AcrIIIA1 is a protein-RNA anti-CRISPR complex that targets core Cas and accessory nucleases.

Clustered regularly-interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins protect bacteria and archaea from their viruses, and anti-CRISPRs (Acrs) are small virus-encoded proteins that inhibit CRISPR-Cas immunity. Over 80 families of Acrs have been described to date; however, only three of these subvert Type III CRISPR-Cas immunity. Type III systems employ a complex network of Cas and accessory nucleases to degrade viral nucleic acids.

Recurring and emerging themes in prokaryotic innate immunity.

A resurgence of interest in the pathways that bacteria use to protect against their viruses (i.e. phages) has led to the discovery of dozens of new antiphage defenses.

Microbial genome mining expedition unearths trove of antiviral defenses.

In this issue of Cell Host & Microbe, Millman et al. use bioinformatic and genetic approaches to discover 21 novel antiviral immune systems in prokaryotes. Remarkably, many of these systems bear homology to components of the human innate immune system, suggesting an evolutionary tie between prokaryotic and eukaryotic antiviral defenses.

A unique mode of nucleic acid immunity performed by a multifunctional bacterial enzyme.

The perpetual arms race between bacteria and their viruses (phages) has given rise to diverse immune systems, including restriction-modification and CRISPR-Cas, which sense and degrade phage-derived nucleic acids. These complex systems rely upon production and maintenance of multiple components to achieve antiphage defense. However, the prevalence and effectiveness of minimal, single-component systems that cleave DNA remain unknown.