See also (Oostergetel et al. 2007) for further images. Size bar equals 25 nm Recently, cryo-electron microscopy was performed on intact chlorosomes of C. tepidum embedded in a thicker layer of vitreous ice to reveal the arrangement of BChl sheets in wild-type chlorosomes and in chlorosomes from the triple mutant bchQRU (Gomez Maqueo Chew et al. 2007), which contains a well-defined

>95% homogeneous BChl d (Oostergetel et al. 2007). End-on views of chlorosomes fixed in a vertical position gave see more a direct clue to the packing of the sheets. They show the presence of multi-lamellar tubules of variable diameter (10–30 nm) with some non-tubular locally curved lamellae in between (Fig. 3). In the bchQRU mutant, most chlorosomes contain two tubular domains, as can be deduced from the banding pattern of the 2-nm striations. Overall, the cryo-electron microscopy

data show that the C. tepidum chlorosomes comprise Selleck CH5424802 multi-lamellar tubular domains extending over most of the length of the chlorosome, embedded in a less well-ordered matrix of smaller curved lamellar domains. The notion of multi-walled cylinders is consistent with the results from both freeze-fracture experiments done several decades ago and the more recent cryo-EM observations. Molecular organization of chlorophylls In addition to the 2-nm lamellar structure, cryo-EM images of C. tepidum chlorosomes and their calculated diffraction patterns indicated the presence of a smaller spaced regular structure in the direction of the long axis (Fig. 4). In wild-type chlorosomes, a weak periodicity of 1.25 nm is present (red arrow in Fig. 4b), in the bchQRU mutant a relatively strong 0.83 nm regular structure is evident from the diffraction pattern (Fig. 4d) and also directly visible in the image (Fig. 4c, inset). These cryo-EM observations provide constraints filipin concerning possible packing modes of the BChl molecules in the multi-lamellar tubes. Fig. 4 Analysis of the interior of the chlorosome of Chlorobaculum tepidum. a Image of an unstained, ice-embedded chlorosome from the wild-type. b Calculated diffraction pattern from the image of frame a. A bright

but unsharp reflection spot (white arrow) indicates an average spacing between lamellae of 2.1 nm, which is also directly visible in the image of frame a. A sharp layer line at 1.25 nm (red arrow) indicates a specific internal repeating distance of 1.25 nm of the lamellae, caused by a specific packing of BChls. A thin but distinct reflection at 3.3 nm (green arrow) is assigned to a spacing of protein molecules of the baseplate. c Image of an unstained, ice-embedded chlorosome from the bclQRU mutant. d Calculated diffraction pattern from the image of c. The white and green arrows indicate structural elements as in the pattern of frame b. The sharp layer line (red arrow) now indicates a specific internal repeating distance of 0.83 nm, instead of 1.25 nm as in the wild-type.