Fig. 2

Biogenesis mechanism of intronic circRNA. a Circular RNA from group I introns: first, an exogenous guanosine (G) attacks the 5′-terminus of the intron as nucleophile and the 5′-exon is cut off due to the transesterification; Second, the 3′-hydroxyl of the free exon attacks the 5′-terminus of the 3′-exon as nucleophile, producing a linear intron; Third, a 2′-hydroxyl close to the 3′-terminus of the linear intron attacks a phosphodiester bond close to the 5′-terminus, producing an RNA lariat circularized with 2′,5′-phosphodiester and releasing the 5′-terminal sequence; b Circular RNA from group II introns: the pre-mRNA releases the 3′-exon, then the 2′-hydroxyl of the 3′-terminus attacks the 5′-terminus of the intron, producing an circular RNA circularized with 2′,5′-phosphodiester; c Intron RNA lariat: the pre-mRNA is spliced by a spliceosome, producing an RNA lariat circularized with 2′,5′-phosphodiester