REGULAR PAPER Supramolecular organization of photosynthetic membrane proteins in the chlorosome-containing bacterium Chloroflexus aurantiacus David Bı ´na • Zdenko Gardian • Frantis ˇek Va ´cha • Radek Litvı ´n Received: 31 January 2014 / Accepted: 8 April 2014 / Published online: 24 April 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract The arrangement of core antenna complexes (B808-866-RC) in the cytoplasmic membrane of filamen- tous phototrophic bacterium Chloroflexus aurantiacus was studied by electron microscopy in cultures from different light conditions. A typical nearest-neighbor center-to-cen- ter distance of *18 nm was found, implying less protein crowding compared to membranes of purple bacteria. A mean RC:chlorosome ratio of 11 was estimated for the occupancy of the membrane directly underneath each chlorosome, based on analysis of chlorosome dimensions and core complex distribution. Also presented are results of single-particle analysis of core complexes embedded in the native membrane. Keywords Photosynthesis  LH1  Chlorosome  Macromolecular crowding  Electron microscopy  Chloroflexus aurantiacus Abbreviations B808-B866 The core antenna of Chloroflexus aurantiacus Cfx Chloroflexus FAP Filamentous anoxygenic phototrophs LH1, LH2 Light-harvesting complexes of purple bacteria P Primary donor of the reaction center Q A 1st quinone acceptor of the reaction center RC Reaction center Introduction The photosynthetic apparatus of filamentous anoxygenic phototrophs (FAP) such as Chloroflexus (Cfx.) aurantiacus appears to be a peculiar combination of components from purple bacteria and green sulfur bacteria. At the heart of the photosystem is a (mena-)quinone-reducing purple-like reaction center (RC), composed of two subunits corre- sponding to L and M proteins of purple bacterial RC, binding 3 bacteriochlorophyll a (BChl a) and 3 molecules of bacteriopheophytin (Ovchinnikov et al. 1988a, b; Blankenship et. al 1983). The notable absence of the third subunit, H, makes it the simplest known photosystem II- type reaction RC. The RC is surrounded by a circular multimeric antenna composed of dimers of helices in a manner of LH1 but binding three, instead of two, molecules of BChl a per dimeric subunit, arranged in two spectrally distinct groups, monomeric B808, and excitonically coupled B866, similar to LH2. Uncertainty concerning the number of antenna subunits remains. In the work of Xin et al. (2005), the core complex was envisioned as consisting of a dimer of RCs, surrounded by a 12 subunit antenna based on biochemical methods and analysis of electron microscopy images showing a particle of *22 nm diameter. However, later works did not support this interpretation. Utilizing small- angle X-ray scattering Tang et al. (2010) found a particle with diameter of *13 nm, close to the purple bacterial LH1-RC complex (Roszak et al. 2003). Moreover, analysis D. Bı ´na (&)  Z. Gardian  F. Va ´cha  R. Litvı ´n Faculty of Science, University of South Bohemia, Branis ˇovska ´ 31, 37005 C ˇ eske ´ Bude ˇjovice, Czech Republic e-mail: bina@umbr.cas.cz D. Bı ´na  Z. Gardian  F. Va ´cha  R. Litvı ´n Biology Centre ASCR vvi, Institute of Plant Molecular Biology, Branis ˇovska ´ 31, 37005 C ˇ eske ´ Bude ˇjovice, Czech Republic 123 Photosynth Res (2014) 122:13–21 DOI 10.1007/s11120-014-0006-8