A RIBONUCLEASE FROM PHYSARUM BIOCHEMICAL PROPERTIES AND SYNTHESIS IN MITOTIC CYCLE

A ribonuclease from the myxomycete Physarum polycephalum has been partially purified and characterized; it is called Physarum ribonuclease I. It is an extracellular enzyme, has a pH optimum of 4.0, is heat labile, stable over a wide range of pH values; and has a molecular weight of about 31 000 daltons, as determined by gel filtration. No metal ions are required for its activity. RNA serves as substrate, while native or denatured DNA are resistant. Poly U and poly A are rapidly degraded to cyclic mononucleotides and slowly to 3′-mononucleotides. Poly C is very slowly degraded to cyclic cytidine monophosphate. Dinucleotide phosphates are rapidly hydrolyzed, except those of the form CpN. After prolonged incubation, RNA is degraded primarily to 3′-AMP, 3′-UMP, 3′-GMP and cyclic 2′,3′-CMP. The enzyme is an endonuclease. It may be useful for sequence studies of RNA, particularly for further partial degradation of T1 fragments. The activity of ribonuclease I was studied in the highly synchronous mitotic cycle of Physarum. Enzyme activity doubles stepwise shortly before the middle of each interphase. This rise depends on protein synthesis de novo. © 1969.