Metal Ion Interaction with Cosubstrate in Self-Splicing of Group I Introns

摘要

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 metal ion interacts with the cosubstrate and thereby influences the efficiency of catalysis in the first step of splicing. This suggests the existence of a metal ion that catalyses the nucleophilic attack of the cosubstrate. Of particular significance is that the transesterification reactions of the first step of splicing with 2′-amino-2′-deoxyguanosine 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 possibility that more than one of these have a direct catalytic role. A working model for a two-metal-ion mechanism in the transesterification steps is suggested.