Abstract
There are three genes encoding the D1 protein (PsbA) of photosystem II (PSII) in a thermophilic cyanobacterium T. elongatus genome. The psbA1 gene mainly expresses under normal cultivation conditions, whereas the expression of psbA3 is induced by high light. It has been shown that natures of the PsbA1- and PsbA3-PSII significantly differ, which would be due to different 21 amino acid residues among 344 ones constituting of PsbA. Especially, the oxygen-evolving activity of PsbA3-PSII is ca. 1.7 times higher than that of the other. By investigating the redox potential Em of the primary electron acceptor pheophytin (Ph), we concluded that the difference in the activity is due to the difference that Em(Ph) of the PsbA3-PSII is more positive by 17 mV than that of the other, which would be induced by substitution of the 130th residue. Thermoluminescence measurements on the PSIIs, however, implied that the different activity cannot be explained simply by the difference in the Em(Ph) values. In this work, by measuring Em of the secondary acceptor QA, we will discuss, based on the free energy change from Ph to QA, on the relationship between the energetics and activity of the PSIIs.
