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![[Figure 1]](vo5001fig1mag.jpg)
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Figure 1
Comparison of simulated ESP and experimental cryo-EM maps for two representative residues. Experimental ESP maps contoured at three sequential levels (+6σ, blue; +12σ, cyan; +18σ, green) (a, d) and simulated ESP maps (b, e) for the previously assigned N7-acetyl-G3897 and O4-propanyl-U1348 nucleotides; (c, f) cryo-EM maps with fitted Mg2+. The comparison illustrates that densities close to the N7, O6 or O4 atoms of guanosines, adenines or uridines, respectively, can be misinterpreted due to the positive charge of hydrated Mg2+ ions that appear notably larger in cryo-EM maps compared with X-ray crystallographic maps. (g) Comparison of neighbouring residues with and without an Mg2+ ion bound; even at the higher contour level of the cryo-EM map the density remains continuous due to the positive charge that is only partially compensated by the nucleotide base and the coordinating water molecules. |
![[Figure 1]](vo5001fig1.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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