Abstract
Etioplasts of 4.5-day-old dark-grown barley synthesize and accumulate most of the membrane and nearly all the soluble polypeptides of mature chloroplasts of light-grown seedlings. Etioplasts do not synthesize a limited set of chloroplast-encoded polypeptides which are major constituents of chloroplast thylakoid membranes: two chlorophyll apoproteins of photosystem I (68 and 65 kDa), two chlorophyll apoproteins of photosystem II (47 and 43 kDa), and a 32-kDa polypeptide which has now been identified as the psbA gene product. Throughout development in the dark, etioplasts were unable to synthesize the chlorophyll apoproteins of photosystem I and II or the psbA gene product despite the presence of significant transcript levels for psbA and psaA-psaB (encode for photosystem I chlorophyll apoproteins). Light was not required for the synthesis of ribulose bisphosphate carboxylase large subunit with the highest rate of large subunit synthesis occurring in young dark-grown seedlings. Illumination of 4.5-day-old dark-grown barley rapidly induced the synthesis of the chlorophyll apoproteins and the psbA gene product at a time when transcript levels for psbA and psaA-psaB did not increase appreciably. Therefore, during the early stages of light-induced development the synthesis of the chlorophyll apoproteins of photosystem I and psbA gene product is regulated at the translational level. With continued chloroplast development in the light, the synthesis of the chlorophyll apoproteins of photosystem I and II decline rapidly as does the synthesis of the large subunit of ribulose bisphosphate carboxylase. The decline in polypeptide synthesis correlated with a decline in rbcL and psaA-psaB transcript levels and a light-dependent decline in plastid rRNA content. In contrast, synthesis of the psbA gene product was maintained throughout light-induced chloroplast development which correlated with the maintenance of psbA transcript levels. However, light is not strictly required for psbA transcript accumulation since psbA transcript levels increased slightly with continued plastid development in dark-grown seedlings.
Highlights
These plastids have synthesized and accumulated mostof the soluble and membrane polypeptides foundin light-grown chloroplasts[16].Whendark-grown plants are illuminatedp,rotochlorophyllide is reduced to chlospite the presence of significant transcript levels for rophyll, chlorophyll apoproteins are synthesized, and chloropsbA and psaA-psaB (encode for photosystem I chlo- phyll-protein complexes are assembled [16,17,18]
Light is not strictly required for psbA transcript accumulation since psbA transcript levels increased slightly with continued plastid development indark-grown the 65-70-kDa chlorophyll apoproteins of PSI,’ a43-kDa chlorophyll apoprotein of PSII, and a 32-34-kDa membrane polypeptide. [35S]Methionine pulse-labelingand Northern assays showed that the synthesis of some of the light-induced proteins was regulated at the level of translation [16]
The 32-34-kDa membrane polypeptide whichshowed lightinduced translation was similar in molecular weight to the psbA gene product.Thisresult was surprising since psbA transcript levels did notincreaseduringthefirsthour of illumination whereas the translation of the 32-34-kDa polypeptide increased dramatically
Summary
OF psbA,psaA-psaB, AND rbcL IN DARK-GROWNAND (Received for publication, September2, 1986). In monocots such as barley, plastid biogenesis proceeds in the dark to a point where the number of plastids/cell approaches that of mature light-grownleaves [15]and towhere etioplasts have reached 70% of the volume of the mature chloroplast[16] These plastids have synthesized and accumulated mostof the soluble and membrane polypeptides (both nuclear-encoded and chloroplast-encoded) foundin light-grown chloroplasts[16].Whendark-grown plants are illuminatedp,rotochlorophyllide is reduced to chlospite the presence of significant transcript levels for rophyll, chlorophyll apoproteins are synthesized, and chloropsbA and psaA-psaB The abbreviationsused are: PSI, photosystem I; PSII, photosystem 11; Hepes, 4-(2-hydroxyethyl)-l-piperazineethanesulfoniaccid; SDS-PAGE, sodiumdodecyl sulfate-polyacrylamide gel electrophoresis
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