Restriction-modification (RM) systems represent a minimal and ubiquitous biological system of self/non-self discrimination in prokaryotes , which protects hosts from exogenous DNA . The mechanism is based on the balance between methyltransferase (M) and cognate restriction endonuclease (R). M tags endogenous DNA as self by methylating short specific DNA sequences called restriction sites, whereas R recognizes unmethylated restriction sites as non-self and introduces a double-stranded DNA break . Restriction sites are significantly underrepresented in prokaryotic genomes [4-7], suggesting that the discrimination mechanism is imperfect and occasionally leads to autoimmunity due to self-DNA cleavage (self-restriction) . Furthermore, RM systems can promote DNA recombination  and contribute to genetic variation in microbial populations, thus facilitating adaptive evolution . However, cleavage of self-DNA by RM systems as elements shaping prokaryotic genomes has not been directly detected, and its cause, frequency, and outcome are unknown. We quantify self-restriction caused by two RM systems of Escherichia coli and find that, in agreement with levels of restriction site avoidance, EcoRI, but not EcoRV, cleaves self-DNA at a measurable rate. Self-restriction is a stochastic process, which temporarily induces the SOS response, and is followed by DNA repair, maintaining cell viability. We find that RM systems with higher restriction efficiency against bacteriophage infections exhibit a higher rate of self-restriction, and that this rate can be further increased by stochastic imbalance between R and M. Our results identify molecular noise in RM systems as a factor shaping prokaryotic genomes.
This work was funded by an HFSP Young Investigators’ grant. M.P. is a recipient of a DOC Fellowship of the Austrian Academy of Science at the Institute of Science and Technology Austria. R.O. and Y.W. were supported by the Platform for Dynamic Approaches to Living System from MEXT, Japan. We wish to thank I. Kobayashi for providing us with the EcoRI and EcoRV plasmids, and A. Campbell for providing us with the λ vir phage. We thank D. Siekhaus and C. Uhler and members of the C.C.G. and J.P. Bollback laboratories for in-depth discussions. We thank B. Stern for comments on an earlier version of the manuscript. We especially thank B.R. Levin for advice and comments, and the anonymous reviewers for significantly improving the manuscript.
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Pleska M, Qian L, Okura R, et al. Bacterial autoimmunity due to a restriction-modification system. Current Biology. 2016;26(3):404-409. doi:10.1016/j.cub.2015.12.041
Pleska, M., Qian, L., Okura, R., Bergmiller, T., Wakamoto, Y., Kussell, E., & Guet, C. C. (2016). Bacterial autoimmunity due to a restriction-modification system. Current Biology. Cell Press. https://doi.org/10.1016/j.cub.2015.12.041
Pleska, Maros, Long Qian, Reiko Okura, Tobias Bergmiller, Yuichi Wakamoto, Edo Kussell, and Calin C Guet. “Bacterial Autoimmunity Due to a Restriction-Modification System.” Current Biology. Cell Press, 2016. https://doi.org/10.1016/j.cub.2015.12.041.
M. Pleska et al., “Bacterial autoimmunity due to a restriction-modification system,” Current Biology, vol. 26, no. 3. Cell Press, pp. 404–409, 2016.
Pleska M, Qian L, Okura R, Bergmiller T, Wakamoto Y, Kussell E, Guet CC. 2016. Bacterial autoimmunity due to a restriction-modification system. Current Biology. 26(3), 404–409.
Pleska, Maros, et al. “Bacterial Autoimmunity Due to a Restriction-Modification System.” Current Biology, vol. 26, no. 3, Cell Press, 2016, pp. 404–09, doi:10.1016/j.cub.2015.12.041.
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