A ribozyme that lacks cytidine

The RNA-world hypothesis proposes that, before the advent of DNA and protei n, life was based on RNA, with RNA serving as both the repository of geneti c information and the chief agent of catalytic function(1). An argument aga inst an RNA world is that the components of RNA lack the chemical diversity necessary to sustain life. Unlike proteins, which contain 20 different ami noacid subunits, nucleic acids are composed of only four subunits which hav e very similar chemical properties. Yet RNA is capable of a broad range of catalytic functions(2-7). Here we show that even three nucleic-acid subunit s are sufficient to provide a substantial increase in the catalytic rate. S tarting from a molecule that contained roughly equal proportions of all fou r nucleosides, we used in vitro evolution to obtain an RNA ligase ribozyme that lacks cytidine. This ribozyme folds into a defined structure and has a catalytic rate that is about 10(5)-fold faster than the uncatalysed rate o f template-directed RNA ligation.