PSII generates strong oxidants used for water oxidation. The secondary electron donor, YZ, is Tyr161 on PSII reaction center D1 protein and mediates electron transfer from the oxygen-evolving Mn4CaO5 cluster to the primary electron donor, P680. The latest PSII crystal structure revealed the presence of a hydrogen bond network around YZ, which is anticipated to play important roles in the electron and proton transfer reactions. YZ forms a hydrogen bond with His190 which in turn forms a hydrogen bond with Asn298 on D1 protein. Although functional roles of YZ and His190 have already been characterized, little is known about the functional role of Asn298. Here we have generated 19 mutants from a green alga Chlamydomonas reinhardtii, in which the Asn298 has been substituted by each of the other 19 amino acid residues. All mutants showed significantly impaired or no photosynthetic growth. Seven mutants capable of photosynthetic growth showed oxygen-evolving activity although at a significantly reduced rate. Interestingly the oxygen-evolving activity of these mutants was markedly photosensitive. The 19 mutants accumulated PSII at variable levels and showed a light-induced electron transfer reaction from 1,5-diphenylcarbazide (DPC) to 2,6-dichlorophenolindophenol (DCIP), suggesting that Asn298 is important for the function and photoprotection of the Mn4CaO5 cluster.