Functional Diversity of the Rhodanese Homology Domain

M. D. Wolfe,Farzana Ahmed,G. Lacourciere,C. T. Lauhon,T. Stadtman,T. Larson

Published 2004 in Journal of Biological Chemistry

ABSTRACT

Escherichia coli has eight genes predicted to encode sulfurtransferases having the active site consensus sequence Cys-Xaa-Xaa-Gly. One of these genes, ybbB, is frequently found within bacterial operons that contain selD, the selenophosphate synthetase gene, suggesting a role in selenium metabolism. We show that ybbB is required in vivo for the specific substitution of selenium for sulfur in 2-thiouridine residues in E. coli tRNA. This modified tRNA nucleoside, 5-methylaminomethyl-2-selenouridine (mnm5se2U), is located at the wobble position of the anticodons of tRNALys, tRNAGlu, and tRNA1Gln. Nucleoside analysis of tRNAs from wild-type and ybbB mutant strains revealed that production of mnm5se2U is lost in the ybbB mutant but that 5-methylaminomethyl-2-thiouridine, the mnm5se2U precursor, is unaffected by deletion of ybbB. Thus, ybbB is not required for the initial sulfurtransferase reaction but rather encodes a 2-selenouridine synthase that replaces a sulfur atom in 2-thiouridine in tRNA with selenium. Purified 2-selenouridine synthase containing a C-terminal His6 tag exhibited spectral properties consistent with tRNA bound to the enzyme. In vitro mnm5se2U synthesis is shown to be dependent on 2-selenouridine synthase, SePO3, and tRNA. Finally, we demonstrate that the conserved Cys97 (but not Cys96) in the rhodanese sequence motif Cys96-Cys97-Xaa-Xaa-Gly is required for 2-selenouridine synthase in vivo activity. These data are consistent with the ybbB gene encoding a tRNA 2-selenouridine synthase and identifies a new role for the rhodanese homology domain in enzymes.

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