We present here a detailed analysis of the mechanism of photoinduced electron and proton transfer in the planar pyrrole-pyridine hydrogen-bonded system, a model for the photochemistry of hydrogen bonds in DNA base pairs. Two different crossings, an avoided crossing and a conical intersection, are the key steps for forward and backward electron and proton transfer providing to the system photostability against UV radiation by restoring the system in its initial electronic and geometric structure.