A genetically engineered increase in fatty acid unsaturation in Synechococcus sp. PCC 7942 allows exchange of D1 protein forms and sustenance of photosystem II activity at low temperature

Photosystem II (PSII)-reaction-center protein D1 is encoded by three psbA genes in Synechococcus sp. PCC 7942. The psbA gene encodes D1 form I (D1:1) and the psbAIII and pshAlll genes encode the transiently expressed D1 form II (D1:2). We have studied the role of membrane-lipid unsaturation in the expression of psbA genes at low temperature, using a Synechococcus transformant with an increased unsaturation level of membrane lipids.Transfer of the cells from 32 degrees C to 25 degrees C under growth light resulted in the exchange of D1:1, the prevailing form, for D1:2 in the wild- type bacterium and the transformant, with no loss of PSII activity.Lowering the temperature further to 18 degrees C caused a drastic decrease in PSII activity in the wild-type bacterium, whereas the transformant was much less affected. Similar decreases in psbAI transcripts and loss of D1 : 1 occurred in both strains at 18 degrees C, with concomitant accumulation of psbAII/III transcripts, the latter event being especially prominent in the wild-type bacterium. However, the wild-type bacterium was incapable of accumulating D1:2 to compensate for the loss of D1:l, which resulted in disassembly of PSII at low temperature. These results imply translational rather than transcriptional regulation of psbA gene expression in Synechococcus 7942 at low temperature, and demonstrate the crucial role of the degree of membrane-lipid unsaturation in promotion of exchange of the D1 protein forms and thus the sustenance of PSII function at low temperature.