ORGANIZATION OF THE LIGHT-HARVESTING COMPLEX OF PHOTOSYSTEM-I AND ITSASSEMBLY DURING PLASTID DEVELOPMENT

Photosystem I (PSI) holocomplexes were fractionated to study the organ ization of the light-harvesting complex I (LHC I) pigment-proteins in barley (Hordeum vulgare) plastids. LHC Ia and LHC Ib can be isolated a s oligomeric, presumably trimeric, pigment-protein complexes. The LHC la oligomeric complex contains both the 24- and the 21.5-kD apoprotein s encoded by the Lhca3 and Lhca2 genes and is slightly larger than the oligomeric LHC Ib complex containing the Lhca1 and Lhca4 gene product s of 21 and 20 kD. The synthesis and assembly of LHC I during light-dr iven development of intermittent light-grown plants occurs rapidly upo n exposure to continuous illumination. Complete PSI complexes are dete cted by nondenaturing Deriphat (disodium N-dodecyl-beta-iminodipropion ate-160)-PAGE after 2 h of illumination, and their appearance correlat es with that of the 730- to 740-nm emission characteristic of assemble d LHC I. However, the majority of the newly synthesized LHC I apoprote ins are present as monomeric complexes in the thylakoids during the ea rly hours of greening. We propose that during development of the proto chloroplast the LHC I apoproteins are first assembled into monomeric p igmented complexes that then aggregate into trimers before becoming at tached to the pre-existing core complex to form a complete PSI holocom plex.