RESCUE OF ABASIC HAMMERHEAD RIBOZYMES BY EXOGENOUS ADDITION OF SPECIFIC BASES

We have synthesized 13 hammerhead ribozyme variants, each containing a n abasic residue at a specific position of the catalytic core. The act ivity of each of the variants is significantly reduced. In four cases, however, activity can be rescued by exogenous addition of the missing base. For one variant, the rescue is 300-fold; for another, the rescu e is to the wild-type level. This latter abasic variant (<(G10.01X)und er bar>) has been characterized in detail. Activation is specific for guanine, the base initially removed. In addition, the specificity for guanine versus adenine is substantially altered by replacing C with U in the opposite strand of the ribozyme. These results show that a bind ing site for a small, noncharged ligand can be created in a preexistin g ribozyme structure. This has implications for structure-function ana lysis of RNA, and leads to speculations about evolution in an ''RNA wo rld'' and about the potential therapeutic use of ribozymes.