SUBSTRATE SPECIFICITY OF RIBONUCLEASE FROM ASPERGILLUS SAITOI

1. The substrate specificity of Ribonuclease M [EC 2.7.7.17, Ribo-nucleate nucleotido-2'-transferase (cyclizing)], which was obtained from Aspergillus saitoi, was studied in detail using several kinds of RNA's and sixteen diribonucleoside monophosphates (XpY's, where X or Y indicates one of the four main ribonucleoside residues), as the substrates.2. The Km value of this enzyme for the XpY's differed depending on the nucleoside residues, X or Y. With different nucleoside residues the order of the Km values was: uridine>guanosine>cytidine>adenosine. On the other hand, the maximum velocity (Vmax) for XpY was lowest when either of X or Y was adenosine.3. It was found that, when mixtures of several XpY's were used as substrates of this enzyme, the XpY's giving the smallest Km value with the enzyme were split firstest.4. A spectrophotometric procedure was developed to follow the enzymatic transesterification of the substrates, XpY's, with purine nucleo-side as X.5. Similar kinetic studies were made on ribonuclease T2 [EC 2.7.7.17] from Aspergillus orizae. © 1968 by the Journal of Biochemistry.