To define the role of the Δ3-trans-hexadecenoic acid (16:1-trans) in the thylakoid membrane, restoration experiments were carried out using a Photosystem II (PS II)-lacking but low-fluorescent mutant, mf 2, of Chlamydomonas reinhardtii. This mutant is unable to synthesize 16:1-trans, lacks an oligomeric form of the main light-harvesting chlorophyll (Chl) a + b antenna, CP II, and shows an impaired regulation of the excitation energy distribution. Whole cells were incubated for 39 h in the presence of liposomes of 16:1-trans-containing phosphatidylglycerol (PG-16:1-trans), in the light at 25°C. Then lipids and Chl-protein complexes were analyzed and low-temperature fluorescence emission spectra, both in state I and in state II (oxidized and reduced plastoquinone pool), were measured. The results indicated: (1) a relatively important content of 16:1-trans specifically incorporated in the chloroplast phosphatidylglycerol (PG); (2) an appreciable amount of CP II oligomeric form; (3) the occurrence of a clear state II-state I transition, as shown by a ratio of the CP II fluorescence at 682 nm to the photosystem I fluorescence at 712 nm, which was 3.8-times higher in state I than in state II. These restorations were not observed when cells of mf 2 were incubated in the presence of palmitate-containing PG, of oleate-containing phosphatidylcholine or of PG-16:1-trans + cycloheximide. It is concluded that: (1) the oligomeric form of CP II is essential for a good excitation energy transfer towards the PS II region and, consequently, to a good state II-state I transition in the distribution of excitation energy; (2) PG-16:1-trans probably plays an essential role in stabilizing neo-formed CP II oligomers during the assembly of new Chl-protein complexes in the chloroplast. © 1990.
