COMPARATIVE ELECTROPHORETIC STUDIES ON PROTEIN OF CHLOROPLAST AND CYTOPLASMIC RIBOSOMES OF SPINACH LEAVES

1. 1. Chloroplast and cytoplasmic ribosomes of Spinacia oleracea have been compared by analytical ultracentrifugation in 2H2O. The s°20,w values assigned to monosomes of chloroplast and cytoplasmic origin are 67 and 78 S, respectively. Polymers of the 67- and 78-S series have also been obtained. Calculations based on their sedimentation coefficients indicate that the mass of chloroplast ribosomes is 1.3-1.4 times smaller than that of cytoplasmic ribosomes. 2. 2. Proteins isolated from chloroplast ribosomes by the LiCl-urea method are electrophoretically dissimilar from those extracted in a similar manner from cytoplasmic ribosomes. Furthermore, chloroplast ribosomes contain (in addition to the usual basic proteins) 8-10 acidic components that are not detected in the corresponding cytoplasmic ribosomes. It is suggested that different cystron(s) direct the synthesis of the ribosomal proteins in the two ribosome classes. 3. 3. Chloroplast ribosomes dissociate into a large (46 S) and a small (30 S) subunit by dialysis against 0.05 mM Mg2+. Upon dissociation, the ribosomal proteins become distributed between the subunit particles in a mode similar to that of the 70-S ribosomes of bacterial origin, i.e., only few proteins are shared by both subunits and the 70-S patterns arise from the qualitative complementation of subunit-specific proteins. 4. 4. Cytoplasmic ribosomes are incompletely dissociated by dialysis against Mg2+-free buffers. However, in the presence of EDTA they dissociate into a large (56 S) and a small (36 S) subunit. Upon dissociation, the ribosomal proteins become distributed between the subunit particles in a mode resembling that of the 80-S ribosomes of animal source, i.e., the larger subunits contain essentially all the proteins which are found in the undissociated ribosome, whereas the 36-S subunits are simply characterized by the absence of several protein components. © 1969.