The DarTG toxin-antitoxin system provides phage defence by ADP-ribosylating viral DNA

The DarTG toxin-antitoxin system protects bacteria against phage infection via ADP-ribosylation of the viral DNA, and this can be evaded by phages via mutation of their DNA polymerase or the gp61.2 anti-DarT factor. Toxin-antitoxin (TA) systems are broadly distributed, yet poorly conserved, genetic elements whose biological functions are unclear and controversial. Some TA systems may provide bacteria with immunity to infection by their ubiquitous viral predators, bacteriophages. To identify such TA systems, we searched bioinformatically for those frequently encoded near known phage defence genes in bacterial genomes. This search identified homologues of DarTG, a recently discovered family of TA systems whose biological functions and natural activating conditions were unclear. Representatives from two different subfamilies, DarTG1 and DarTG2, strongly protected E. coli MG1655 against different phages. We demonstrate that for each system, infection with either RB69 or T5 phage, respectively, triggers release of the DarT toxin, a DNA ADP-ribosyltransferase, that then modifies viral DNA and prevents replication, thereby blocking the production of mature virions. Further, we isolated phages that have evolved to overcome DarTG defence either through mutations to their DNA polymerase or to an anti-DarT factor, gp61.2, encoded by many T-even phages. Collectively, our results indicate that phage defence may be a common function for TA systems and reveal the mechanism by which DarTG systems inhibit phage infection.

Automated summary

The DarTG toxin-antitoxin system protects bacteria against phage infection by ADP-ribosylating viral DNA. The system can be evaded by phages through mutation of their DNA polymerase or the gp61.2 anti-DarT factor. A bioinformatics search identified homologues of DarTG, a family of TA systems that provide strong protection against phages. The toxin of DarTG modifies viral DNA upon phage infection, preventing replication and virion production. Phages have evolved mutations to overcome DarTG defense. These findings suggest that phage defense may be a common function of TA systems.