EVIDENCE FROM IN-VIVO MANIPULATIONS OF LIPID-COMPOSITION IN MUTANTS THAT THE DELTA(3)-TRANS-HEXADECENOIC ACID-CONTAINING PHOSPHATIDYLGLYCEROL IS INVOLVED IN THE BIOGENESIS OF THE LIGHT-HARVESTING CHLOROPHYLL AB-PROTEIN COMPLEX OF CHLAMYDOMONAS-REINHARDTII/

The phosphatidylglycerol containing the unusual Delta(3)-trans hexadec enoic fatty acid is specifically found in photosynthetic membranes of eukaryotic organisms. Its involvement in the biogenesis and the struct ure of the light-harvesting chlorophyll a/b-protein complex has been e videnced by in vivo targeting this lipid to photosynthetic membranes o f Chlamydomonas reinhardtii mutants lacking this lipid. In the mf1 and mf2 mutants, this deficiency results in (a) the absence of the oligom eric light-harvesting complex of photosystem 2; (b) an extensive desta cking of thylakoid membranes; (c) a very low 77-K fluorescence emissio n in the photosystem-2 region. We show in this paper that these defici encies result from modifications in the pigment and polypeptide compos itions of the photosystem-2 light-harvesting complex; it contains less chlorophyll b and some of its constitutive polypeptides are absent or reduced in amount, while immunologically related polypeptides of lowe r molecular mass accumulate. The direct involvement of the lack of tra ns-C16:1-phosphatidylglycerol in these deficiencies is evidenced by th e partial restoration of normal characteristics of the light-harvestin g complex (pigment and polypeptide composition, oligomerization) after liposome-mediated, in vivo incorporation of this lipid into the photo synthetic membranes of the mf2 mutant. Trans-C16:1-phosphatidylglycero l, therefore, is involved in the biogenesis of the photosystem-2 light -harvesting chlorophyll a/b-protein complex through a mechanism that m ay prevent degradation processes. Its contribution to the structural c onformation of neosynthesized monomers and to their organization into stable oligomeric form is discussed.