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Abstract:
The structure of the photosynthetic reaction center (RC) from Rhodopseudomonas viridis is known to high resolution. It contains a firmly bound tetraheme cytochrome from which electrons are donated to a special pair (P) of bacteriochlorophylls, which is photooxidized upon absorption of light. Tyrosine at position 162 of the L-subunit of the reaction center (L 162 Y) is a highly conserved residue positioned halfway between P and the proximal heme group (c-559) of the cytochrome. By specific mutagenesis this residue was exchanged against the amino acids phenylalanine (F), 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 [Laussermair & Oesterhelt (1992) EMBO J. 11, 777-783]. They were shown biochemically to synthesize all four subunits of the RC (cytochrome, subunits L, M, and H) and to assemble them correctly into the membrane. The structures 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 changed by the mutations. The kinetics of electron transfer from the heme group to the special pair were measured in chromatophores by flash absorption. As found earlier in the wild type (Y) several exponential components were needed to fit the data. For the dominant fastest phase, the half-time varies from 147 to 1000ns, in the order M,F,Y,W, H, L, G, T. We conclude that the tyrosine residue at position L162 is not required for fast electron transfer from c-559 to P+.
