Comparative Kinetic Analysis of Structural Variants of the Hairpin Ribozyme Reveals Further Potential to Optimize Its Catalytic Performance

The hairpin ribozyme derived from the minus strand of the satellite RNA associated with the tobacco ringspot virus is one of the small catalytic RNAs that has been shown to catalyze trans-cleavage reactions. There is much interest in designing hairpin ribozymes with improved catalytic activity for the development of new therapeutic agents. Extensive mutagenesis studies as well as in vitro selection experiments have been performed to define the structure and optimize its catalytic activity. This communication describes a comparative kinetic analysis of four structural variants, introduced, either alone, or in combination, into the hairpin ribozyme. We have shown that extension of the helix 2 from 4 to 6 bp resulted in a significant decrease in K_M. Furthermore, the combination of this extension with the simultaneous stabilization of helix 4, led to a more than two-fold increase in the catalytic efficiency. This variant showed a 15-fold reduction in the K_M value in respect to the wildtype ribozyme. This could be of great interest for the in vivo application of this catalytic motif. The 9-bp enlargement of helix 4 implied about a three-fold improvement in the catalytic activity. Similarly, the U₃₉C substitution brought up the efficiency of the ribozyme slightly. However, introduction of nucleotides at the hinge region between A and B domains reduced the catalytic activity. This reduction was gradually increased with the number of nucleotides. Results obtained with variants carrying more than one modification always agreed with the ones obtained from each single variant.