1. Ribonuclease U1 splits only the phosphodiester bonds of guanosine 3′-phosphates in RNA. It may be regarded as a guanyloribonuclease [ribonucleate (guanine nucleotide)-2′-transferase (cyclizing), EC 2.7.7.26] similar to ribonuclease T1 (Egami, Takahashi & Uchida, 1964). It seems to be identical with the extracellular ribonuclease described by Glitz & Dekker (1963, 1964a,b). 2. Ribonucleases U2 and U3 are novel enzymes with a strict specificity. They split the internucleotide bonds between purine 3′-nucleotides and 5′-hydroxy groups of adjacent nucleotides in RNA with the intermediary formation of purine nucleoside 2′,3′-(cyclic)-phosphates, which are slowly hydrolysed to purine 3′-nucleotides. So they may be classified as ‘puryloribonucleases [ribonucleate (purine nucleotide)-2′-transferase (cyclizing)]’. Double-stranded RNA is scarcely split by ribonucleases U2 and U3. 3. Ribonuclease U4 has no absolute base specificity, and produces the mononucleotides 3′-adenylate, 3′-guanylate, 3′-cytidylate and 3′-uridylate from RNA.
