Abstract
A free α-carboxylate of C-terminal Ala344 of the D1 protein (Ala344) has been proposed to be responsible for ligating Mn ion. We constructed Synechocystis sp. PCC 6803 mutants, in which D1-Ala344 was replaced with aspartate (Ala344Asp-stop), asparagines (Ala344Asn-stop), glutamate (Ala344Glu-stop) or glutamine (Ala344Gln-stop). All mutants grew photoautotrophically under low-light (Gln > Glu = Asn > Asp), but only Ala344Gln-stop grew under high-light. Ala344Gln-stop showed normal-like low-temperature S2 ESR and mid-frequency FTIR S2/S1 difference spectra. In contrast, the symmetric carboxylate stretching bands for the putative carboxylate ligands to the Mn cluster in the FTIR S2/S1 difference spectra were changed in other mutants. The results suggest that the presence of a bulky or negatively charged group near the C-terminal carboxylate leads to modification of the properties of the Mn cluster, in compatible with the view that the D1 C-terminal α-carboxylate is the ligand for the Mn cluster.
