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. Author manuscript; available in PMC: 2017 Jul 1.
Published in final edited form as: Wiley Interdiscip Rev RNA. 2016 Apr 12;7(4):512–526. doi: 10.1002/wrna.1349

Figure 2.

Figure 2

Effects of counterions on RNA. (A) SAXS profiles of 25 bp RNA duplexes at different Mg2+ concentrations. These curves have been normalized by [RNA] to enable a comparison of their shapes. The profile measured at low salt without Mg2+ yields a sharp ‘downturn’ at the lowest q, indicating strong repulsive inter-particle interactions. The peak reflects the mean separation between highly charged duplexes in solution. As the [Mg2+] increases, the intensity rises at the lowest q. At first this increase signals decreasing repulsion; at higher [Mg2+] the continued increase reflects end-to-end association of RNA duplexes. (B) Side view of a pRNA pentameric ring superimposed with the cryo-EM envelopes of the pRNA and the pRNA-bound gp16 ATPase 85, in a functional structure. The construction of the pentamer is mediated by base pairing between the Lce and Ld loops of adjacent pRNA mononers, forming 5 superhelix scaffolds. (C) SAXS profiles of pRNA in solutions with varying salts. In Na+, SAXS profiles of dilute solutions of pRNA demonstrate weak repulsive interactions between monomers. The dramatic increase in the signal at low q, when 2mM Mg2+ is added, suggests strong intermolecular interactions between pRNA monomers.