In investigations on self-splicing in the group I intron of the pre-mRNA from the nrdB gene of bacteriophage T4 it was found that 2'-amino-2'-deoxyguanosine can replace guanosine as cofactor. This is the first guanosine-analogue with a modification in the 2'-position and substantial activity in a group I self-splicing reaction. The results suggest that the 2'-amino and 2'-hydroxy groups of the cosubstrates have some properties in common, which are important for binding as well as for catalysis.