Fig. 6.

Riboswitches for B₁₂ and Co(II).

A, The binding site of AdoCbl and AqCbl in the AdoCbl riboswitch (Thermoanaerobacter tengcongensis; PDB 4GMA) and env8AqCbl riboswitch (marine metagenome; PDB 4FRN), respectively (adapted from [236]) . Although similar orientations of the J3/4 strand (red) are observed, the differential positioning of the central J6/3 strand (teal) enables accommodation of the deoxyadenosyl moiety (Ado) of AdoCbl which base pairs with A162 in the AdoCbl riboswitch, but is occluded by A20 and A68 in env8AqCbl [148]. B, The Co(II)-binding sites in the NiCo riboswitch from Erysipelotrichaceae bacterium (PDB 4RUM). The four Co(II) ions bind with octahedral co-ordination geometry and Co(II) ions I and II are co-ordinated by four conserved guanine nucleotides (G46, G47, G87, G88). Site III requires G45 which also forms the outer co-ordination sphere of site II implicating an allosteric linkage between sites I-III [148].