Humans have good reasons to worry about our epigenetics. DNA decorations have big impacts on our gene expression and health, and we can inadvertently change them through such lifestyle choices as smoking, having parents, or getting old. We multicellular organisms can, at least, breathe easy that epigenetics aren’t contagious, but it turns out the same is not true for bacteria.
Bacteria methylate their DNA just like eukaryotes do, and methylation patterns can control gene expression in these bugs too. In one particularly nice E. coli example, DNA methylation switches expression of the agn43 gene between bistable “on” and “off” states. When the promoter is unmethylated, the repressor OxyR binds and blocks both transcription and methylation.
If OxyR happens to get distracted for long enough, however, the dam methylase can sneak in and decorate the promoter with a few N6-methyladenosines, which block OxyR and set a romantic enough scene to turn on the promoter.
Knowing that bacteria can swap genes, Jung-Eun Shin, Chris Lin, and Han Lim at UC Berkeley wondered if bugs can swap epigenetic information as well. Using the agn43 promoter as an assay, they found that epigenetic gene expression states can indeed transfer between cells, via both phage transduction and electroporation! In the study:
- The team first devised a system to amplify the agn43 signal, by placing said promoter in front of the T7 RNA polymerase in donor strains, and then placing T7 RNAP-driven CFP in recipient strains.
- Using donor strains defective in OxyR or dam methylase, they created methylated and unmethylated donor phage, respectively.
- When recipient cells got unmethylated T7 RNAP genes, less than 1% of them expressed the RNAP and turned on CFP. However, when recipients got methylated promoters, two-thirds of them turned on!
- The gene expression state was due to DNA methylation only, because electroporation of PCR products, either raw or in vitro methylated, produced similar results.
- By placing a gene linked to fitness downstream of the bistable promoter, the team could make epigenetic transfer provide either a growth advantage or a defect.
It’s not clear how common or important epigenetic transfer is in the real world, but life does seem to harness everything that’s possible, and the authors offer some speculation on how this might influence evolution and how it could be harnessed to control gene expression. In any case, next time you sit down for a healthy meal while not smoking, be glad you don’t have to worry about catching epigenetic states from the person sitting next to you at the cafe!
To horizontally transfer this epigenetic information into your own paper library, head over to Nucleic Acids Research, 2016.