METAL-ION INTERACTION WITH COSUBSTRATE IN SELF-SPLICING OF GROUP-I INTRONS

The catalytic mechanism for self-splicing of the group I intron in the pre-mRNA from the nrdB gene in bacteriophage T4 has been investigated using 2'-amino-2'-deoxyguanosine or guanosine as cosubstrates in the presence of Mg2+, Mn2+ and Zn2+. The results show that a divalent meta l ion interacts with the cosubstrate and thereby influences the effici ency of catalysis in the first step of splicing. This suggests the exi stence of a metal ion that catalyses the nucleophilic attack of the co substrate. Of particular significance is that the transesterification reactions of the first step of splicing with 2'-amino-2'-deoxyguanosin e as cosubstrate are more efficient in mixtures containing either Mn2 or Zn2+ together with Mg2+ than with only magnesium ions present. The experiments in metal ion mixtures show that two (or more) metal ions are crucial for the self-splicing of group I introns and suggest the p ossibility that more than one of these have a direct catalytic role. A working model for a two-metal-ion mechanism in the transesterificatio n steps is suggested.