Evidence that ThiI, an enzyme shared between thiamin and 4-thiouridine biosynthesis, may be a sulfurtransferase that proceeds through a persulfide intermediate

J Biol Chem. 2000 Mar 24;275(12):8283-6. doi: 10.1074/jbc.275.12.8283.


ThiI is an enzyme common to the biosynthetic pathways leading to both thiamin and 4-thiouridine in tRNA. Comparison of the ThiI sequence with protein sequences in the data bases revealed that the Escherichia coli enzyme contains a C-terminal extension displaying sequence similarity to the sulfurtransferase rhodanese. Cys-456 of ThiI aligns with the active site cysteine residue of rhodanese that transiently forms a persulfide during catalysis. We investigated the functional importance of this sequence similarity and discovered that, like rhodanese, ThiI catalyzes the transfer of sulfur from thiosulfate to cyanide. Mutation of Cys-456 to alanine impairs this sulfurtransferase activity, and the C456A ThiI is incapable of supporting generation of 4-thiouridine in tRNA both in vitro and in vivo. We therefore conclude that Cys-456 of ThiI is critical for activity and propose that Cys-456 transiently forms a persulfide during catalysis. To accommodate this hypothesis, we propose a general mechanism for sulfur transfer in which the terminal sulfur of the persulfide first acts as a nucleophile and is then transferred as an equivalent of S(2-) rather than S(0).

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Bacterial Proteins*
  • Cyanides / metabolism
  • Escherichia coli Proteins*
  • Models, Chemical
  • Molecular Sequence Data
  • RNA, Transfer / metabolism*
  • Sequence Homology, Amino Acid
  • Sulfides / metabolism*
  • Sulfurtransferases / metabolism*
  • Thiamine / biosynthesis*
  • Thiosulfate Sulfurtransferase / metabolism
  • Thiosulfates / metabolism
  • Thiouridine / metabolism*


  • Bacterial Proteins
  • Cyanides
  • Escherichia coli Proteins
  • Sulfides
  • Thiosulfates
  • persulfides
  • Thiouridine
  • RNA, Transfer
  • Sulfurtransferases
  • ThiI protein, bacteria
  • thiI protein, E coli
  • Thiosulfate Sulfurtransferase
  • Thiamine