Bioinformatic and experimental evidence for suicidal and catalytic plant THI4s

Biochem J. 2020 Jun 12;477(11):2055-2069. doi: 10.1042/BCJ20200297.


Like fungi and some prokaryotes, plants use a thiazole synthase (THI4) to make the thiazole precursor of thiamin. Fungal THI4s are suicide enzymes that destroy an essential active-site Cys residue to obtain the sulfur atom needed for thiazole formation. In contrast, certain prokaryotic THI4s have no active-site Cys, use sulfide as sulfur donor, and are truly catalytic. The presence of a conserved active-site Cys in plant THI4s and other indirect evidence implies that they are suicidal. To confirm this, we complemented the Arabidopsistz-1 mutant, which lacks THI4 activity, with a His-tagged Arabidopsis THI4 construct. LC-MS analysis of tryptic peptides of the THI4 extracted from leaves showed that the active-site Cys was predominantly in desulfurated form, consistent with THI4 having a suicide mechanism in planta. Unexpectedly, transcriptome data mining and deep proteome profiling showed that barley, wheat, and oat have both a widely expressed canonical THI4 with an active-site Cys, and a THI4-like paralog (non-Cys THI4) that has no active-site Cys and is the major type of THI4 in developing grains. Transcriptomic evidence also indicated that barley, wheat, and oat grains synthesize thiamin de novo, implying that their non-Cys THI4s synthesize thiazole. Structure modeling supported this inference, as did demonstration that non-Cys THI4s have significant capacity to complement thiazole auxotrophy in Escherichia coli. There is thus a prima facie case that non-Cys cereal THI4s, like their prokaryotic counterparts, are catalytic thiazole synthases. Bioenergetic calculations show that, relative to suicide THI4s, such enzymes could save substantial energy during the grain-filling period.

Keywords: cereal grains; proteomics; thiamin; thiazole synthase.

Publication types

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

MeSH terms

  • Arabidopsis Proteins* / chemistry
  • Arabidopsis Proteins* / genetics
  • Arabidopsis Proteins* / metabolism
  • Arabidopsis* / enzymology
  • Arabidopsis* / genetics
  • Catalysis
  • Computational Biology
  • Escherichia coli / enzymology
  • Escherichia coli / genetics
  • Genetic Complementation Test
  • Ligases* / chemistry
  • Ligases* / genetics
  • Ligases* / metabolism
  • Models, Molecular*
  • Plants, Genetically Modified* / enzymology
  • Plants, Genetically Modified* / genetics
  • Protein Domains
  • Thiamine* / biosynthesis
  • Thiamine* / genetics
  • Thiazoles / metabolism*


  • Arabidopsis Proteins
  • Thiazoles
  • Ligases
  • Thiamine