Stoichiometric model of the photosynthetic unit of Ectothiorhodospira halochloris

Abstract

A stoichiometric model of the photosynthetic unit of Ectothiorhodospira halochloris has been obtained by means of scanning transmission electron microscope mass determination and mass mapping in conjunction with polyacrylamide gel electrophoresis. One reaction center, consisting of four single polypeptides, including one cytochrome, is surrounded by six identical light-harvesting complexes, each containing three polypeptides with 2:2:2 stoichiometry. This stoichiometric model was incorporated into the three-dimensional structure of the photosynthetic unit as derived from surface relief reconstructions of the two surfaces of shadowed membranes. The reaction center protrudes substantially from both membrane surfaces and has the cytochrome attached to the periplasmic face in a noncentrosymmetric fashion. The reaction center may assume various orientations within the photosynthetic complexes.The photosynthetic membranes of bacteriochlorophyll bcontaining bacteria, particularly of Rhodopseudomonas viridis and Ectothiorhodospira halochloris, have previously been studied by electron microscopy and image processing, and several models of the photosynthetic unit have been proposed (1-5). They agree as far as the hexagonal arrangement ofthe complexes in the membranes with lattice spacings of approximately 13 nm is concerned, and in the gross morphology of the photosynthetic units, which consist of a central core, the reaction center (RC), surrounded by a ring of light-harvesting (LH) complexes. The models differ, however, with respect to the number and arrangement of LH subunits.In this study we present a low-resolution stoichiometric model of the photosynthetic unit from E. halochloris. We have applied scanning transmission electron microscope (STEM) mass determination (6) and mass mapping (7). These methods are-capable of determining the masses of small discrete features within a large protein complex; it is in fact the only way of measuring masses of membrane constituents in situ without disintegrating the structures of interest. Mass mapping, in conjunction with polyacrylamide gel electrophoresis (IAGE) of the photosynthetic membranes, enabled us to determine the stoichiometric polypeptide composition in the photosynthetic unit. The three-dimensional structure of the photosynthetic complex was obtained by surface relief reconstruction (8) from unidirectionally shadowed membranes.

Supramolecular arrangement of Rhodospirillum rubrum B880 holochrome as studied by radiation inactivation and electron paramagnetic resonance.

“…While we cannot offer any definitive model, our data are compatible with a ringlike structure like that proposed for the photosynthetic membranes of Bchl b-containing Rhodopseudomonas viridis and Ectothiorhodospira halochloris (31)(32)(33)(34). The latter structure was deduced by scanning transmission electron microscopy to be formed of a ring of six identical light-harvesting complexes, each containing three polypeptides with (aJ32y2) stoichiometry (34).…”

8.5 Å Projection Map of the Light-Harvesting Complex I from Rhodospirillum Rubrum Reveals a Ring Composed of 16 Subunits

“…Biological significance Early electron microscopy and digital image processing at low resolution (40-50 A) of negatively stained photosynthetic membranes from Rhodopseudomonas viridis (Miller, 1982;Stark et al, 1984) and Ectothiorhodospira halochloris (Engelhardt et al, 1986), which contain BChl b as the major photosynthetic pigment, revealed the membranes to be composed of hexagonally packed 120 A particles which appeared to contain six to 12 lightharvesting units (these organisms possess only LHC I) arranged in a ring around a central reaction centre. Each unit of the ring was suggested to be composed of an ax3-heterodimer, yielding a value of 24 mol BChl b/mol reaction centre.…”

Structural Role of PufX in the Dimerization of the Photosynthetic Core Complex of Rhodobacter sphaeroides

“…We suggest that PufX could act as a symmetry breaker to prevent the clustering of RC-LH1 complexes into large hexagonal arrangements, as observed for several species of purple bacteria without pufX (6,8,15,46). Due to this suggestion, PufX-induced core dimerization would facilitate indirectly the functional interplay of RC-LH1 core complexes with other components of the photosynthetic unit like LH2 and bc 1 complex.…”