= RNA ligation
{c}
<RNA ligation> joins compatible <RNA> termini by forming a new <phosphodiester bond>. Joining two fragments extends the connected chain; joining the two ends of one fragment produces <circular RNA>. The end chemistry matters: an <ATP-dependent RNA ligase> can join a $3'$ <hydroxyl group> to a $5'$ phosphate, whereas <RtcB> uses a different combination of terminal groups.<RNA ligase ATP in IUBMB Enzyme Nomenclature>[EC6.5.1.3, Reaction and Comments]{ref}<RtcB in IUBMB Enzyme Nomenclature>[EC6.5.1.8, Reactions1–2]{ref}
An unmodified linear four-<nucleotide> fragment has three backbone links, and a six-<nucleotide> fragment has five. One intermolecular ligation gives ten <nucleotides> connected by $3+5+1=9$ links in a linear chain. Circularizing that chain adds one more backbone link while retaining ten <nucleotides>. Counting chain units, connecting bonds and separate molecules therefore answers three different questions.
A 1972 experiment made end joining measurable using radioactively labelled <RNA>. After reaction with a purified <RNA ligase>, labelled terminal phosphate became resistant to alkaline phosphatase. Additional tests with enzymes attacking from chain ends supported formation of circular poly(A) under the studied conditions.<Purification and properties of bacteriophage T4-induced RNA ligase by R. Silber and others>[Abstract, phosphate-label protection and exonuclease tests]{ref} A single protected label would show a chemical change at that end; the complementary end-accessibility tests made the circular-product interpretation stronger.
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