Studies on extracellular ribonucleases of Ustilago sphaerogena. Characterization of substrate specificity with special reference to purine-specific ribonucleases

Abstract

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