Comparison of B12 box structure models. In addition to the model of the AdoCbl riboswitch aptamer structure presented here , an alternative model that folds the highly conserved B12 box sequence (highlighted in red) into a 'facultative stem-loop' has been proposed . The core of the AdoCbl riboswitch aptamer is shown with abbreviated peripheral helices and without the optional P8-P10-P11 domain for comparison with the alternative secondary structure model. The upper model is supported by multiple base pairs with significant MI scores between B12 box bases and remote positions. In it, a portion of the B12 box also forms part of an internal T-loop motif between P6 and P7. Each diagram uses the symbols described in the legend to Figure 5.