Novel RNA catalysts for the Michael reaction

Background: In vitro selected ribozymes with nucleotide synthase, peptide a nd carbon-carbon bond forming activity provide insight into possible scenar ios on how chemical transformations mag. have been catalyzed before protein enzymes had evolved. Metabolic pathways based on ribozymes may have existe d at an early stage of evolution. Results: We have isolated a novel ribozyme that mediates Michael-adduct for mation at a Michael-acceptor substrate, similar to the rate-limiting step o f the mechanistic sequence of thymidylate synthase. The kinetic characteriz ation of this catalyst revealed a rate enhancement by a factor of similar t o 10(5). The ribozyme shows substrate specificity and can act as an intermo lecular catalyst which transfers the Michael-donor substrate onto an extern al 20-mer RNA oligonucleotide containing the Michael-acceptor system. Conclusions: The ribozyme described here is the first example of a catalyti c RNA with Michael-adduct forming activity which represents a key mechanist ic step in metabolic pathways and other biochemical reactions. Therefore, p reviously unforeseen RNA-evolution pathways can be considered, for example the formation of dTMP from dUMP. The substrate specificity of this ribozyme may also render it useful in organic syntheses. (C) 2001 Elsevier Science Ltd. All rights reserved.