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.
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.
The DarTG defense system is a toxin-antitoxin (TA) system that provides defense against bacteriophages by ADP-ribosylating viral DNA, thereby preventing replication and the production of mature virions. This system consists of two subfamilies, DarTG1 and DarTG2, which protect against different phages. When infected by specific phages, the DarT toxin, a DNA ADP-ribosyltransferase, is released, modifying viral DNA and inhibiting replication.
## Molecular mechanism
DarT uses NAD+ to ADP-ribosylates tymidines on ssDNA, while DarG catalyses the reverse reaction. ADP-ribosylation of ssDNA prevents DNA replication and triggers the cell's SOS response. While initially proposed to work on bacterial ssDNA as a TA system :ref{doi=10.1016/j.molcel.2016.11.014}, Leroux et al. :ref{doi=10.1038/s41564-022-01153-5} show that it mostly modifies viral DNA and therefore block viral replication and perturb the transcription of phage genes. They conclude that "DarTG does not ultimately kill the host cell as in a conventional Abi mechanism, but instead acts to thwart phage replication directly."
## Example of genomic structure
## Example of genomic structure
The DarTG system is composed of 2 proteins: DarT and, DarG.
The DarTG system is composed of 2 proteins: DarT and, DarG.
