Expression of the far-red D1 protein or introduction of conserved far-red D1 residues into Synechocystis sp. PCC 6803 impairs Photosystem II

The wavelengths of light harvested in oxygenic photosynthesis are similar to 400-700 nm. Some cyanobacteria respond to far-red light exposure via a process called far-red light photoacclimation which enables absorption of light at wavelengths >700 nm and its use to support photosynthesis. Far-red-light-induced changes include up-regulation of alternative copies of multiple proteins of Photosystem II (PS II). This includes an alternative copy of the D1 protein, D1(FR). Here, we show that D1(FR) introduced into Synechocystis sp. PCC 6803 (hereafter Synechocystis 6803) can be incorporated into PS II centres that evolve oxygen at low rates but cannot support photoautotrophic growth. Using mutagenesis to modify the psbA2 gene of Synechocystis 6803, we modified residues in helices A, B, and C to be characteristic of D1(FR) residues. Modification of the Synechocystis 6803 helix A to resemble the D1(FR) helix A, with modifications in the region of the bound ss-carotene (Car(D1)) and the accessory chlorophyll, ChlZ(D1), produced a strain with a similar phenotype to the D1(FR) strain. In contrast, the D1(FR) changes in helices B and C had minor impacts on photoautotrophy but impacted the function of PS II, possibly through a change in the equilibrium for electron sharing between the primary and secondary plastoquinone electron acceptors Q(A) and Q(B) in favour of Q(A)(-). The addition of combinations of residue changes in helix C indicates compensating effects may occur and highlight the need to experimentally determine the impact of multiple residue changes.

Automated summary

The wavelengths of light harvested in oxygenic photosynthesis are typically between 400 and 700 nm. Cyanobacteria can adapt to far-red light by up-regulating alternative copies of various proteins, including D1(FR), to support photosynthesis. Incorporating D1(FR) into Synechocystis 6803 resulted in low rates of oxygen evolution and an inability to support photoautotrophic growth. Mutations in helix A, resembling D1(FR) residues, produced a similar phenotype, while changes in helices B and C had minor effects on photoautotrophy but impacted the function of PS II.