IUBMB Enzyme Nomenclature


Accepted name: ribonuclease U2

Reaction: [RNA] containing adenosine + H2O = an [RNA fragment]-3'-adenosine-3'-phosphate + a 5'-hydroxy-ribonucleotide-3'-[RNA fragment] (overall reaction)
(1a) [RNA] containing adenosine = an [RNA fragment]-3'-adenosine-2',3'-cyclophosphate + a 5'-hydroxy-ribonucleotide-3'-[RNA fragment]
(1b) an [RNA fragment]-3'-adenosine-2',3'-cyclophosphate + H2O = an [RNA fragment]-3'-adenosine -3'-phosphate
2 [RNA] containing guanosine + H2O = an [RNA fragment]-3'-guanosine-3'-phosphate + a 5'-hydroxy-ribonucleotide-3'-[RNA fragment] (overall reaction)
(2a) [RNA] containing guanosine = an [RNA fragment]-3'-guanosine-2',3'-cyclophosphate + a 5'-hydroxy-ribonucleotide-3'-[RNA fragment]
(2b) an [RNA fragment]-3'-guanosine-2',3'-cyclophosphate + H2O = an [RNA fragment]-3'- guanosine-3'-phosphate

Other name(s): purine specific endoribonuclease; ribonuclease U3; RNase U3; RNase U2; purine-specific ribonuclease; purine-specific RNase; Pleospora RNase; Trichoderma koningi RNase III; ribonuclease (purine)

Systematic name: [RNA]-purine 5'-hydroxy-ribonucleotide-3'-[RNA fragment]-lyase (cyclicizing; [RNA fragment]-3'-purine-nucleoside -2',3'-cyclophosphate-forming and hydrolysing)

Comments: The enzyme secreted by the fungus Ustilago sphaerogena cleaves at the 3'-phosphate group of purines, and catalyses a two-stage endonucleolytic cleavage. The first reaction produces 5'-hydroxy-phosphooligonucletides and 3'-phosphooligonucleotides ending in Ap or Gp with 2',3'-cyclic phosphodiester, which are released from the enzyme. The enzyme then hydrolyses these cyclic compounds in a second reaction that takes place only when all the susceptible 3',5'-phosphodiester bonds have been cyclised. The second reaction is a reversal of the first reaction using the hydroxyl group of water instead of the 5'-hydroxyl group of ribose. The overall process is that of a phosphorus-oxygen lyase followed by hydrolysis to form the 3'-nucleotides.

Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number:


1. Glitz, D.G. and Dekker, C.A. Studies on a ribonuclease from Ustilago sphaerogena. I. Purification and properties of the enzyme. Biochemistry 3 (1964) 1391-1399. [PMID: 14230791]

2. Glitz, D.G. and Dekker, C.A. Studies on a ribonuclease from Ustilago sphaerogena. II. Specificity of the enzyme. Biochemistry 3 (1964) 1399-1406. [PMID: 14230792]

3. Uchida, T. and Egami, F. Microbial ribonucleases with special reference to RNases T1, T 2, N 1, and U2. In: Boyer, P.D. (Ed.), The Enzymes, 3rdedn, vol.4, Academic Press, New York, 1971, pp. 205-250.

4. Martinez-Ruiz, A., Garcia-Ortega, L., Kao, R., Onaderra, M., Mancheno, J.M., Davies, J., Martinez del Pozo, A. and Gavilanes, J.G. Ribonuclease U2: cloning, production in Pichia pastoris and affinity chromatography purification of the active recombinant protein. FEMS Microbiol. Lett. 189 (2000) 165-169. [PMID: 10930732]

[EC created 1978 as, modified 1981, transferred 2018 to EC]

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