TEMPLATE SWITCHING BETWEEN PNA AND RNA OLIGONUCLEOTIDES

THE origin of the RNA world(1) is not easily understood, as effective prebiotic syntheses of the components of RNA, the beta-ribofuranoside- 5'-phosphates, are hard to envisage(2). Recognition of this difficulty has led to the proposal(1,3) that other genetic systems, the componen ts of which are more easily formed, may have preceded RNA. This raises the question of how transitions between one genetic system and anothe r could occur. Peptide nucleic acid (PNA) resembles RNA in its ability to form double-helical complexes stabilized by Watson-Crick hydrogen bonding between adenine and thymine and between cytosine and guanine(4 -6), but has a backbone that is held together by amide rather than by phosphodiester bonds. Oligonucleotides based on RNA are known to act a s templates that catalyse the non-enzymatic synthesis of their complem ents from activated mononucleotides(7-9), we now show that RNA oligonu cleotides facilitate the synthesis of complementary PNA strands and vi ce versa. This suggests that a transition between different genetic sy stems can occur without less of information.