Yet More Evidence of Collusion: a New Viral Defense System Encoded by Phage CarolAnn.

Temperate phages play important roles in the physiology of their bacterial hosts and establish a lysogenic relationship with the host through which prophage-expressed genes confer new phenotypes. A key phenotype is prophage-mediated defense against heterotypic viral attack, in which temperate phages collude with their bacterial host to prevent other phages from attacking, sometimes with exquisite specificity. Such defense systems have been described in and phages but are likely widespread throughout the microbial community. Here, we describe a novel prophage-mediated defense system encoded by phage CarolAnn, which defends against infection by unrelated phages grouped in cluster CZ. CarolAnn genes and are coexpressed with the repressor and are necessary and sufficient to confer defense against phage Kita and its close relatives. Kita and these relatives are targeted through Kita gene , a gene that is of unknown function but which is the location of defense escape mutations that overcome CarolAnn defense. Expression of Kita gene is toxic to in the presence of CarolAnn genes and , suggesting that defense may be mediated by an abortive infection type of mechanism. CarolAnn genes and are distant relatives of mycobacteriophage Sbash genes and , respectively, which also confer viral defense but use a different targeting system. Prophage-mediated viral defense systems play a key role in microbial dynamics, as lysogeny is established relatively efficiently, and prophage-expressed genes can strongly inhibit lytic infection of other, unrelated phages. Demonstrating such defense systems in suggests that these systems are widespread and that there are a multitude of different systems with different specificities for the attacking phages.