The presence and synthesis of the NADPH-protochlorophyllide oxidoreductase in barley leaves with a high temperature-induced deficiency of plastid ribosomes

High-temperature-induced deficiency of plastid ribosomes in barley plants (Hordeum vulgare L.) was used as a system for studying the role of the cytoplasm in the synthesis of the NADPH-protochlorophyllide oxidoreductase. The enzyme is present in 33° C-grown plants. The failure of high-temperature-grown plants to accumulate chlorophyll during illumination is not caused by the absence of the protochlorophyllide-reducing enzyme. The synthesis of the NADPH-protochlorophyllide oxidoreductase was studied by feeding ³⁵S]methionine to the seedling and by following the incorporation of the radioactively labeled amino acid into plastid proteins. The NADPH-protochlorophyllide oxidoreductase was labeled in high-temperature-grown barley plants to the same extent as in control plants grown at 25° C. It is concluded that the 36,000-M_r polypeptide of the NADPH-protochlorophyllide oxidoreductase is synthesized outside the plastid on cytoplasmic 80S ribosomes.