Light-Harvesting Chlorophyll a/b Complexes: Interdependent Pigment Synthesis and Protein Assembly.

Abstract

The biogenetic interdependence of light-harvesting chlorophyll (Chl) alb proteins (LHCPs) and antenna pigments has been analyzed for two nuclear mutants of Chlamydomonas that have low levels of Chl b, neoxanthin, and loroxanthin. In mutant PA2.1, the apoprotein precursors (pLHCP 11) of the major light-harvesting complex LHC II were synthesized at approximately wild-type rates, processed to their mature size, and rapidly degraded. Because the bulk of labile LHCP II in PA2.1 was soluble, a thylakoid integration factor apparently is defective in this strain. Chl a, Chl b, neoxanthin, and loroxanthin synthesis and accumulation were coordinately reduced in PA2.1, indicating that LHCP II play important regulatory or substrate roles in de novo synthesis of these pigments. Mutant GE2.27 is impaired principally in Chl b synthesis but nonetheless accumulated wild-type levels of all LHCPs. Topology studies of the GE2.27 LHCP II demonstrated that their insertion into thylakoids was incomplete even though they were not structurally altered. Thus, Chl b formation mediates conformational changes of LHCP II after thylakoid integration is initiated. GE2.27 also exhibited very low rates of neoxanthin synthesis and was unable to accumulate loroxanthin. Revertant GE2.27 strains with varying capacities for Chl b formation provided additional evidence that neoxanthin synthesis and accumulation are coupled with the final steps of LHCP II integration into thylakoids. We propose that biogenesis of LHC includes interdependent pigment synthesidassembly events that occur during LHCP integration into the thylakoid membrane and that defects in these events account for the pleiotropic characteristics of many Chl b-deficient mutants.

Antisense Expression and Overexpression of Biotin Carboxylase in Tobacco Leaves

“…Furthermore, antisense expression of the small subunit in tobacco resulted in the coordinate decrease of the large subunit (Rodermel et al, 1988). It has also been reported that in chlorophyll biosynthetic mutants, light-harvesting chlorophyll alb proteins are rapidly degraded because of their inability to assemble the correct pigment-protein complex (Plumley and Schmidt, 1995).…”

Lhcb Transcription Is Coordinated with Cell Size and Chlorophyll Accumulation (Studies on Fluorescence-Activated, Cell-Sorter-Purified Single Cells from Wild-Type and immutans Arabidopsis thaliana)

“…* Corresponding author; e-mail rodermel8iastate.edu; fax 1-515-294-1337. chlorophyll pigments that bind CAB are synthesized in the chloroplast by posttranslationally imported nuclear gene products (reviewed by Bartley and Scolnik, 1995;von Wettstein et al, 1995). It is thought that pigment binding occurs concurrently with or shortly after apoprotein integration in the thylakoid membrane (Paulsen et al, 1993;Cohen et al, 1995;Kuttkat et al, 1995;Plumley and Schmidt, 1995).…”

“…Analyses of mutants with apoprotein or pigment deficiencies have established that pigment cofactors, especially chlorophyll b, are required for the stabilization of many, but not all, CAB apoproteins in the thylakoid membrane (Somerville, 1986;Flachmann and Kiihlbrandt, 1995;KrÓ1 et al, 1995;Plumley and Schmidt, 1995;Preiss and Thornber, 1995;von Wettstein et al, 1995). This indicates that posttranslational mechanisms play a central role in coordinating pigment and CAB apoprotein biosynthesis.…”

Simultaneous regulation of antenna size and photosystem I/II stoichiometry in Arabidopsis thaliana

“…Chlorophylls a and b are indispensable for the stabilization of LHC proteins in land plants (Plumley and Schmidt 1995) and Chlamydomonas reinhardtii (Polle et al 2000). Indeed, the LHCII trimer is absent in the Arabidopsis ch1 mutant, which does not accumulate chlorophyll b due to mutation of the chlorophyll a oxygenase (CAO) gene (Espineda et al 1999;Tanaka et al 1998).…”