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Group I intron from the cyanobacterium Anabaena sp. PCC 7120 catalyzes phosphodiester bond formation using a triphosphate on the 5'-terminal nucleotide, like protein polymerases and engineered ribozymes. This ribozyme forms a unique circular RNA that incorporates the exogenous guanosine cofactor added during self-splicing.
 A natural ribozyme with 3',5' RNA ligase activity.
Vicens, Q. and T. R. Cech
Nat Chem Biol 5(2): 97-9.(2009)