Abstract
A templated RNA synthesis is characterized in which G5′pp5′G accelerates synthesis of A5′pp5′A from pA and chemically activated ImpA precursors. Similar acceleration is not observable in the presence of UppU, CppC, AppG, AppA, or pG alone. Thus, it seems likely that AppA is templated by GppG via a form or forms of G:A base-pairing. AppA also appears, more slowly, via a previously known untemplated second-order chemical route. Such AppA synthesis requires only ordinary near-neutral solutions containing monovalent and divalent salts, and rates are only slightly sensitive to variation in pH. Templated synthesis rates are first order in pA, ImpA, and template GppG; thus third order overall. Therefore, this reaction resembles cross-templating of AppA on poly(U), but is notably slower and less sensitive to temperature. Viewing AppA as a coenzyme analog, GppG templating provides a simpler molecular route, termed para-templating, to encoded chemical functions. Para-templating can also arise from a single, localized nucleobase geosynthetic event which yields purines. It requires only a single backbone-forming chemistry. Thus it may have appeared earlier and served as evolutionary precursor for more complex forms of encoded genetic expression.
Highlights
Coenzyme-RNAsExtensive experimental evidence supports the hypothesis of coenzyme activity during and after an RNA world
Increased synthesis with GppG added to the reaction containing 5′ [32P]A is clearly seen and quantitated by phosphorimaging
GppG stimulates the assembly of AppA from pA and an activated form of pA (Fig. 2B)
Summary
Extensive experimental evidence supports the hypothesis of coenzyme activity during and after an RNA world. Bona fide coenzymes can be synthesized at the 5′ terminus of RNA molecules via ribozyme activity (Huang et al 2000), or assembled from nucleotides bound on a ribozyme surface (Huang et al 1998). They can be added by ribozymes to RNAs carrying a specific sequence tag (Jadhav and Yarus 2002), or acquired at a 5′ terminal position by acting as initiators of transcription (Coleman and Huang 2005). From 5′-coenzyme-terminated sequence libraries, ribozymes that mimic the modern biosynthesis of acyl-CoA’s can be selected (Coleman and Huang 2002; Jadhav and Yarus 2002). Ancient reliance on cofactor RNAs for oxidoreduction is further supported by selection of NAD-ribozymes that both oxidize and reduce a substrate (Tsukiji et al 2003, 2004)
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