Cross-backbone templating; ribodinucleotides made on poly(C).

G(5')pp(5')G synthesis from pG and chemically activated 2MeImpG is accelerated by the addition of complementary poly(C), but affected only slightly by poly(G) and not at all by poly(U) and poly(A). This suggests that 3'-5' poly(C) is a template for uncatalyzed synthesis of 5'-5' GppG, as was poly(U) for AppA synthesis, previously. The reaction occurs at 50 mM mono- and divalent ion concentrations, at moderate temperatures, and near pH 7. The reactive complex at the site of enhanced synthesis of 5'-5' GppG seems to contain a single pG, a single phosphate-activated nucleotide 2 MeImpG, and a single strand of poly(C). Most likely this structure is base-paired, as the poly(C)-enhanced reaction is completely disrupted between 30 and 37 °C, whereas slower, untemplated synthesis of GppG accelerates. More specifically, the reactive center acts as would be expected for short, isolated G nucleotide stacks expanded and ordered by added poly(C). For example, poly(C)-mediated GppG production is very nonlinear in overall nucleotide concentration. Uncatalyzed NppN synthesis is now known for two polymers and their complementary free nucleotides. These data suggest that varied, simple, primordial 3'-5' RNA sequences could express a specific chemical phenotype by encoding synthesis of complementary, reactive, coenzyme-like 5'-5' ribodinucleotides.