Ribonuclease T1
= Ribonuclease T1
{title2=EC 4.6.1.24}
<Ribonuclease T1> is a guanosine-selective <ribonuclease>: it cleaves susceptible <phosphodiester bonds> on the 3′ side of <guanosine> residues in single-stranded <RNA>. Its two-stage chemistry first creates an <RNA cyclic phosphate end> and a 5′-hydroxyl end, then converts the cyclic end to a 3′-phosphate end by <hydrolysis>.<Ribonuclease T1 in IUBMB Enzyme Nomenclature>[EC 4.6.1.24, Reaction 1a–1b and Comments]{ref}
The current classification is \b[EC 4.6.1.24], a <lyase> entry. The former EC 3.1.27.3 entry was transferred in 2020 because the primary cleavage step is a phosphorus–oxygen lyase reaction. Merely describing the final overall reaction as hydrolysis hides that first step.<Ribonuclease T1 in IUBMB Enzyme Nomenclature>[Systematic name, Comments and entry history; former EC 3.1.27.3 transfer notice]{ref}
An invented sequence example makes its selectivity concrete. Read `ACGUAAGCU` from 5′ to 3′ and suppose all eligible internal bonds are accessible and completely cleaved. Cutting after each G gives `ACG|UAAG|CU`: fragments of three, four and two <nucleotides>, respectively. The nine nucleotides are redistributed among three fragments rather than disappearing. For a linear chain, $k$ distinct internal cuts produce $k+1$ fragments.
The letters alone do not guarantee that a folded <RNA> exposes every candidate bond. Sequence preference and physical accessibility jointly affect a cleavage pattern; the toy calculation explicitly assumes complete accessibility.
This selectivity also made <ribonuclease T1> useful in historical sequence comparisons. A 1977 study characterized <16S ribosomal RNA> from ten methane-producing microbial species through the fragments produced by T1 digestion, then compared the resulting catalogs.<Classification of methanogenic bacteria by 16S ribosomal RNA characterization by George E. Fox and others>[Abstract, T1 digestion and comparative oligonucleotide characterization]{ref} Selective cleavage converted a large molecule into a collection of experimentally comparable sequence pieces.