Chlorophyll apoprotein accumulation in higher plant chloroplasts is controlled by light-dependent chlorophyll' formation. Dark-grown plants lack chlorophyll and chlorophyl apoproteins. However, the plastid genes' encoding the chlorophyll apoproteins are transcribed; chlorophyll apoprotein mRNA accumulates and associates with polysomes in plastids of dark-grown plants. Pulse-labeling assays revealed a population of short-lived proteins in plastids of dark-grown plants. One of these transiently labeled proteins was CP43, a chlorophyll apoprotein associated with photosystem II. Plse chase assays showed that newly synthesized CP43 was rapidly degraded in plastids of dark-grown plants, which lack chlorophyll. In contrast, CP43 synthesized in plastids from illuminated plants was stable. The synthesis of D1, a chlorophyll apoprotein of the photosystem II reaction center, was also analyzed in plastids of dark-grown and illuminated plants. Radiolabel accumulation into full-length D1 was only 'detected in plastids of illuminated plants. However, Dl translation intermediates of 15-25 kDa were detected in both plastid populations. Pulse-chase assays showed that the 15-to 25-kDa D1 translation products were precursors of mature D1 in plastids of illuminated plants. In contrast, in plastids of darkgrown plants, the 15-to 25-kDa translation intermediates were converted into a 23-kDa polypeptide previously suggested to be a proteolytic product of Dl. These results indicate that chlorophyll produced in illuminated plants stabilizes D1 nascent polypeptides, which allows accumulation of mature DM.Chlorophyll is the key chromophore involved in plant photosynthetic light reactions. This chromophore mediates lightdependent charge separation in photosystem I (PSI) and photosystem II (PSII) and serves as the primary lightharvesting pigment in these photosystems. Chlorophyll is noncovalently associated with at least 12 different membrane-bound proteins of the thylakoid. PSII reaction centers contain two plastid-encoded chlorophyll-binding proteins, D1 although radiolabeling studies failed to detect amino acid incorporation into D1, CP43, CP47, and the P700 chlorophyll apoproteins (10). Within 5 min after dark-grown plants are illuminated, amino acid incorporation into D1, CP43, CP47, and the P700 chlorophyll apoproteins can be detected, and the chlorophyll apoproteins begin to accumulate (11,14,20). The light-dependent activation 'of chlorophyll apoprotein accumulation is controlled by protochlorophyllide reductase and requires formation of chlorophyll a (11,20
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