A type II Thoeris anti-phage defense enacts growth arrest independent of NAD depletion

Thoeris immune systems protect bacteria against invading phages through production of a signaling molecule by ThsB that activates the effector ThsA. Across the four defined types, the two most dominant (I and II) have been associated with abortive infection, with type I acting through the depletion of the essential coenzyme NAD+. Here, we show that a previously uncharacterized type II Thoeris system from Escherichia coli ATCC 25922 deviates from this paradigm. This system consists of the effector protein ThsA and two distinct signaling proteins ThsB1 and ThsB2. Heterologous expression of thsA and thsB2 confers anti-phage defense, while thsB1 is cytotoxic when expressed in common E. coli lab strains. Furthermore, while phage infection drives growth arrest, we could not detect any measurable decrease in NAD+ levels as well as standard markers of cell death. Together, these results suggest that Thoeris contains even greater functional diversity within the defined system types.