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

Abstract

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, DchlP, 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 Dhpt 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 DmenA mutant also grows photoautotrophically, albeit rather slowly and only at low light intensities. Under strong irradiation, DmenA retained its chlorophyll content, indicative of stable photosystems. DchlP may only be cultured photomixotrophically (due to the instability of both photosystems I and II). The increased accumulation of myxoxanthophyll in DchlP cells indicates photooxidative stress even under moderate illumination. Under high-light conditions, DchlP 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. © 2012 The Authors. The Plant Journal © 2012 Blackwell Publishing Ltd.