EXTRACELLULAR RIBONUCLEASE FROM RHIZOPUS STOLONIFER - CHARACTERISTICSOF AN ATYPICAL - GUANYLIC ACID PREFERENTIAL - ENZYME FROM RIBONUCLEASE-T2 FAMILY

An extracellular ribonuclease from Rhizopus stolonifer (designated as RNase Rs) was purified to homogeneity by chromatography on DEAE-cellul ose followed by CM-cellulose. The Mr of the purified enzyme determined by gel filtration and SDS-PAGE is 25 000 and 28 200, respectively. RN ase Rs is a glycoprotein and contains 10.5% neutral sugar. It is an ac idic protein with a pI of 5.0 and has a blocked N-terminus. The optimu m pH and temperature are 5.5 and 45 degrees C, respectively. RNase Rs shows high stability between pH 6.0-10.0. Divalent cations like Zn2+, Hg2+ and Cu2+ inhibit the enzyme activity whereas, mononucleotides doe s not have any significant effect. The enzyme cleaves RNA to 3'-mono-n ucleotides via 2',3'-cyclic nucleotides, with preferential liberation of 2',3'-cyclic GMP, suggesting that RNase Rs is a guanylic acid prefe rential cyclizing RNase. Moreover, cyclic nucleotides generated are hi ghly resistant to further hydrolysis. (C) 1998 Elsevier Science B.V.