Evidence for the Direct Binding of Polyamines to a Ribonuclease that Hydrolyzes Ribonucleic Acid at Uridylic Acid Residues

The Citrobacter ribonuclease, an enzyme that cleaves preferentially phosphodiester bonds of ribonucleic acid at uridylic acid residues, has been shown to bind the polyamines, putrescine, spermidine, and spermine. The binding of spermidine insures the maintenance of enzyme activity in the presence of the polynucleotide, polyguanylic acid, at a concentration which is ordinarily completely inhibitory. Enzyme activity, if inhibited by polyguanylic acid, could, however, be restored by spermidine, which appears to effect a removal of the polynucleotide from the enzyme surface. The substitution of the polyamine for the polynucleotide was used to obtain two spermidine-bound peptides from tryptic digests of a polyguanylic acid-enzyme complex. Amino acid analysis of the isolated peptides showed that one consisted of a chain having a minimum length of 17 residues but composed of 10 different amino acids. The other had a minimum length of 8 residues and contained only 6 different amino acids. In both cases, the ratio of peptide to polyamine was approximately 1 to 1.