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The core complex ofRhodobacter sphaeroidesis formed by the association of the light-harvesting antenna 1 (LH1) and the reaction center (RC). The PufX protein is essential for photosynthetic growth; it is locatedwithin the core in a 1 : 1 stoichiometry with the RC. PufX is required for a fast ubiquinol exchange between the QBsite of the RC and the Qo site of the cytochromebc1complex. | Eur. J. Biochem. 269 1877-1885 2002 FEBS 2002 doi 10.1046 j.1432-1033.2002.02834.x Role of the N- and C-terminal regions of the PufX protein in the structural organization of the photosynthetic core complex of Rhodobacter sphaeroides Francesco Francis1 2 Jun Wang1 Hans Zischka1 t Giovanni Venturoli2 and Dieter Oesterhelt1 1Department of Membrane Biochemistry Max-Planck-Institute for Biochemistry Martinsried Germany department of Biology Laboratory of Biochemistry and Biophysics University of Bologna Italy The core complex of Rhodobacter sphaeroides is formed by the association of the light-harvesting antenna 1 LH1 and the reaction center RC . The PufX protein is essential for photosynthetic growth it is located within the core in a 1 1 stoichiometry with the RC. PufX is required for a fast ubiquinol exchange between the QB site of the RC and the Qo site of the cytochrome bc1 complex. In vivo the LH1-PufX-RC complex is assembled in a dimeric form where PufX is involved as a structural organizer. We have modified the PufX protein at the N and the C-terminus with progressive deletions. The nine mutants obtained have been characterized for their ability for photosynthetic growth the insertion of PufX in the core LH1-RC complex the stability of the dimers and the kinetics of flash-induced reduction of cytochrome b561 of the cytochrome bc1 complex. Deletion of 18 residues at the N-terminus destabilizes the dimer in vitro without preventing photosynthetic growth. The dimer or a stable dimer does not seem to be a necessary requisite for the photosynthetic phenotype. Partial C-terminal deletions impede the insertion of PufX while the complete absence of the C-terminus leads to the insertion of a PufX protein composed of only its first 53 residues and does not affect the photosynthetic growth of the bacterium. Overall the results point to a complex role of the N and C domains in the structural organization of the core complex the N-terminus is suggested to be responsible .
