ACTIVE HYBRID 80 S PARTICLES FORMED FROM SUBUNITS OF RAT, RABBIT AND PROTOZOAN (TETRAHYMENA PYRIFORMIS) RIBOSOMES

Ribosomes of rat skeletal muscle and rat liver were dissociated in 880 mm-potassium chloride at 28 °C in the presence of magnesium ions (12.5 mm); at 4 °C the subunits formed aggregates. In contrast, the subunita formed from rabbit muscle and Tetrahymena pyriformis ribosomes in the same ionic circumstances did not aggregate at 4 °C. The ability of rat muscle ribosomal subunits to combine with subunits of other 80 s ribosomes was determined by density-gradient analysis and by estimation of polyuridylic acid-directed polypeptide synthesis. Both hybrids formed and were active when rat muscle and rat liver subunits were combined. Subunits from rat and rabbit skeletal muscle ribosomes also formed active hybrids, despite a difference in the 60 s particles of the two species; the rat 60 s subunit has a far greater tendency to form 90 s dimers at low temperatures. However, only one hybrid was detected by density-gradient analysis when rat muscle and Tetrahymena subunits were mixed. The small subunits of the protozoan and the large subunits of rat formed 80 s particles, which synthesized polyphenylalanine at the same rate as re-associated Tetrahymena ribosomes. The 40 s of rat and the 60 s of the protozoan did not appear to form 80 s particles, but the mixture had some protein synthetic activity in the presence of polyuridylic acid. The ribosomal proteins contained in the subunits of rat muscle and Tetrahymena ribosomes were markedly different. © 1969.