= RNA ligase
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= RNA ligases
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{synonym}
An <RNA ligase> is an <enzyme> that catalyzes covalent joining of <RNA> ends. Its usefulness depends on the terminal groups it recognizes and the chemistry that activates the joining reaction. The activity classified as EC6.5.1.3, an <ATP-dependent RNA ligase>, joins $3'$ <hydroxyl group> and $5'$ phosphate termini while converting <ATP> to <adenosine monophosphate> and diphosphate.<RNA ligase ATP in IUBMB Enzyme Nomenclature>[Reaction and three-step mechanism]{ref}
<RtcB> illustrates another route. It joins a $3'$ phosphate to a $5'$ <hydroxyl group> using <GTP>, and can process an <RNA cyclic phosphate end> before sealing. The substrate ends appropriate for one <RNA ligase> therefore need not be appropriate for another.<RtcB in IUBMB Enzyme Nomenclature>[EC6.5.1.8, Reactions1–2 and Comments]{ref}
For a specified assay, imagine two <RNA> fragments held in the correct alignment. One pair has the terminal groups accepted by the chosen <RNA ligase>; an otherwise sequence-identical pair carries a blocked end. A failure to join the second pair could reflect end chemistry even though the letters and alignment match. A meaningful comparison separates recognition, activation and final <phosphodiester bond> formation. Historically, purification of a T4-induced enzyme and tests of its labelled products connected the joining activity to a biochemical preparation.<Purification and properties of bacteriophage T4-induced RNA ligase by R. Silber and others>[Abstract, enzyme purification and product tests]{ref} That functional evidence goes beyond assigning an enzyme name from sequence resemblance alone.
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