ISOGUANOSINE SUBSTITUTION OF CONSERVED ADENOSINES IN THE HAMMERHEAD RIBOZYME

Isoguanosine has been incorporated into a 34-mer hammerhead ribozyme b y the solid-phase phosphoramidite method, using an acetamidine base pr otecting group. The activity of the hammerhead ribozyme when singly mu tated to isoguanosine at the adenosine positions 6, 9, and 13 was 1-2- fold less than the wild-type activity. Mutations to 2-aminopurine ribo nucleoside at positions 9 and 13 were 5-fold reduced in activity, but that at position 6 was approximately 30-fold reduced. These results su pport the view that the 6-amino functions of A6, A9, and A13 are not v ery important for catalysis. The 2-position of A6 tolerates a carbonyl function but not an amino group, whereas A9 and A13 tolerate both fun ctional groups. The tolerance of a 2-amino group at A9 and A13 makes G (anti)/A(anti) Watson-Crick type base mispairing for G12/A9 and A13/G8 unlikely.