The electronic mechanism and the origin of the unidirectionality of the electron transfer from photoexcited special pair to bacteriopheophytin in the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps) viridis are studied theoretically by using the SAC(symmetry adapted cluster)-CI (configuration interaction) method. The effects of the surrounding proteins are considered by using the point charge model. The L-branch selectivity of the electron transfer is explained by the asymmetry of the transfer integral, an electronic factor, which originates from a small structural asymmetry of the PSRC: the L-side chromophores are locally closer than the M-side ones, though the average separations are almost the same. The smallness of the charge recombination rate is attributed to the difference in the electron localization between the LUMO and HOMO of special pair. Protein effects on the unidirectionality are quite small as far as the electrostatic model is valid, though the proteins keep the three-dimensional arrangement of the chromophores in the PSRC. A mutation experiment for realizing M-side selectivity is suggested.