PARENT SESSION

Symposium S3A Bacteriochlorophyll based antenna systems
Tuesday August 31st, 2004 10:20 AM-12:20 PM Room 511D
Chair: Neil Hunter
Co-Chair: Richard Cogdell

Cryo-electron microscopy and x-ray scattering of chlorosomes from green sulfur bacteria: evidence for lamellar organization of bacteriochlorophyll aggregates. Jakub Psencik^*,1, Teemu Ikonen², Pasi Laurinmäki³, Michael Merckel⁴, Sarah Butcher³, Ritva Serimaa², Roman Tuma³, ¹ Department of Chemical Physics and Optics, Prague 2, Czech Republic² Division of X-ray Physics, Helsinki, Finland³ Institute of Biotechnology & Department of Biosciences, Helsinki, Finland⁴ Helsinki Bioenergetics Group, Helsinki, Finland

ABSTRACT- Chlorosomes of green photosynthetic bacteria constitute the most efficient light harvesting complexes found in nature. In addition, the chlorosome is the only known photosynthetic system where the majority of pigments (bacteriochlorophyll, BChl) is not organized in pigment-protein complexes but instead is assembled into aggregates. Because of the unusual organization, the chlorosome structure has not been resolved and only models, in which BChl pigments were organized into large rods, were proposed on the basis of freeze fracture electron microscopy and spectroscopic constraints. We have obtained the first high-resolution images of chlorosomes from the green sulfur bacterium Chlorobium tepidum by cryo-electron microscopy (EM). EM images revealed dense striations ∼2 nm apart. X-ray scattering from chlorosomes exhibited a feature with the same ∼2 nm spacing. No evidence for the rod models was obtained. The observed spacing and tilt-series EM projections are compatible with a lamellar model, in which BChl molecules aggregate into semi-crystalline lateral arrays. The diffraction data further indicate that arrays are built from BChl dimers. The arrays form undulating lamellae, which, in turn, are held together by interdigitated esterifying alcohol tails, carotenoids and lipids. The lamellar model is consistent with earlier spectroscopic data and provides insight into chlorosome self-assembly.

KEY WORDS: chlorosome, green bacteria, bacteriochlorophyll aggregates