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Journal Article

The size of the immune repertoire of bacteria


Nourmohammad,  Armita
Max Planck Research Group Statistical physics of evolving systems, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Bradde, S., Nourmohammad, A., Goyal, S., & Balasubramanian, V. (2020). The size of the immune repertoire of bacteria. Proceedings of the National Academy of Sciences of the United States of America, 117(10), 5144-5151. doi:10.1073/pnas.1903666117.

Cite as: http://hdl.handle.net/21.11116/0000-0005-BF5C-E
Some bacteria and archaea possess an immune system, based on the CRISPR-Cas mechanism, that confers adaptive immunity against viruses. In such species, individual prokaryotes maintain cassettes of viral DNA elements called spacers as a memory of past infections. Typically, the cassettes contain several dozen expressed spacers. Given that bacteria can have very large genomes and since having more spacers should confer a better memory, it is puzzling that so little genetic space would be devoted by prokaryotes to their adaptive immune systems. Here, assuming that CRISPR functions as a long-term memory-based defense against a diverse landscape of viral species, we identify a fundamental tradeoff between the amount of immune memory and effectiveness of response to a given threat. This tradeoff implies an optimal size for the prokaryotic immune repertoire in the observational range.