DarTG

Contributors: Ernest Mordret

Description

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 thymidine on ssDNA, while DarG catalyzes 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 () , Leroux et al. () show that it mostly modifies viral DNA and therefore blocks 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

The DarTG is composed of 2 proteins: DarT and DarG.

Here is an example found in the RefSeq database:

The DarTG system in Leptodesmis sichuanensis (GCF_021379005.1, NZ_CP075171) is composed of 2 proteins DarT (WP_233744308.1) DarG (WP_233744309.1)

Distribution of the system among prokaryotes

Among the 22,803 complete genomes of RefSeq, the DarTG is detected in 952 genomes (4.17 %).

The system was detected in 386 different species.

Proportion of genome encoding the DarTG system for the 14 phyla with more than 50 genomes in the RefSeq database.

Structure

DarTG

Example 1

Experimental validation

      
graph LR;
    Leroux_2022[LeRoux et al., 2022] --> Origin_0
    Origin_0[Escherichia coli strain C7] --> Expressed_0[Escherichia coli]
    Expressed_0[Escherichia coli] ----> RB69 & T5 & SECphi18 & Lust
    subgraph Title1[Reference]
        Leroux_2022
end
    subgraph Title2[System origin]
        Origin_0
end
    subgraph Title3[Expression species]
        Expressed_0
end
    subgraph Title4[Protects against]
        RB69
        T5
        SECphi18
        Lust
end
    style Title1 fill:none,stroke:none,stroke-width:none
    style Title2 fill:none,stroke:none,stroke-width:none
    style Title3 fill:none,stroke:none,stroke-width:none
    style Title4 fill:none,stroke:none,stroke-width:none

    
::

References

10.1038/s41564-022-01153-5
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10.1016/j.molcel.2016.11.014
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10.1016/j.celrep.2020.01.014
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10.1038/s41586-021-03825-4
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