Two-carbon folate cycle of commensal Lactobacillus reuteri 6475 gives rise to immunomodulatory ethionine, a source for histone ethylation

FASEB J. 2019 Mar;33(3):3536-3548. doi: 10.1096/fj.201801848R. Epub 2018 Nov 19.


Colonization of the gut by certain probiotic Lactobacillus reuteri strains has been associated with reduced risk of inflammatory diseases and colorectal cancer. Previous studies pointed to a functional link between immunomodulation, histamine production, and folate metabolism, the central 1-carbon pathway for the transfer of methyl groups. Using mass spectrometry and NMR spectroscopy, we analyzed folate metabolites of L. reuteri strain 6475 and discovered that the bacterium produces a 2-carbon-transporting folate in the form of 5,10-ethenyl-tetrahydrofolyl polyglutamate. Isotopic labeling permitted us to trace the source of the 2-carbon unit back to acetate of the culture medium. We show that the 2C folate cycle of L. reuteri is capable of transferring 2 carbon atoms to homocysteine to generate the unconventional amino acid ethionine, a known immunomodulator. When we treated monocytic THP-1 cells with ethionine, their transcription of TNF-α was inhibited and cell proliferation reduced. Mass spectrometry of THP-1 histones revealed incorporation of ethionine instead of methionine into proteins, a reduction of histone-methylation, and ethylation of histone lysine residues. Our findings suggest that the microbiome can expose the host to ethionine through a novel 2-carbon transporting variant of the folate cycle and modify human chromatin via ethylation.-Röth, D., Chiang, A. J., Hu, W., Gugiu, G. B., Morra, C. N., Versalovic, J., Kalkum, M. The two-carbon folate cycle of commensal Lactobacillus reuteri 6475 gives rise to immunomodulatory ethionine, a source for histone ethylation.

Keywords: ethenyltetrahydrofolate; lysine ethylation; microbiome; posttranslational modification; probiotic bacteria.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Amino Acids / metabolism
  • Carbon / metabolism*
  • Cell Proliferation / physiology
  • Cells, Cultured
  • Culture Media / metabolism
  • Ethionine / metabolism*
  • Folic Acid / metabolism*
  • Histones / metabolism*
  • Homocysteine / metabolism
  • Humans
  • Immunomodulation / physiology*
  • Lactobacillus reuteri / metabolism*
  • Methionine / metabolism
  • Methylation
  • Microbiota / physiology
  • Probiotics / metabolism
  • THP-1 Cells / metabolism
  • Tumor Necrosis Factor-alpha / metabolism


  • Amino Acids
  • Culture Media
  • Histones
  • Tumor Necrosis Factor-alpha
  • Homocysteine
  • Carbon
  • Folic Acid
  • Methionine
  • Ethionine