Inhibition of Phenylalanine Incorporation by Polyuridylic Acid

Abstract

ADDITION of polyuridylic acid (poly U) to cell-free systems prepared from a number of tissues results in enhanced incorporation of phenylalanine into polypeptide material^1–4. Arnstein et al.² noted that, when poly U was added to a reticulocyte ribosome system, the incorporation of other amino-acids was not inhibited and they concluded that natural messenger RNA (m-RNA) was not displaced from the ribosomes by the poly U. They questioned whether the poly U effect on phenylalanine incorporation had any physiological significance. Weinstein et al.⁵ later showed that incorporation of some amino-acids was inhibited when poly U was added to reticulocyte ribosomes. The present report concerns an instance when poly U, added to rat liver polysomes prepared in the absence of magnesium, actually inhibited phenylalanine incorporation. It is argued that this anomalous occurrence, paradoxically, strengthens the concept that poly U stimulation of phenylalanine incorporation is a reflexion of physiological events.