A natural ribozyme with 3',5' RNA ligase activity.

Using electrophoresis, sequencing and enzymatic digestion, we show that the group I intron from the cyanobacterium Anabaena sp. PCC 7120 catalyzes phosphodiester bond formation using a triphosphate on the 5'-terminal nucleotide, much like protein polymerases and engineered ribozymes. In the process, this ribozyme forms a unique circular RNA that incorporates the exogenous guanosine cofactor added during self-splicing. This finding may have relevance to a prebiotic RNA world and to modern biology.