An adenosyl-cobalamin (coenzyme-B12)-repressed translational enhancer in the cob mRNA of Salmonella typhimurium

Expression of the cobalamin (Cbl) biosynthetic cob operon in Salmonella typ himurium is repressed by the end-product. This regulation is conferred main ly at the translational level and involves a cobalamin-induced folding of a n RNA hairpin that sequesters the ribosomal binding site (RBS) of the cob m RNA and prevents translation initiation. A combined structural and mutation al analysis shows that a cis-acting translational enhancer (TE) element, lo cated 83 nucleotides upstream of the Shine-Dalgarno sequence in the 5'-untr anslated region (5'-UTR) of the cob mRNA, is required to unfold the inhibit ory RBS hairpin in the absence of cobalamin. The TE element, which consists of 5 nucleotides, is proposed to confer its enhancer function in the absen ce of cobalamin by interacting with nucleotides in the stem of the RBS hair pin. This interaction destabilizes the RNA hairpin and allows ribosome bind ing. In the presence of cobalamin, the enhancer function is inhibited. As a result, the RBS hairpin forms and prevents translation initiation. Several additional RNA hairpins in the 5'-UTR were also identified and are suggest ed to be important for repression. The above data suggest that normal cobal amin repression of the cob operon requires that the 5'-UTR has a defined se condary and tertiary structure.