The mechanism of electron transfer (ET) from the primary to the secondary quinone of bacterial photosynthetic reaction centers is discussed on the basis of theoretical computations of the minimum energy nuclear configurations and ET coupling elements, and quantum dynamic simulations of elementary reaction steps. For ET to occur via tunneling, unreasonably high values of the electronic coupling elements or very stringent energy conditions, i.e., tight degeneracy (within a few cm(-1)) between the initial and final vibronic states, are necessary, both for the direct and through-bridge (superexchange) routes. The assumption of tight degeneracy significantly slows down the process so that other competitive processes, such as proton transfer from the H-bonded HisM219 to the primary quinone, can take place. All these results suggest that the iron-histidine bridge can play an important role in the ET mechanism.