RIBOSOMAL SUBUNIT-POLYRIBOSOME CYCLE IN PROTEIN SYNTHESIS OF EMBRYONIC SKELETAL MUSCLE

Thigh muscle from 14-day-old chicken embryos consists predominantly of young multinucleated myofibers having a broad and characteristic distribution of polyribosomes which are not attached to membranes. Autoradiographic experiments suggest that the fibers are the major site of muscle ribosome synthesis. Accordingly, large polyribosomes synthesizing fiber-specific proteins become highly labeled with radioactive ribosomal ribonucleic acid after embryos are incubated with isotopic ribonucleic acid precursors. Pulse labeling of polysomal nascent peptide chains with radioactive amino acids showed that the largest polysomes synthesize much larger proteins than those made on small polysomes. Furthermore, the time required to synthesize a complete protein increases linearly with the size of polypeptide chain. Following intravenous injection of embryos with [32P]phosphate, analysis was made of the flow of radioactive ribosomal ribonucleic acid into polysomes of various sizes, into single ribosomes, and into subribosomal particles. The results suggest that equilibration of active subribosomal particles with even the largest polyribosomes is so rapid that their specific radioactivities remain nearly equal throughout the labeling. In contrast with its rapid cycling through polysomes, radioactive ribosomal ribonucleic acid enters into the pool of single ribosomes at a very slow rate. It is concluded that at least 90% of the embryonic muscle single ribosomes, and possibly all of them, do not participate in the ribosomal subunitpolyribosome cycle which accompanies protein synthesis. © 1969, American Chemical Society. All rights reserved.