Membrane lipid composition and restoration of photosynthesis during low temperature acclimation in Synechococcus sp. strain PCC 7942

We compared temperature acclimation of the cyanobacterium Synechococcus sp. strain PCC 7942 and two psbA inactivation mutants, R2K1 and R2S2C3, follow ing shifts from 37 to 25 degrees C. Wild-type cultures incubated in the dar k at 25 degrees C showed no chill-induction of lipid desaturation, probably because the lipid acclimation is dependent on photosynthesis. Incubation i n the light at 25 degrees C, however, induced considerable increases in mem brane lipid desaturation, and within 24 h the monoenoic fatty acids increas ed from about 46 to about 57%. In parallel with this desaturation the ratio of monogalactosyldiacylglycerol to digalactosyldiacylglycerol (MGDG/DGDG) increased. Both of these lipid changes increase the repulsive forces of the hydrophobic chains of the membrane lipids and thereby alter the physical p roperties of the membrane. As expected, under irradiation this temperature shift also induced a reversible replacement of the constitutive photosystem II protein, D1:1, with an alternative stress form, D1:2. Photosynthesis de creased to 42% of the control level within the initial 2 h of cold incubati on. but later recovered. The D1:2 protein accumulated to high levels betwee n 2 and 4 h after the temperature shift, when desaturation of membrane lipi ds and MGDG/DGDG ratio had not yet increased significantly. Much of this ac cumulated D1:2 protein was in a higher molecular mass form, termed D1:2*, w hich is probably an unprocessed precursor form of the protein. In contrast to the wild-type cells the psbA inactivation mutants, R2K1 and R2S2C3 did n ot accumulate any precursor form of D1 protein either at the optimal or low growth temperature. The R2S2C3 mutant strain expresses only the constitutive D1:1 protein and s uffered severe photoinhibition following the temperature shift. Nevertheles s; R2S2C3 eventually recovered some photosynthetic activity, induced lipid desaturation and slowly resumed growth ar 25 degrees C, thus demonstrating acclimation to the lower growth temperature. The R2K1 mutant synthesizes only the D1:2 stress form of D1 protein and mai ntained oxygen evolution at a high level (ca 70% of a control rate) after t he low temperature shift. Chill-induced lipid desaturation and increase in MGDG/DGDG ratio did proceed but, for unknown reasons the strain did nor res ume growth at the lower temperature. The physical properties of the membran e lipids were not the limiting factor for growth resumption. Our results demonstrate that in the wild-type the chill-induced desaturatio n of membrane lipids follows after the exchange of the two forms of the D1 proteins, but the D1 exchange results in accumulation of unprocessed D1:2* polypeptides until the lipid composition later acclimates. We also show tha t the lipid desaturation process in Synechococcus sp. strain PCC 7942 is de pendent upon photosynthetic activity.