The specific activity of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) increases 30-to 50-fold when dark-grown pea seedlings are shifted into the light. The large subunit (LS) of this multimeric protein is known to be synthesized in the chloroplast, but plastids from dark-grown cells contain relatively low levels of LS. However, despite the low level of LS synthesis in the plastids of dark-grown plants, these organelles contain significant levels of LS mRNA. Hybridization studies showed that the amount of LS mRNA increased about 3-fold, relative to total plant RNA, when dark-grown plants were illuminated. This increase in LS mRNA can be accounted for by a similar increase in chloroplast genome copy number. It was found that the amount of translatable LS mRNA per ltg of plastid RNA is similar when isolated from either dark-grown plants or dark-grown plants subjected to light. These results suggest that although light can increase the level of LS mRNA by increasing the copy number of this gene, the primary regulation of LS synthesis by light in pea chloroplasts is at the level of translation.

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The two subunits of the renal brush border enzyme, y-glutamyl transpeptidase (EC 2.3.2.2), are derived from a single-chain propeptide. The membrane-spanning domain consists of a hydrophobic sequence near its NHz-terminus and the protein is oriented with its NHz-terminus on the cytoplasmic side. The enzyme is synthesized without a cleavable signal sequence. Translocation and insertion of this enzyme have been shown to be dependent on the signal recognition particle and presumably require the same translocation machinery that other secretory and membrane proteins use for these processes.y-Glutamyl transpeptidase; Brush border enzyme; Membrane insertion; Signal sequence; Signal recognition particle