The binding of polyamines and of ethidium bromide to tRNA

Abstract

The binding of spermidine and ethidium bromide to mixed tPNA and phenylalanine tRNA has been studied under equilibrium conditions. The numbers and classes of binding sites obtained have been compared to those found in complexes isolated by gel filtration a low ionic strength. The latter complexes contain 10–11 moles of either spermidine or ethidium per mole of tRNA; either cation is completely displaceable by the other. In ethidium complexes, the first 2–3 moles are bound in fluorescent binding sites; the remaining 7–8 molecules bind in non-fluorescent form. At least one of the binding sites for spermidine appears similar to a binding site for fluorescent ethidium. Similar results are found with E. coli formylmethionine tRNA. Spermine, in excess of 18–20 moles per mole tRNA, causes precipitation of the complex. Putrescine does not form isolable complexes with yeast tRNA and displaces ethidium less readily from preformed ethidium-tRNA complexes.