A novel ligase ribozyme was in vitro selected from a random-sequence RNA library including N(6)-aminohexyl-modified adenine residues in place of natural adenine residues. This ribozyme mediated the formation of a phosphodiester bond with a DNA oligonucleotide through condensation with a 5'-triphosphate moiety on the ribozyme. Among the clones isolated from this selection, one was shown to accelerate ligation about 250-fold compared to the original random-sequence RNA library. Almost no rate acceleration was observed when the N(6)-aminohexyl-groups on adenine residues were omitted. Furthermore, ligation was dependent on the presence of a template DNA oligonucleotide that bridged the two strands.