Accepted name: ribonuclease U2
Reaction: (1) [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.
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, 3rd edn, 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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