Chlorophyll a phytylation is required for the stability of photosystems I and II in the cyanobacterium Synechocystis sp PCC 6803

In oxygenic phototrophic organisms, the phytyl 'tail' of chlorophyll a is formed from a geranylgeranyl residue by the enzyme geranylgeranyl reductase. Additionally, in oxygenic phototrophs, phytyl residues are the tail moieties of tocopherols and phylloquinone. A mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking geranylgeranyl reductase, Delta chlP, was compared to strains with specific deficiencies in either tocopherols or phylloquinone to assess the role of chlorophyll a phytylatation (versus geranylgeranylation). The tocopherol-less Delta hpt strain grows indistinguishably from the wild-type under 'standard' light photoautotrophic conditions, and exhibited only a slightly enhanced rate of photosystem I degradation under strong irradiation. The phylloquinone-less Delta menA mutant also grows photoautotrophically, albeit rather slowly and only at low light intensities. Under strong irradiation, Delta menA retained its chlorophyll content, indicative of stable photosystems. Delta chlP may only be cultured photomixotrophically (due to the instability of both photosystems I and II). The increased accumulation of myxoxanthophyll in Delta chlP cells indicates photo-oxidative stress even under moderate illumination. Under high-light conditions, Delta chlP exhibited rapid degradation of photosystems I and II. In conclusion, the results demonstrate that chlorophyll a phytylation is important for the (photo) stability of photosystems I and II, which, in turn, is necessary for photoautotrophic growth and tolerance of high light in an oxygenic environment.