Sheep in wolves' clothing: Temperate T7-like bacteriophages and the origins of the Autographiviridae

Bacteriophage T7 is an extensively studied virulent phage, and its taxonomic family, the Autographiviridae, is broadly synonymous with a strictly virulent lifestyle. It is difficult to imagine how a T7-like phage could function in a "domesticated" temperate lifestyle, in which it is incorporated into the host's genome. Here we describe two temperate T7-like bacteriophages: ProddE, a Desulfovibrio phage, and Pasto, an Agrobacterium phage. Each contains recognizable T7-like proteins in the canonical T7-like gene order, but with the addition of lysogeny gene modules. While ProddE contains a phage-like repressor, Pasto lysogeny appears to be controlled by a novel MarR-like transcriptional regulator. In addition, we identify similar T7-like prophage elements in a wide variety of Gram-negative bacterial genomes and a small number of Gram-positive genomes. Identification of these elements in diverse bacterial species raises interesting evolutionary questions about the origins of T7-like phages and which lifestyle, temperate or virulent, is the ancestral form.