Regulation of synthesis and accumulation of chlorophyll-apoproteins during light-induced chloroplast development in barley

Abstract

The ribosome binding sites on chloroplast mRNAs in vivo were determined by toeprint analyses of rbcL mRNA and psbA mRNA associated with chloroplast polysomes. The binding of E. coli 30S ribosomes to rbcL and psbA mRNAs was also tested, suggesting that the selection of psbA mRNA involves a mechanism different from rbcL mRNA or bacterial mRNA. The translation initiation complexes for psbA, psaA and rbcL mRNAs were detected in 4.5 day-old dark-grown barley seedlings. Illumination of these plants for up to 16 h did not cause an increase in the abundance of these initiation complexes, indicating that translation initiation is not a control point for the regulation of chlorophyll-apoprotein accumulation during this stage of plastid development. Ribosomes associated with chlorophyll-apoprotein mRNAs were shown to run-off to a similar extent in the presence and absence of chlorophyll a. Translation intermediates of chlorophyll-apoproteins, D1, CP43 and CP47, were found to be associated with polysomes in etioplasts in the absence of chlorophyll a and were shown to be rapidly degraded. In contrast, when chlorophyll a was synthesized in etioplasts, the stability of these intermediates was increased and mature proteins accumulated. These results suggest that cotranslational binding of chlorophyll to the chlorophyll-apoprotein confers the increased stability on the apoprotein, allowing mature chlorophyll-apoprotein to accumulate. The translation intermediates of D1 were found to correlate with ribosome pausing sites. No difference in ribosome pausing on psbA mRNA was found at 0 to 1 h of illumination of dark-grown plants. The ribosome pausing on psbA mRNA was increased at 16 or 72 h of illumination. Similar correlations between ribosome pausing and translation intermediates were also observed during translation of CP43 and CP47. Ribosome pausing may facilitate co-translational binding of cofactors such as chlorophyll to chlorophyll-apoproteins and aid the integration of chlorophyll-apoproteins into thylakoid membranes. The vir-115 nuclear gene product was demonstrated to stabilize D1 translation intermediates which increase as a function of chloroplast development. The vir115 protein was proposed to be a chaperonin-like factor which facilitates correct folding and membrane insertion.