Anti-CRISPRs on the march

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Science  12 Oct 2018:
Vol. 362, Issue 6411, pp. 156-157
DOI: 10.1126/science.aav2440

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Viruses and other mobile genetic elements (MGEs) are ubiquitous to all cellular life-forms, and thus, evolution of life is a perennial host-parasite arms race (1). Nearly all cellular organisms have evolved diverse, multilayer defense systems against parasites (2). Arguably, adaptive immunity is the most elaborate, efficient, and specific form of such defense. In almost all archaea and about one-third of bacteria, adaptive immunity is mediated by CRISPR-Cas systems that incorporate fragments of viral genomes into CRISPR arrays and use processed transcripts of these inserted fragments (spacers) to recognize and destroy the cognate viruses (3). The efficiency of CRISPR-Cas is such that when a virus is recognized by the immune system, it has no chance to survive. And yet, viruses thrive in all microbial communities. Virus genomes mutate fast, so the most straightforward way to avoid adaptive immunity is mutational escape. However, the hosts keep adapting, so in addition to simply attempting to outpace immunity, viruses evolve active antidefense—in particular, multiple anti-CRISPR proteins (Acrs) (see the figure) (4, 5). On pages 240 and 236 of this issue, Marino et al. (6) and Watters et al. (7), respectively, expand the collection of Acrs, which may have application in the regulation of gene editing.