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Philos Trans R Soc Lond B Biol Sci. 2019 May 13;374(1772):20180090. doi: 10.1098/rstb.2018.0090.

Different genetic and morphological outcomes for phages targeted by single or multiple CRISPR-Cas spacers.

Author information

1 Department of Microbiology and Immunology, University of Otago , PO Box 56, Dunedin 9054 , New Zealand.
2 Otago Centre for Electron Microscopy, University of Otago , PO Box 56, Dunedin 9054 , New Zealand.
3 Molecular Cryo-Electron Microscopy Unit, Okinawa Institute of Science and Technology, Graduate University , Onna, Okinawa , Japan.
4 Department of Biochemistry, University of Cambridge , Cambridge CB2 1QW , UK.
5 Laboratory of Microbiology, Department of Agrotechnology and Food Sciences, Wageningen University , 6708 HB Wageningen , The Netherlands.
6 Bio-Protection Research Centre, University of Otago , Dunedin , New Zealand.


CRISPR-Cas systems provide bacteria and archaea with adaptive immunity against genetic invaders, such as bacteriophages. The systems integrate short sequences from the phage genome into the bacterial CRISPR array. These 'spacers' provide sequence-specific immunity but drive natural selection of evolved phage mutants that escape the CRISPR-Cas defence. Spacer acquisition occurs by either naive or primed adaptation. Naive adaptation typically results in the incorporation of a single spacer. By contrast, priming is a positive feedback loop that often results in acquisition of multiple spacers, which occurs when a pre-existing spacer matches the invading phage. We predicted that single and multiple spacers, representative of naive and primed adaptation, respectively, would cause differing outcomes after phage infection. We investigated the response of two phages, ϕTE and ϕM1, to the Pectobacterium atrosepticum type I-F CRISPR-Cas system and observed that escape from single spacers typically occurred via point mutations. Alternatively, phages escaped multiple spacers through deletions, which can occur in genes encoding structural proteins. Cryo-EM analysis of the ϕTE structure revealed shortened tails in escape mutants with tape measure protein deletions. We conclude that CRISPR-Cas systems can drive phage genetic diversity, altering morphology and fitness, through selective pressures arising from naive and primed acquisition events. This article is part of a discussion meeting issue 'The ecology and evolution of prokaryotic CRISPR-Cas adaptive immune systems'.


CRISPR-Cas; bacteriophages; phage evolution; phage morphology; tape measure protein

[Available on 2020-05-13]

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