title: The linkage between magnesium binding and RNA folding.
abstract: Understanding the linkage between Mg(2+) binding and RNA folding requires a proper theoretical model describing the energetics of Mg(2+) binding to the folded and unfolded states of RNA. Our current understanding of Mg(2+) binding to these different RNA states derives from empirical thermodynamic models that depend on a number of unjustified assumptions. We present a rigorous theoretical model describing the linkage between RNA folding and magnesium ion binding. In this model, based on the non-linear Poisson-Boltzmann (NLPB) equation, the stabilization of RNA by Mg(2+) arises from two distinct binding modes, diffuse binding and site binding. Diffusely bound Mg(2+) are described as an ensemble of hydrated ions that are attracted to the negative charge of the RNA. Site-bound Mg(2+) are partially desolvated ions that are attracted to electronegative pockets on the RNA surface. We explore two systems, yeast tRNA(Phe) and a 58-nucleotide rRNA fragment, with different Mg(2+) binding properties. The NLPB equation accurately describes both the stoichiometric and energetic linkage between Mg(2+) binding and RNA folding for both of these systems without requiring any fitted parameters in the calculation. Moreover, the NLPB model presents a well-defined physical description of how Mg(2+) binding helps fold an RNA. For both of the molecules studied here, the relevant unfolded state is a disordered intermediate state (I) that contains stable helical secondary structure without any tertiary contacts. Diffusely bound Mg(2+) interact with these secondary structure elements to stabilize the I state. The secondary structural elements of the I state fold into a compact, native tertiary structure (the N state). Diffuse binding plays a dominant role in stabilizing the N state for both RNAs studied. However, for the rRNA fragment, site-binding to a location with extraordinarily high electrostatic potential is also coupled to folding. Our results suggest that much experimental data measuring the linkage between Mg(2+) binding and RNA folding must be reinterpreted.
authors.affiliation: Department of Chemistry, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA. vmisra@umich.edu
authors.forename: Vinod K
authors.initials: VK
authors.lastname: Misra
authors.forename: David E
authors.initials: DE
authors.lastname: Draper
mesh_terms: D004926:Escherichia coli / Q000235:genetics; D006860:Hydrogen Bonding; D008274:Magnesium / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D016012:Poisson Distribution; D012313:RNA / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D012338:RNA, Ribosomal, 23S / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012360:RNA, Transfer, Phe / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D055672:Static Electricity; D013816:Thermodynamics; D015003:Yeasts / Q000235:genetics
chemical_list: D012338:RNA, Ribosomal, 23S; D012360:RNA, Transfer, Phe; D012313:RNA; D008274:Magnesium