Dissociation reassociation and phenylalanine incorporation by chloroplast and cytoplasmic wheat-leaf ribosomes.

Abstract

Ribosome preparations from wheat seedlings were separated by zonal centrifugation to yield monomers of 80 S (cytoplasmic) and 70 S (chloroplast). When centrifuged through gradients containing high KCl-to-MgCl2 buffer, the 80 S ribosomes dissociated into subunits of 61 S, 49 S, and 42 S, while the 70 S monomers gave subunits of 50 S and 30 S. After zonal separation of the subunits, they could be reassociated into parent ribosomes in the presence of MgCl2. Both 80 S and 70 S ribosomes actively synthesized polyphenylalanine in the presence of poly U. The rate of synthesis was similar for both species. Incorporation of phenylalanine by 80 S and 70 S ribosomes was inhibited by puromycin, was not inhibited by cycloheximide, and only the 70 S species was affected by chloramphenicol, indicating that the latter was of chloroplast origin.