POLYRIBOSOMES ASSOCIATED WITH PRIMARY MEMBRANES OF PEA ETIOCHLOROPLASTS - THEIR LOCALIZATION AND BIOGENESIS OF THE CHLOROPLAST MEMBRANE SYSTEM

The fraction of primary membranes was isolated from etiochloroplasts o f pea seedlings (Pisum sativum L.) at an early stage of development, a nd a protein-synthesizing apparatus is shown to be associated with it. The fraction was separated by centrifugation in a stepped sucrose con centration gradient into five subfractions, two of which (the lower on es) are represented by fragments of primary membranes containing polyr ibosome-like structures, while the upper ones contain the same structu res partially covered by a membrane. Polyribosomes of all subfractions have a uniform structural type. They consist of expanded networks who se outlines are wholly inscribed in the fret-like structures formed at subsequent stages. Treating the fraction of primary membranes with tr yspin gave a subfraction of reticulate polyribosomes that is sensitive to RNase treatment, possesses a higher RNA/protein ratio, and has a h igher rate of incorporation of C-14-labelled amino acids than in the s tarting fraction of primary membranes. The localization of polyribosom es associated with primary membranes differs from that established for the rough reticulum of eukaryotes. Instead of on the surface, they ar e found inside the membrane and can be released only as a result of br eakage and removal of the membrane component. Treatment with agents th at remove polyribosomes from membranes (0.5 M KCl and puromycin) is in effective in the case of primary membranes. The authors develop the co ncept that a new early stage exists in the biogenesis of primary membr anes, one that is characterized by the appearance in them of expanded reticulate polyribosomes which precede the fret-like structures and ar e the basis of their formation.