Circularization of linear viroid RNA via 2′-phosphomonoester, 3′, 5′-phosphodiester bonds by a novel type of RNA ligase from wheat germ and Chlamydomonas

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.