Isolated tetrameric particles (166S) derived from the crystalline lattices known to appear in hypothermic chicken embryos consist of mature 80S ribosomes which contain all species of ribosomal RNA and a complete set of ribosomal proteins.

Ribosome tetramers are not a special type of polysomes since in solutions of high ionic strengths (500 mM KCl and 50 nM triethanolamine-HCl buffer) containing 5 mM MgCl2 they dissociate into 40S and 60S ribosomal subunits, without the need of puromycin, and at a concentration of Mg++ higher than 3 mM they are not disassembled by mild RNase treatment. Tetramers spontaneously disassemble into 80S monomers when the Mg++ concentration is lowered to 1 mM at relatively low ionic strength. Tetramers failed to couple in vitro puromycin-3H into an acid-insoluble product, indicating the lack of nascent polypeptide chains. Although tetramers have no endogenous messenger RNA activity, they can be programmed in vitro with polyuridylic acid (poly U) to synthesize polyphenylalanine. All ribosomes within a tetramer can accept poly U, without the need of disassembly of the tetramers into monomers or subunits.

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