Light-induced EPR spectra of reaction centers from Rhodobacter sphaeroides at 80 K:: Evidence for reduction of QB by B branch electron transfer in native reaction centers

Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides capture solar energy by electron transfer from primary donor D to quinone acceptor Q(B) through the active A branch of electron acceptors. The light-induced electron paramagnetic resonance (EPR) spectrum from native RCs that had Fe2+ replaced by Zn2+ was investigated at cryogenic temperature (80 K, 35 GHz). In addition to the light-induced signal due to the formation of D(+center dot)Q(A)(-center dot) observed previously, a small fraction (ca. 5%) of the signal displayed very different characteristics. The signal was absent in RCs in which the Q(B) was displaced by the inhibitor stigmatellin. Its decay time (tau = 6 s) was the same as observed for D+*Q(B)(-center dot) in mutant RCs lacking Q(A), which is significantly slower than for D(+center dot)Q(A)(-center dot) (T = 30 ms). Its EPR spectrum was identical to that of D(+center dot)Q(B)(-center dot). The quantum efficiency for forming the major component of the signal was the same as that found for mutant RCs lacking Q(A) (0= 0.2%) and was temperature independent. These results are explained by direct photochemical reduction of via B branch electron transfer in a small fraction of native RCs.