title: RNA enzymes with two small-molecule substrates.
abstract: BACKGROUND
  The 'RNA world' hypothesis posits ancient organisms employing versatile catalysis by RNAs. In particular, such a metabolism would have required RNA catalysts that join small molecules. Such anabolic reactions now occur very widely, for example in phospholipid, terpene, amino acid and nucleotide synthetic pathways in modern organisms. Present RNA systems, however, do not perform such reactions using substrates that do not base pair. Here we ask whether this lack is a methodological artifact due to the practice of selection-amplification, or a fundamental property of active sites reconstructed within RNA structures.
  
  
  RESULTS
  Three rationally modified RNA enzymes, Iso6-G, Iso6-2G and Iso63G, catalyze the formation of (5'-->5') polyphosphate-linked oligonucleotides in trans. One of these, Iso6-G RNA, has a specific substrate site for a guanosine triphosphate, GTP, dGTP or ddGTP, and one nonspecific substrate site for a terminal-phosphate-containing small molecule. This ribozyme catalyzes multiple turnovers, proceeding at a constant rate. Guanosine specificity is probably not attributable to Watson-Crick base pairing.
  
  
  CONCLUSIONS
  Ribozymes can readily bind multiple small-molecule substrates simultaneously and catalyze reactions that build up larger products, apparently independent of substrate-RNA Watson-Crick base pairing. RNA enzymes therefore parallel proteins, which often overcome the entropic difficulties of positioning multiple small substrates for catalysis of anabolic reactions. These results support the idea of a complex ancestral metabolism based on RNA catalysis.
authors.affiliation: Department of Molecular, Cellular and Developmental Biology University of Colorado at Boulder Boulder CO 80309-0347, USA.
authors.forename: F
authors.initials: F
authors.lastname: Huang
authors.forename: Z
authors.initials: Z
authors.lastname: Yang
authors.forename: M
authors.initials: M
authors.lastname: Yarus
mesh_terms: D000465:Algorithms; D020029:Base Pairing; D001665:Binding Sites; D005784:Gene Amplification; D005819:Genetic Markers; D006160:Guanosine Triphosphate / Q000378:metabolism; D007700:Kinetics; D009841:Oligonucleotides / Q000378:metabolism; D012313:RNA / Q000096:biosynthesis* / Q000235:genetics*; D016337:RNA, Catalytic / Q000096:biosynthesis* / Q000235:genetics*; D015719:Single-Strand Specific DNA and RNA Endonucleases / Q000378:metabolism
chemical_list: D005819:Genetic Markers; D009841:Oligonucleotides; D016337:RNA, Catalytic; D012313:RNA; D006160:Guanosine Triphosphate; D015719:Single-Strand Specific DNA and RNA Endonucleases