Nucleotides that are essential but not conserved; a sufficient L-tryptophan site in RNA

  1. Rob Knight3,4
  1. 1Department of Molecular, Cellular and Developmental Biology, University of Colorado at Boulder, Boulder, Colorado 80309, USA
  2. 2Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic
  3. 3Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, USA
  4. 4Howard Hughes Medical Institute, University of Colorado at Boulder, Boulder, Colorado 80309, USA

Abstract

Conservation is often used to define essential sequences within RNA sites. However, conservation finds only invariant sequence elements that are necessary for function, rather than finding a set of sequence elements sufficient for function. Biochemical studies in several systems—including the hammerhead ribozyme and the purine riboswitch—find additional elements, such as loop–loop interactions, required for function yet not phylogenetically conserved. Here we define a critical test of sufficiency: We embed a minimal, apparently sufficient motif for binding the amino acid tryptophan in a random-sequence background and ask whether we obtain functional molecules. After a negative result, we use a combination of three-dimensional structural modeling, selection, designed mutations, high-throughput sequencing, and bioinformatics to explore functional insufficiency. This reveals an essential unpaired G in a diverse structural context, varied sequence, and flexible distance from the invariant internal loop binding site identified previously. Addition of the new element yields a sufficient binding site by the insertion criterion, binding tryptophan in 22 out of 23 tries. Random insertion testing for site sufficiency seems likely to be broadly revealing.

Keywords

Footnotes

  • Reprint requests to: Rob Knight, Department of Chemistry and Biochemistry, University of Colorado at Boulder, UCB 215, Boulder, CO 80309, USA; e-mail: rob{at}spot.colorado.edu; fax: (303) 492-7744.

  • Article published online ahead of print. Article and publication date are at http://www.rnajournal.org/cgi/doi/10.1261/rna.2220210.

  • Received April 13, 2010.
  • Accepted June 22, 2010.

Freely available online through the RNA Open Access option.

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