Anti-CRISPR-Associated Proteins Are Crucial Repressors of Anti-CRISPR Transcription

Phages express anti-CRISPR (Acr) proteins to inhibit CRISPR-Cas systems that would otherwise destroy their genomes. Most acr genes are located adjacent-to anti-CRISPR-associated (aca) genes, which encode proteins with a helix- turn- helix DNA- binding motif. The conservation of aca genes has served asa signpost for the identification of acr genes, but the function of the proteins encoded by these genes has not been investigated. Here we reveal that anacr- associated promoter drives high levels of acr transcription immediately after phage DNA injection and that Aca proteins subsequently repress this transcription.Without Aca activity, this strong transcription is lethal to a phage. Our results demonstrate how sufficient levels of Acr proteins accumulate early in the infection process to inhibit existing CRISPR-Cas-complexes in the host cell. They also imply that the conserved role of Aca proteins is to mitigate the deleterious effects of strong constitutive transcription from acr promoters.