RESPONSE OF RAT LIVER POLYSOMES TO ADDED HOMOPOLYNUCLEOTIDES - REMOVAL OF INACTIVE RIBOSOMES

The addition of poly U, poly I and poly A to rat liver polysomes promotes the incorporation of a number of amino acids not coded for either normally or ambiguously by the added polynucleotides. Part of the stimulation is due to protection by the added polynucleotides of the endogenous messenger from nuclease attack, but we suggest most of this stimulation is caused by the removal of inactive or inoperative ribosomes from the native polysomes. The active ribosomes left are then able to translate more of the endogenous messenger. Low concentrations of puromycin stimulate translation of polysomes that have stopped incorporating, probably by removing a block constituted by inoperative ribosomes that are unable to release their peptide chains. Stimulation of phenylalanine incorporation is observed on addition of poly U to rat liver polysomes in a cell-free system. To see whether the ribosomes present as polysomes or the monomeric ribosomes contaminating the polysomes were responsible for the stimulation, use was made of poly I to block the initiation of ribosomes prior to the addition of poly U. Evidence from this type of experiment and also experiments with fractionated polysomes indicates that the stimulation is partly due to active monomeric ribosomes present in the polysome preparation and partly due to active monomeric ribosomes derived from the polysomes during the process of translation. © 1969.