Abstract
Cyanobacteria perform oxygenic photosynthesis using the water:plastoquinone oxidoreductase Photosystem II (PS II), a catalyst responsible for water splitting. A core protein of PS II is D1 which provides the majority of the ligands to the water splitting complex and binds several other key ligands within PS II. The D1 protein is highly conserved in Cyanobacteria and photosynthetic eukaryotes. Phylogenetic analyses have shown that there are multiple, distinct types of the D1 protein in Cyanobacteria which are upregulated in response to a variety of physiological and environmental conditions. These forms of the D1 protein may fine-tune photosynthetic activity in response to environmental conditions ranging from high light to the subjective dark. In this study, these previous phylogenetic analyses were expanded to include the D1 proteins from 206 cyanobacterial strains. This extended analysis of the D1 protein family identified distinct D1 sequences that separates into two additional sub-groups.
One of these sub-groups (D1INT) predominantly occurs in the genomes of nitrogen-fixing heterocystous cyanobacteria and the psbA gene encoding this D1 was shown in this study to be upregulated during nitrogen induced hormogonia differentiation in the heterocystous cyanobacterium, Nostoc punctiforme. The second sub-group (D1FR) is associated with the far-red light photoacclimation (FaRLiP) response which allows cyanobacteria to utilise far-red / near infrared light, unavailable to the majority of oxygenic photosynthetic organisms.
This project characterised how both D1INT and D1FR alter photosynthetic performance when expressed in the model cyanobacterium, Synechocystis sp. PCC 6803. Both proteins were incorporated into PS II centres, but retarded the performance of PS II. D1INT-containing PS II centres were assembled at lower levels and more prone to photoinhibition than PS II centres with the native D1 protein. A strain with only D1FR-containing centres was not photoautotrophic, both in the presence and absence of chlorophyll f synthase and a variant of the D2 protein associated with the FaRLiP response, D2FR. Insertion of characteristic residues associated with the D1FR protein in the native D1 protein of Synechocystis sp. PCC 6803 showed that alterations to the D1 protein impact the physiological performance of PS II and provide a rationale for the abolishment of photoautotrophy in the D1FR background.