Fig. 1.

Ribozyme replication and the error threshold. (A) A complete replication cycle for the hammerhead ribozyme consists of copying the template (−) strand to produce the catalytic (+) strand (Top arrow) and copying the (+) to produce the (−) strand (Bottom arrow). While earlier RNA polymerase ribozymes could perform the top reaction to some extent, ribozyme 71-89 performs the full replication cycle with high fidelity. (B and C) Starting from the hammerhead ribozyme sequence, cycles of full replication (“R”) and selection for catalytic activity of the (+) strand (“S”) cause evolution. In this depiction, colored dots represent different sequences, with blue representing the wild-type hammerhead sequence and other colors representing mutants (with lower activity) generated during replication. If the mutation rate u is lower than the critical error threshold value, selection can corral the population in sequence space, maintaining the wild-type information despite replication errors (B). However, if u is too high, selection cannot beat the mutational overload and information is lost to an error catastrophe, and sequences diverge (C).