Non-enzymatic template-directed synthesis of RNA copolymers

Cytosine-rich RNA copolymers facilitate the template-directed synthesis of complementary oligomers from mononucleotide 5′-phospho-2-methylimidazolides (Fig. 1). The efficiency of this reaction falls sharply as the ratio of cytosine to non-cytosine in the template is decreased. This is a severe limitation for self-replication because it means that any cytosine-rich polynucleotide that can serve as a good template will produce a cytosine-poor complementary strand that is unable to direct further rounds of synthesis. Studies with low-ratio random copolymer templates have shown that the efficiency can be increased by adjusting initial monomer concentrations and by providing additional activated monomers during later stages of the reaction. The oligomeric reaction products can be studied in detail using high performance liquid chromatography. It is possible to separate oligonucleotides on the basis of chain length and base composition. Thus a wealth of information is available to characterize the distribution of products over the course of the reaction and under a variety of reaction conditions.