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The btuB riboswitch is a regulatory RNA sequence controlling gene expression of the outer membrane B12 transport protein BtuB by specifically binding coenzyme B12 (AdoCbl) as its natural ligand. The B12 sensing riboswitch class is known to accept various B12 derivatives, leading to a division into two riboswitch subclasses, dependent on the size of the apical ligand. Here we focus on the role of side chains b and e on affinity and proper recognition, i. e. correct structural switch of the btuB RNA, which belongs to the AdoCbl‐binding class I. Chemical modification of these side chains disturbs crucial hydrogen bonds and/or electrostatic interactions with the RNA, its effect on both affinity and switching being monitored by in‐line probing. Chemical modifications at sidechain b of vitamin B12 show larger effects indicating crucial B12‐RNA interactions. When introducing the same modification to AdoCbl the influence of any side‐chain modification tested is reduced. This renders the impact of the adenosyl‐ligand for B12‐btuB riboswitch recognition clearly beyond the known role in affinity. [ABSTRACT FROM AUTHOR]

Riboswitches are structural elements of mRNA involved in the regulation of gene expression by responding to specific cellular metabolites. To fulfil their regulatory function, riboswitches prefold into an active state, the so-called binding competent form, that guarantees metabolite binding and allows a consecutive refolding of the RNA. Here, we describe the folding pathway to the binding competent form as well as the ligand free structure of the moaA riboswitch of E. coli. This RNA proposedly responds to the molybdenum cofactor (Moco), a highly oxygen-sensitive metabolite, essential in the carbon and sulfur cycles of eukaryotes. K+- and Mg2+-dependent footprinting assays and spectroscopic investigations show a high degree of structure formation of this RNA already at very low ion-concentrations. Mg2+ facilitates additionally a general compaction of the riboswitch towards its proposed active structure. We show that this fold agrees with the earlier suggested secondary structure which included also a long-range tetraloop/tetraloop-receptor like interaction. Metal ion cleavage assays revealed specific Mg2+-binding pockets within the moaA riboswitch. These Mg2+ binding pockets are good indicators for the potential Moco binding site, since in riboswitches, Mg2+ was shown to be necessary to bind phosphate-carrying metabolites. The importance of the phosphate and of other functional groups of Moco is highlighted by binding assays with tetrahydrobiopterin, the reduced and oxygen-sensitive core moiety of Moco. We demonstrate that the general molecular shape of pterin by its own is insufficient for the recognition by the riboswitch. The metal-ion dependent folding pathway of the ligand-free moaA riboswitch was studied towards the formation of the active, molybdenum cofactor-binding competent fold. This fold was further characterized by various footprinting methods while Tb(III) cleavage identified specific Mg2+ binding sites. Binding experiments with pterins complete this study. [Display omitted] • The metal-ion dependent folding of a molybdenum cofactor riboswitch is described. • The binding competent fold forms at very low Mg2+-concentrations. • A tetraloop/tetraloop-receptor like interaction stabilizes the tertiary structure. • Specific Mg2+ binding sites were identified. • The pterin moiety of the molybdenum cofactor is not sufficient for recognition/refolding of the riboswitch. [ABSTRACT FROM AUTHOR]