A novel type of RNA ligase activity in extracts of wheat germ or Chlamydomonas requires 2′,3′-cyclic phosphate and 5′-phosphate ends for ligation to form a 2′-phosphomonoester, 3′,5′-phosphodiester bond. Using 5′-32P-labeled linear PSTV, we demonstrate that RNase T1-nicked viroid predominantly forms -G3′-P2′-P-5′U-bonds. Natural linear PSTV, however, forms main--G3′-P2′-P-5′A-bonds upon enzymatic circularization. We show that natural linear PSTV RNA has nicks between C181 and A182, or between C348 and A349, and that consequently C181 and C348 carry 2′,3′-cyclophosphate termini.
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