title: Two independently selected capping ribozymes share similar substrate requirements.
abstract: We report the isolation and characterization of a second capping ribozyme, called 6.17. This ribozyme has substrate requirements that are very similar to a previously isolated capping ribozyme called Iso6. Both ribozymes promote capping and cap exchange reactions with a broad range of nucleotide substrates. The ribozymes mediate a reaction where the terminal phosphate of the nucleotide substrate attacks the alpha-phosphate found at the ribozyme's 5' terminus. This reaction involves the release of pyrophosphate during capping or a nucleotide during cap exchange. The second-order rate constants for 6.17 and Iso6 depend strongly on the length of the phosphate group found on the nucleotide substrate. Nucleoside diphosphates or triphosphates are efficiently utilized, while monophosphates are used approximately 20-fold less efficiently by both ribozymes. These ribozymes also have rates that increase as pH is decreased. Despite these similarities, the ribozymes are not identical and 6.17 performs optimally when incubated with divalent magnesium ions, while Iso6 displays a preference for calcium ions. Further, the ribozymes have globally different secondary structures; 6.17 has a complicated pseudoknot structure consisting of five helical elements, while Iso6 likely consists of four helical elements. We hypothesize that capping proceeds via an invariant phosphate dependent mechanism that imposes a nearly identical "catalytic fingerprint" on these two distinct ribozymes.
authors.affiliation: Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
authors.forename: Hani S
authors.initials: HS
authors.lastname: Zaher
authors.forename: R Ammon
authors.initials: RA
authors.lastname: Watkins
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Unrau
mesh_terms: D020029:Base Pairing; D001483:Base Sequence; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008969:Molecular Sequence Data; D009711:Nucleotides / Q000378:metabolism; D039381:RNA Cap-Binding Proteins / Q000235:genetics*; D016337:RNA, Catalytic / Q000235:genetics* / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D009711:Nucleotides; D039381:RNA Cap-Binding Proteins; D016337:RNA, Catalytic