Changes in midpoint potentials of hemes by SD-mutagenesis in tetraheme cytochrome subunit of photosynthetic reaction center complex in purple bacteria

Abstract

The RC-bound tetraheme cytochrome subunit of a purple bacterium, Blastochloris viridis, was successfully synthesized in Rubrivivax gelatinosus cells. The redox midpoint potentials (E_m's) of 4 hemes in the chimeric RC were almost identical with those in the B. viridis RC, in which the hemes are linearly arranged from the core proteins as c559 (+380 mV), c552 (+30 mV), c556 (+320 mV), and c553 (-50 mV). To clarify the physiological significance of such high-low-high-low arrangement of E_m's of hemes, amino acids located near the hemes were mutationary replaced to change their E_m's. More than 20 mutants were obtained. In a mutant, R264L, in which the 264th Arg was replaced by Leu, the E_m of c559 was lowered to 130 mV and P⁺ was rereduced with a rate 20 times slower than in the native chimera when c556 works as the donor. This mutant, however, grow photosynthetically.