Role of the psbA Gene Family of PSII from the Thermophilic Cyanobacterium Thermosynechococcus elongatus

Abstract

The first step of the light reaction in photosynthesis is the light-driven oxidation of water into molecular oxygen and protons. This process is catalysed by the membrane protein complex pho-tosystem 2 (PS2). Most of the cofactors needed for this process are associated with the D1 core protein of PS2. In contrast to higher plants, all sequenced cyanobacteria possess a gene family coding for the D1 protein. The number of genes and gene products varies species-dependant. In case of the thermophilic cyanobacterium Thermosynechococcus elongatus, there are three gene copies each coding for a distinct protein. As shown for other cyanobacteria, the exchange of these D1 proteins seems to be a protective mechanism under high light stess conditions. Quantitative real time PCR analysis revealed an exchange of the psbA transcription within 30 minutes of high light conditions (500 µE). While the transcription of psbA1 decreases from 90 % to 1.5 %, the psbA3 transcription increases from 9 % to 98 %. The transcription of psbA2 seems to be unaffected (1%). Preliminary functional characterisation of isolated PS2 complexes containing only one of the D1 proteins revealed differences in the Q B redox potential in thermoluminescence measurement.