Cell Host & Microbe
Volume 28, Issue 1, 8 July 2020, Pages 23-30.e5
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Short Article
Critical Anti-CRISPR Locus Repression by a Bi-functional Cas9 Inhibitor

https://doi.org/10.1016/j.chom.2020.04.002Get rights and content
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Highlights

  • Listeria anti-CRISPR protein AcrIIA1 serves as an anti-CRISPR and a vital autorepressor

  • The strong early acr promoter must be repressed for maximal phage fitness

  • AcrIIA1 allows prophages to tune Acr expression to Cas9 levels

  • AcrIIA1 homologs have been co-opted by host bacteria as “anti-anti-CRISPRs”

Summary

Bacteriophages must rapidly deploy anti-CRISPR proteins (Acrs) to inactivate the RNA-guided nucleases that enforce CRISPR-Cas adaptive immunity in their bacterial hosts. Listeria monocytogenes temperate phages encode up to three anti-Cas9 proteins, with acrIIA1 always present. AcrIIA1 binds and inhibits Cas9 with its C-terminal domain; however, the function of its highly conserved N-terminal domain (NTD) is unknown. Here, we report that the AcrIIA1NTD is a critical transcriptional repressor of the strong anti-CRISPR promoter. A rapid burst of anti-CRISPR transcription occurs during phage infection and the subsequent negative feedback by AcrIIA1NTD is required for optimal phage replication, even in the absence of CRISPR-Cas immunity. In the presence of CRISPR-Cas immunity, full-length AcrIIA1 uses its two-domain architecture to act as a “Cas9 sensor,” tuning acr expression according to Cas9 levels. Finally, we identify AcrIIA1NTD homologs in other Firmicutes and demonstrate that they have been co-opted by hosts as “anti-anti-CRISPRs,” repressing phage anti-CRISPR deployment.

Keywords

CRISPR-Cas
Cas9
bacteriophage
anti-CRISPR
Listeria monocytogenes
autorepression
anti-anti-CRISPR

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