setting. In this study, a taxol-producing endophytic fungus strain MD2 was isolated from the inner bark of Taxus media. Strain MD2 produced taxol when grown in potato dextrose liquid medium. The fungal taxol-which was analyzed by ultraviolet, high-performance liquid chromatography and mass spectrometry-was shown to be identical to authentic taxol and 10-deacetylbaccatin III. Further analysis with nuclear magnetic resonance (NMR) spectroscopy to show the chemical structure of the fungal taxol indicated that the fungal taxol produced an NMR spectrum identical to that of authentic taxol. Strain MD2 was identified as Cladosporium cladosporioides according to morphology of the fungal culture, characteristics of the spores, and analysis of 18S rDNA sequence. In addition, 10-deacetylbaccatin III-10-O-acetyl transferase gene of C. cladosporioides MD2 was cloned for the first time and was shown to share 99% identity with that of T. x media and 97% identity with that of T. wallichiana var. mairei.

Rhodobacter (Rba.) sphaeroides is the most widely used model organism in bacterial photosynthesis. The light-harvesting-reaction center (LH1-RC) core complex of this purple phototroph is characterized by the co-existence of monomeric and dimeric forms, the presence of the protein PufX, and approximately two carotenoids per LH1 αβ-polypeptides. Despite many efforts, structures of the Rba. sphaeroides LH1-RC have not been obtained at high resolutions. Here we report a cryo-EM structure of the monomeric LH1-RC from Rba. sphaeroides strain IL106 at 2.9 Å resolution. The LH1 complex forms a C-shaped structure composed of 14 αβ-polypeptides around the RC with a large ring opening. From the cryo-EM density map, a previously unrecognized integral membrane protein, referred to as protein-U, was identified. Protein-U has a U-shaped conformation near the LH1-ring opening and was annotated as a hypothetical protein in the Rba. sphaeroides genome. Deletion of protein-U resulted in a mutant strain that expressed a much-reduced amount of the dimeric LH1-RC, indicating an important role for protein-U in dimerization of the LH1-RC complex. PufX was located opposite protein-U on the LH1-ring opening, and both its position and conformation differed from that of previous reports of dimeric LH1-RC structures obtained at low-resolution. Twenty-six molecules of the carotenoid spheroidene arranged in two distinct configurations were resolved in the Rba. sphaeroides LH1 and were positioned within the complex to block its channels. Our findings offer an exciting new view of the core photocomplex of Rba. sphaeroides and the connections between structure and function in bacterial photocomplexes in general.