Structural and functional evidence of bacterial antiphage protection by Thoeris defense system via NAD+ degradation

The intense arms race between bacteria and phages has led to the development of diverse antiphage defense systems in bacteria. Unlike well-known restriction-modification and CRISPR-Cas systems, recently discovered systems are poorly characterized. One such system is the Thoeris defense system, which consists of two genes, thsA and thsB . Here, we report structural and functional analyses of ThsA and ThsB. ThsA exhibits robust NAD + cleavage activity and a two-domain architecture containing sirtuin-like and SLOG-like domains. Mutation analysis suggests that NAD + cleavage is linked to the antiphage function of Thoeris. ThsB exhibits a structural resemblance to TIR domain proteins such as nucleotide hydrolases and Toll-like receptors, but no enzymatic activity is detected in our in vitro assays. These results further our understanding of the molecular mechanism underlying the Thoeris defense system, highlighting a unique strategy for bacterial antiphage resistance via NAD + degradation. The Thoeris defense system is a recently discovered bacterial defense system that protects bacteria against phage infection and consists of the two genes thsA and thsB . Here, the authors present the crystal structures of Bacillus cereus ThsA and ThsB and show that ThsA is a NAD + cleaving enzyme.