Directed mutations affecting spectroscopic and electron transfer properties of the primary donor in the photosynthetic reaction center.

Oligonucleotide-mediated mutagenesis has been used to change the histidine residues that act as axial ligands to the central Mg(2+) ions of the "special pair" bacteriochlorophylls in the reaction center of Rhodobacter capsulatus. Histidine-173 of the L subunit has been replaced with glutamine, while histidine-200 of the M subunit has been replaced with glutamine, leucine, or phenylalanine. When leucine or phenylalanine is introduced at M200, one of the special pair bacteriochlorophylls is converted to bacteriopheophytin, which generates a heterodimer at the special pair binding site. The pigment composition of the reaction center is unaltered when either histidine is replaced with glutamine. All of these mutant reaction centers are photochemically active, although the electron transfer properties of heterodimer-containing reaction centers are altered. These mutations begin to define the structural parameters that determine whether bacteriochlorophyll or bacteriopheophytin will be incorporated into the tetrapyrrole binding sites of the photosynthetic reaction center. Our results demonstrate that the properties of the photosynthetic reaction center can be changed by directed mutagenesis, which makes this complex an excellent model for testing theories of electron transfer in biological systems.