The Cobalamin riboswitch, also known as B12 element and AdoCbl riboswitch, is a highly con-served RNA structure that is involved in regulation of coenzyme B12 metabolism in Bacteria. It serves as a riboswitch [12323379] that binds directly to coenzyme B12, or adenosylcobalamin (AdoCbl) to regulate bacterial gene expression through either transcriptional or translational attenuation mechanisms [14704351]. Cobalamin belongs to a class of the most structurally complex cofactors synthesized by bacteria. The distinctive feature of these cofactors is their tetrapyrrole-derived framework with a centrally chelated metal ion (e.g., cobalt in B12). more

Methylcobalamin and AdoCbl, two derivatives of vit-amin B12, are essential cofactors for several important enzymes that catalyze a variety of transmethyla-tion and rearrangement reactions. Among the most prominent vitamin B12-dependent enzymes in Bac-teria are methionine synthase and ribonucleotide reductase. Initially, AdoCbl was shown to inhibit ribosome binding to the leader sequence of the btuB gene of E. coli and Salmonella typhimurium [9190822] and to influence translation of the cob operon of S. typhimurium [11260475]. A conserved sequence motif, called the B12-box, was identified that corresponds to a part of the cobalamin riboswitch. Direct binding of AdoCbl to the leader region of btuB was demonstrated [12323379, 14704351]. Models for the cobalamin-binding structures in the leader regions of these mRNAs were suggested on the basis of chemical probing, mutagenesis and computer prediction [11260475, 12323379] and enhanced by comparative analysis, leading to the prediction of the B12 element structure [12923257]. Multiple alignment of two hundreds of B12 elements from bacterial genomes reveals their common secondary structure and several extended regions of sequence conservation, including the previously known B12-box motif (Fig. 2). The conserved core of B12-element consists of seven helices (P0 to P6) and single-stranded regions with a high degree of sequence conservation. In addition to the conserved core, the B12-element has a number of facultative nonconserved stem-loops, designated Add-I and Add-II, and one internal variable structure, named VS. The B12 elements can be classified into two major types based on the existence of a highly conserved stem-loop region, named BII (Fig. 2). Although most B12 elements are complete, the BII part is absent in a number of genomes (e.g. in all B12 elements of Cyanobacteria) [12923257]. Comparative genomic analysis of B12 regulatory elements resulted in identification of a large number of genes encoding various B12 biosynthesis enzymes and transporters in the reconstructed B12 regulons in Bacteria [12869542]. In particular, numerous new cobalt transporters and chelatases required for the synthesis of B12 were found. The vitamin B12 transporters are widely distributed in bacteria and mostly B12-regulated. Finally, the B12 element was predicted to regulate B12-independent methionine synthase and ribonucleotide reductase isozymes in bacteria that also have corresponding B12-dependent isozymes. These bioinformatics predictions have been later confirmed in Mycobacterium, Streptomyces and Bacillus spp. [17307844, 16547038, 17038623].