RNA 3'-terminal phosphate cyclase activity and RNA ligation in HeLa cell extract

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Abstract
HeLa cell extract contains RNA ligase activity that converts linear polyribonucleotides to covalently closed circles. RNA substrates containing 2′,3′-cyclic phosphate and 5′-hydroxyl termini are circularized by formation of a normal 3′,5′ phosphodiester bond. This activity differs from a previously described wheat germ RNA ligase which circularizes molecules with 2′,3′-cyclic and 5′ phosphate ends by a 2′-phosphomonoester, 3′,5′-phosphodiester linkage (Konarska et al., Nature 293, 112–116, 1981; Proc. Natl. Acad. Sci. USA 79, 1471–1478, 1982). The HeLa cell ligase can also utilize molecules with 3′-phosphate ends. However, in this case ligation is preceded by an ATP-dependent conversion of the 3′-terminal phosphate to the 2′,3′ cyclic form by a novel activity, RNA 3′-terminal phosphate cyclase. Both RNA ligase and RNA 3′-terminal phosphate cyclase activities are also present in extract of Xenopus oocyte nuclei, consistent with a role in RNA processing.