Chlorophyll Regulates Accumulation of the Plastid-Encoded Chlorophyll Proteins P700 and D1 by Increasing Apoprotein Stability

Abstract

Chlorophyll protein accumulation in barley (Hordeum vulgare 1.) chloroplasts is controlled posttranscriptionally by light-induced formation of chlorophyll a. l h e abundance of translation initiation complexes associated with psbA, psaA, and rbcL mRNAs was measured using extension and inhibition analysis in plants grown in the dark for 4.5 d and then illuminated for up to 16 h. Light-induced accumulation of the chlorophyll proteins was not accompanied by changes in the abundance of translation initiation complexes, indicating that regulation of chlorophyll protein accumulation at this stage of development does not occur at the level of translation initiation. Translational runoff assays were performed in the presente of lincomycin, an inhibitor of translation initiation, to determine whether chlorophyll protein accumulation was regulated at the level of translation elongation. l h e extent of ribosome runoff of psaA and psbA mRNAs was similar in the presence or absence of chlorophyll, indicating that chlorophyll did not alter chlorophyll protein translation elongation. Polysome-associated D1 translation intermediates were radiolabeled in the presence or absence of chlorophyll, even though full-length D1 accumulated only in the presence of chlorophyll. Chlorophyll influenced the stability of D 1 translation intermediates to a small extent and greatly increased D 1 stability after release from ribosomes. Overall, these results demonstrate that light-induced chlorophyll biosynthesis triggers the accumulation of the chlorophyll proteins D1 and P700 in barley chloroplasts by enhancement of chlorophyll apoprotein stability.

Light regulates the rate of translation elongation of chloroplast reaction center protein D1

“…In etioplasts isolated from 4.5-day-old dark-grown barley, D1 synthesis is unaltered in light and darkness [7,9,10]. During continuous illumination of dark-grown seedlings, the synthesis rate of D1 remains high; however, at later stages of greening, an increasing number of ribosomes pause at discrete sites during translation elongation and translation intermediates of 15Ϫ 24 kDa accumulate [15,16].…”

“…In the light, analysis of ribosome distribution on psbA mRNA supported an accumulation of ribosomes at the site of initiation, but also revealed that ribosomes pause during translation elongation [14Ϫ16]. Pausing of ribosomes was speculated to improve the efficiency of Chl binding to D1 nascent chains or to enable integration of the D1 protein into the etioplast membranes during greening [9,15].In plastids isolated from transgenic tobacco leaves and in Chlamydomonas reinhardtii, light-dependent accumulation of D1 has been ascribed to regulation at the 5′-untranslated region of the psbA mRNA [17Ϫ19] ; structured RNA elements were characterized to be responsible for translation initiation and proteins were identified that specifically bind to the 5′-untranslated region during translation initiation [17,20,21] nas, ADP-dependent protein phosphorylation was identified to decrease binding of regulatory proteins to the 5′-untranslated region, upon transfer of cells from light to dark, whereas during light-activated generation of a photosynthetic redox potential, binding was enhanced and thioredoxin was suggested to link light and activation of translation initiation [22,23]. In mature spinach chloroplasts, light was shown to increase the reassembly of the D1 protein into photosystem II, whereas darkness increased the accumulation of D1 in polysomes and light was speculated to increase the ribosome run off [24].…”

“…In the light, analysis of ribosome distribution on psbA mRNA supported an accumulation of ribosomes at the site of initiation, but also revealed that ribosomes pause during translation elongation [14Ϫ16]. Pausing of ribosomes was speculated to improve the efficiency of Chl binding to D1 nascent chains or to enable integration of the D1 protein into the etioplast membranes during greening [9,15].…”

The Regulation of Enzymes Involved in Chlorophyll Biosynthesis

“…In addition to Chls a and b, the different chlorophyllprotein complexes also bind carotenoids (Krauss et al, 1993;Lee and Thornber, 1995), which quench Chl triplet states and thus help prevent the formation of harmful oxygen radicals and singlet oxygen (Siefermann-Harms, 1987). There appears to be an excess of nuclear-encoded and plastid-encoded chlorophyll apoproteins that bind free pigment molecules whenever they are synthesized (Kim et al, 1994).…”

Hydrogen Bonding in a Model Bacteriochlorophyll-binding Site Drives Assembly of Light Harvesting Complex

“…In many cases, the BChlhelix interactions exceed the helix-helix interactions and often play a key role in correct folding and assembly of pigment protein complexes (23)(24)(25)(26)(27). Our strategy to study the binding of BChl to membrane-embedded helices and the assembly to stable TMH/BChl pockets is based on the use of model BChl proteins, modified in particular near the BChl-binding sites.…”