Group II introns are catalytic RNA molecules that require divalent metal io
ns for folding, substrate binding, and chemical catalysis. Metal ion bindin
g sites in the group II core have now been elucidated by monitoring the sit
e-specific RNA hydrolysis patterns of bound ions such as Tb3+ and Mg2+. Maj
or sites are localized near active site elements such as domain 5 and its s
urrounding tertiary interaction partners. Numerous sites are also observed
at intron substructures that are involved in binding and potentially activa
ting the splice sites. These results highlight the locations of specific me
tal ions that are likely to play a role in ribozyme catalysis.