Photosynthetic growth despite a broken Q-cycle

Central in respiration or photosynthesis, the cytochrome bc1 and b6f complexes are regarded as functionally similar quinol oxidoreductases. They both catalyse a redox loop, the Q-cycle, which couples electron and proton transfer. This loop involves a bifurcated electron transfer step considered as being mechanistically mandatory, making the Q-cycle indispensable for growth. Attempts to falsify this paradigm in the case of cytochrome bc1 have failed. The rapid proteolytic degradation of b6f complexes bearing mutations aimed at hindering the Q-cycle has precluded so far the experimental assessment of this model in the photosynthetic chain. Here we combine mutations in Chlamydomonas that inactivate the redox loop but preserve high accumulation levels of b6f complexes. The oxidoreductase activity of these crippled complexes is sufficient to sustain photosynthetic growth, which demonstrates that the Q-cycle is dispensable for oxygenic photosynthesis

Structural basis of mechanochemical coupling in a hexameric molecular motor

The P4 protein of bacteriophage φ 12 is a hexameric molecular motor closely related to superfamily 4 helicases. P4 converts chemical energy from ATP hydrolysis into mechanical work, to translocate single-stranded RNA into a viral capsid. The molecular basis of mechanochemical coupling, i.e. how small ∼1 Å changes in the ATP-binding site are amplified into nanometer scale motion along the nucleic acid, is not understood at the atomic level. Here we study in atomic detail the mechanochemical coupling using structural and biochemical analyses of P4 mutants. We show that a conserved region, consisting of superfamily 4 helicase motifs H3 and H4 and loop L2, constitutes the moving lever of the motor. The lever tip encompasses an RNA-binding site that moves along the mechanical reaction coordinate. The lever is flanked by γ-phosphate sensors (Asn-234 and Ser-252) that report the nucleotide state of neighboring subunits and control the lever position. Insertion of an arginine finger (Arg-279) into the neighboring catalytic site is concomitant with lever movement and commences ATP hydrolysis. This ensures cooperative sequential hydrolysis that is tightly coupled to mechanical motion. Given the structural conservation, the mutated residues may play similar roles in other hexameric helicases and related molecular motors. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc