4RUM:
reference: Bacterial riboswitches cooperatively bind ni(2+) or co(2+) ions and control expression of heavy metal transporters., Furukawa K, Ramesh A, Zhou Z, Weinberg Z, Vallery T, Winkler WC, Breaker RR, Mol Cell. 2015 Mar 19;57(6):1088-98. doi: 10.1016/j.molcel.2015.02.009. PMID: 25794617

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