ELECTRON-TRANSFER FROM THE TETRAHEME CYTOCHROME TO THE SPECIAL PAIR IN THE RHODOPSEUDOMONAS-VIRIDIS REACTION-CENTER - EFFECT OF MUTATIONS OF TYROSINE L162

The structure of the photosynthetic reaction center (RC) from Rhodopse udomonas viridis is known to high resolution. It contains a firmly bou nd tetraheme cytochrome from which electrons are donated to a special pair (P) of bacteriochlorophylls, which is photooxidized upon absorpti on of light. Tyrosine at position 162 of the L-subunit of the reaction center (L 162 Y) is a highly conserved residue positioned halfway bet ween P and the proximal heme group (c-559) of the cytochrome. By speci fic mutagenesis this residue was exchanged against the amino acids phe nylalanine 0, glycine (G), methionine (M), leucine (L), tryptophan (W) , threonine (T), and histidine (H). All mutants were expressed in Rps. viridis using a recently established transformation system [Lausserma ir & Oesterhelt (1992) EMBO J. 11, 777-783]. They were shown biochemic ally to synthesize all four subunits of the RC (cytochrome, subunits L , M, and H) and to assemble them correctly into the membrane. The stru ctures of two mutants (L 162 F and L 162 T) were determined and found not to differ significantly from the wild-type structure. All mutants grew photosynthetically. The absorption spectrum of all the mutants is the same as in WT, but the redox potential of P and of c-559 was chan ged by the mutations. The kinetics of electron transfer from the heme group to the special pair were measured in chromatophores by flash abs orption. As found earlier in the wild type (Y) several exponential com ponents were needed to fit the data. For the dominant fastest phase, t he half-time varies from 147 to 1000 ns, in the order M, F, Y, W, H, L , G, T. We conclude that the tyrosine residue at position L 162 is not required for fast electron transfer from c-559 to P+.