Introduction
Rhodospirillum (Rsp.) rubrum is an anoxygenic phototrophic purple bacterium with a long history as a model for the study of bacterial photosynthesis and related metabolic processes. It is unique among purple bacteria by producing both rhodoquinone (RQ) and ubiquinone (UQ)1 as electron carriers and bacteriochlorophyll (BChl) a esterified at the propionic acid side chain by geranylgeraniol (abbreviated as BChl aG) rather than phytol.2 Rsp. rubrum has a single pair of αβ-polypeptides in its core light-harvesting (LH1) complex and lacks both the peripheral light-harvesting (LH2) complex and reaction center
(RC) cytochrome (Cyt) c subunit present in many purple bacteria; thus, Rsp. rubrum is one of the simplest phototrophic bacteria known, in terms of its photosynthetic light reactions. Because the entire Rsp. rubrum LH1 complex and a stable B820 LH1-subunit can be reconstituted using the αβ-polypeptides
and pigment molecules,3−5 both complexes have been intensively studied as models of the bacterial antenna apparatus6 and as such have provided a wealth of information on mechanisms of light energy acquisition, pigment−protein interactions, and assembly of multicomponent complexes. [1]
Structure
Structures of both purified LH1 and the RC-associated core complex (LH1-RC) of Rsp. rubrum have not been obtained at high resolution, and no RC atomic structure is known.
Structure overview of the Rsp. rubrum LH1-RC complex. (a) Side view of the LH1-RC parallel to the membrane plane. (b) Top view of the LH1-RC from the periplasmic side of the membrane. (c) Tilted view of the cofactor arrangement. (d) Superposition of Cα carbons of the LH1 αβpolypeptides between Rsp. rubrum and Tch. tepidum (gray, PDB: 5Y5S). Color scheme: LH1-α, green; LH1-β, slate-blue; L-subunit, magenta; Msubunit, blue; BChl aG in LH1 and special pair, red sticks; Accessory BChl aG, cyan sticks; BPhe aG, light-pink sticks; Spirilloxanthin, yellow sticks; UQ10, blue sticks; RQ-10, green sticks; Fe, magenta ball. Phospholipids and detergents are omitted for clarity
Relevance
Structural highlights
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