In vitro selection of adenine-dependent hairpin ribozymes.

Adenine-dependent hairpin ribozymes were isolated by in vitro selection from a degenerated hairpin ribozyme population. Two new adenine-dependent ribozymes catalyze their own reversible cleavage in the presence of free adenine. Both aptamers have Mg(2+) requirements for adenine-assisted cleavage similar to the wild-type hairpin ribozyme. Cleavage kinetics studies in the presence of various other small molecules were compared. The data suggest that adenine does not induce RNA self-cleavage in the same manner for both aptamers. In addition, investigations of pH effects on catalytic rates show that both adenine-dependent aptamers are more active in basic conditions, suggesting that they use new acid/base catalytic strategies in which adenine could be involved directly. The discovery of hairpin ribozymes dependent on adenine for their reversible self-cleavage presents considerable biochemical and evolutionary interests because we show that RNA is able to use exogenous reactive molecules to enhance its own catalytic activity. Such a mechanism may have been a means by which the ribozymes of the RNA world enlarged their chemical repertoire.