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ABSTRACT
Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. Recently, a lamellar model was proposed for the arrangement of pigment aggregates in Chlorobium tepidum chlorosomes, which contain bacteriochlorophyll (BChl) c as the main pigment. Here we demonstrate that the lamellar organization is also found in chlorosomes from two brown-colored species (Chl. phaeovibrioides and Chl. phaeobacteroides) containing BChl e as the main pigment. This suggests that the lamellar model is universal among green sulfur bacteria. In contrast to green-colored Chl. tepidum, chlorosomes from the brown-colored species often contain domains of lamellar aggregates that may help them to survive in extremely low light conditions. We suggest that carotenoids are localized between the lamellar planes and drive lamellar assembly by augmenting hydrophobic interactions. A model for chlorosome assembly, which accounts for the role of carotenoids and secondary BChl homologs, is presented.
INTRODUCTION
Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. A typical chlorosome is an ellipsoidal body (typically 100-200 nm in length, 20-50 nm in width) that is composed mainly of bacteriochlorophylls and carotenoids with minor contributions from quinones, lipids, and proteins (1,2). The major difference from all other light harvesting complexes is that the main chlorosome pigments, bacteriochlorophyll (BChl) c, d, or e, are not associated with proteins and self-assemble into aggregates. The BChl aggregates were originally proposed to a1ssemble into rodlike elements (3,4). An alternative lamellar model was put forward for the pigment arrangement in Chlombium tepidum chlorosomes (5). Recently, further support for the lamellar model was obtained by a careful fixation of whole cells and electron microscopy (EM) (6). The density patterns and striations observed for most Chl. tepidum chlorosomes in this study were inconsistent with the presence of hexagonally packed, rod-shaped, BChl c aggregates, but could be explained by the lamellar structure of the aggregates. However, it is not obvious whether similar structural features are common to chlorosomes from the other members of Chlorobiaceae, which exhibit different pigment compositions.
Chl. tepidum is a green-colored, green sulfur bacterium containing BChl c as the main pigment, and chlorobactene and OH-chlorobactene as the main carotenoids. For comparison we selected two brown-colored green sulfur bacteria, Chl. phaeovibrioides and Chl. phaeobacteroides, which contain BChl e, isorenieratene, and β-isorenieratene as the main pigments...