Here we describe three novel reactions of the self-splicing group II intron bI1 (the first intron of the COB gene of yeast mitochondria) demonstrating its catalytic versatility: reversal of the first step of the self-splicing reaction catalyzed by a linear form of the intron utilizing the energy of a phosphoanhydride bond for transesterification, ligation of a single-stranded DNA to an RNA, and cleavage of a single-stranded DNA substrate. These results have the following evolutionary implications: use of the alpha-beta bond of a terminal triphosphate for transesterification suggests that an RNA RNA replicase could use mononucleotide triphosphates as precursors, and cleavage of single-stranded DNA and DNA-RNA ligation suggests that excised group II introns might integrate directly into DNA without prior reverse transcription.