Photosynthesis Research 56: 315–328, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 315 Regular paper Orientation and excitonic interactions of the Fenna–Matthews–Olson bacteriochlorophyll a protein in membranes of the green sulfur bacterium Chlorobium tepidum Alexander N. Melkozernov 1,2 , John M. Olson 3 , Yi-Fen Li 1,2 , James P. Allen 1,2 & Robert E. Blankenship 1,2,∗ 1 Department of Chemistry and Biochemistry; 2 Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287-1604, USA; 3 Department of Biochemistry and Molecular Biology, Lederle Graduate Research Center, University of Massachusetts, Amherst MA 01003-4505; ∗ Author for correspondence and reprints Received 19 November 1997; accepted in revised form 2 April 1998 Key words: antenna, bacteriochlorophyll a protein, Chlorobium tepidum, circular dichroism, FMO protein, linear dichroism Abstract Linear and circular dichroism spectra of isolated bacteriochlorophyll a proteins (FMO proteins) and membrane vesicles containing FMO protein from the green sulfur bacterium Chlorobium tepidum were measured at room temperature and 77 K. The orientation of membranes and isolated FMO protein was obtained by gel squeezing. Linear dichroism (LD) data indicate that isolated FMO protein and membrane vesicles associated with the FMO protein are oriented in a similar way in a squeezed polyacrylamide gel. Both samples show a characteristic negative LD band around 814 nm with flanking positive bands at 802 and 824 nm ascribed to the Q y excitonic transitions of BChl a of the FMO protein. This confirms that the C3 symmetry axis of the trimer is perpendicular to the membrane plane, which is supported by the model of the disc-like structure of FMO protein trimers of Cb. tepidum [Li Yi-Fen, Zhou W, Blankenship RE, and Allen JP (1997) J Mol Biol 272: 456–471]. The LD data are consistent with either BChl 3 or 6, but not 7 as the principal contributor to the low temperature band at 825 nm. The low temperature linear and circular dichroism spectra of FMO protein trimers from Chlorobium tepidum show significant differences from the low temperature LD and CD spectra of FMO protein trimers from Prosthecochloris aestuarii. The data are interpreted in terms of somewhat different pigment-protein and pigment-pigment interactions in the two complexes. Abbreviations: BChl – bacteriochlorophyll; BPheo – bacteriopheophytin; CD – circular dichroism; FMO protein – Fenna–Matthews–Olson protein; LD – linear dichroism; RC – reaction center Introduction The bacteriochlorophyll a (BChl a) protein from green sulfur bacteria, also known as the Fenna–Matthews– Olson (FMO) protein, is a water-soluble pigment– protein complex that transfers excitation energy from the chlorosome to the core antenna of the Fe-S-type reaction center. In intact cells, FMO proteins are lo- cated between chlorosomes and the reaction center complexes embedded in the cytoplasmic membrane (Olson et al. 1980; Blankenship et al. 1995; Olson 1998). The structures of two FMO proteins are known in detail. The crystal structure at 1.9 A resolution of the BChl a protein from Prosthecochloris aestuarii (Tronrud et al. 1986; Tronrud and Matthews 1993) and the 2.2 A resolution model of the FMO protein from Chlorobium tepidum (Li et al. 1997) indicate overall similarity in structure. Dracheva et al. (1992) reported 78% identity of polypeptide sequences of the FMO