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

Abstract

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’.

Two Lineages of Pseudomonas aeruginosa Filamentous Phages: Structural Uniformity over Integration Preferences

“…S8 , Supplementary Material online). Interestingly, when the resistance to phages is mediated via multiple spacers, most phages counteract with deletions in genes encoding structural proteins ( Watson et al. 2019 ).…”

The ecology and evolution of microbial CRISPR-Cas adaptive immune systems

“…The measurement of the phage mutation rate using Luria -Delbruck fluctuation tests allowed Chabas et al to suggest that this variation in resistance durability is likely to be driven by heterogeneity in the mutation rate across different protospacers. The evolution of escape mutations was also studied by Watson et al [96] in two lytic phages infecting CRISPR-resistant Pectobacterium atrosepticum. This study contrasted the evolution of the phage against single or multiple spacers.…”

Searching for fat tails in CRISPR-Cas systems: Data analysis and mathematical modeling

“…The most important current application of CRISPR-Cas systems is the genetic engineering of mammals and plants [4], with other applications including the release of modified CRISPR-Cas systems in natural and artificial environments [5][6][7]. A good understanding of the effect of CRISPR on the abundance and diversity of microbial populations as well as the consequences of the release of genetically modified bacteria into the environment can be only achieved via an interdisciplinary approach combining experimental work, mathematical modelling and extensive data analysis [8][9][10][11][12].…”