S. Ravnum et Di. Andersson, An adenosyl-cobalamin (coenzyme-B12)-repressed translational enhancer in the cob mRNA of Salmonella typhimurium, MOL MICROB, 39(6), 2001, pp. 1585-1594
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.