The D2 Protein from the Thermophilic Cyanobacterium S. Vulcanus is Functional in Synechocystis PCC6803

Abstract

The optimal temperature for the primary reactions in photosynthesis - charge separation and stabilization in photosystem II (PSII), is ca. 20–35 °C in all mesophiles, which comprise all land plants. At temperatures above 40 °C the quantum yield of forward electron transport in PSII is reduced due to increasing probability for charge recombination between the reduced PSII quinone acceptors, Q A - and Q B - , and the positively charged S2 and S3 in the oxidized PSII donor side. In contrast, the optimal temperature for electron transport in PSII in thermophilic cyanobacteria is 50–60 °C [1]. One mechanism of adaptation to high temperatures is the capability of the PSII reaction center (RC) to maintain charge separation and stabilization at temperatures that cause destabilization in the mesophiles. In all mesophilic plants and cyanobacteria the peak of the B band of thermoluminescence (TL), which results from charge recombinations occurring in QB-S2 and QB-S3, is approximately at 25–30 °C, and the peak of the A band, which results from recombination in QA-S2 is at 10–15 °C. In comparison, the B band of TL in the thermophilic cyanobacterium Synechococcus vukanus is 55 °C and the A band is 35 °C [2]. These characteristic of electron transport in PSII of thermophilic cyanobacteria imply that, in addition to the significant contribution of the membrane lipids, there are inherent properties of the polypeptide subunits that adapt them to carry out electron transfer in PSII at high temperatures. Since the polypeptide constituents in the RC of PSII that comprise the S states and QB environments are well defined, it is possible to determine the contribution of specific amino acid residues in RC subunits to the stability of the charge pair at higher temperatures. In this work we have replaced the D2 polypeptide in Synechocystis PCC6803 with D2 from the thermophilic cyanobacterium S. vulcanus.