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Molecular Biology and Gene Regulation

Plastid Development in Pisum sativum Leaves during Greening ¹

II. Post-Translational Uptake by Plastids as an Indicator System to Monitor Changes in Translatable mRNA for Nuclear-Encoded Plastid Polypeptides

Department of Cellular and Developmental Biology, Harvard University, Cambridge, Massachusetts 02138

When isolated pea plastids are incubated with translation products of poly(A⁺) mRNA they specifically import precursor molecules of plastid polypeptides. Etioplasts and chloroplasts import the same polypeptides from identical translation products, and, the imported polypeptides can be well resolved by two-dimensional gel electrophoresis. Therefore, the posttranslational uptake system using isolated chloroplasts can monitor changes in the abundance of translatable plastid-targeted messages. Poly(A⁺) mRNA was isolated from peas at various times during greening and analyzed by this technique. (a) After 48 hours of illumination of dark-grown plants, the relative portion of nuclear encoded messages for plastid targeted proteins had increased by a factor of 2. The percentage of polypeptides recovered in the stroma fraction increased from about 50 to 65%. (b) More than 140 imported polypeptide species could be detected in fluorograms of two-dimensional gels, most of which could be identified throughout the time course of greening. At least 37 imported polypeptides decreased and 36 increased in relative abundance during greening of darkgreen plants. (c) In most cases, where differences in translatable messages were seen between dark- and light-grown plants, they were accompanied by parallel changes in polypeptide abundance.

¹ Supported in part by a research grant from the National Institute of General Medical Sciences. It was also supported in part by the Maria Moors Cabot Foundation for Botanical Research of Harvard University.