Septu

Contributors: Florian Tesson

Description

The Septu defense system was discovered in 2018 in Bacillus () . The name of the system is related to Septu (or Sopdu) the ancient Egypt god of the sky and eastern border regions.

The Septu defense system is composed of two proteins PtuA and PtuB: PtuA encoding for an ATPase (AAA15/AAA21) and PtuB with an HNH endonuclease domain.

After the discovery of Septu, two studies discovered separately another form of Septu (, ) . This new subtype encodes a third protein with a reverse transcriptase domain. This system is either referred to as Septu Type II () or as Retron subtype RT-I-A (, ) .

The Septu defense system is part of a large category of systems that encodes the AAA15/21 ABC ATPase () .

Molecular mechanism

The structure of the PtuAB complex shows a stoichiometry of 6:2 (A:B) () . The 6 PtuA proteins form a horseshoe-like trimer of dimers. The CTD terminal domains of the external PtuA bind to the PtuB endonuclease.

The PtuB is an HNH endonuclease like Cas9 () that cleaves the phage DNA () .

Interestingly, the ATPase domain of PtuA has no ATPase activity. However, the experimentally determined structure shows that 4 ATPs bind inside the PtuA hexamer () . The authors then show that the endonuclease activity of PtuAB is down-regulated by ATP in a concentration-dependent manner.

Authors () hypothesize that the PtuAB complex is inactive in noninfected cells by the physiological concentration of ATP. During phage infection, the pool of ATP can be depleted leading to the activation of the PtuAB complex and the phage DNA cleavage by PtuB HNH endonuclease.

Example of genomic structure

The Septu is composed of 2 proteins: PtuA and PtuB.

Here is an example found in the RefSeq database:

The Septu system in Burkholderia pyrrocinia (GCF_001028665.1, NZ_CP011503) is composed of 2 proteins PtuB (WP_148669058.1) PtuA (WP_082146056.1)

Distribution of the system among prokaryotes

Among the 22,803 complete genomes of RefSeq, the Septu is detected in 2130 genomes (9.34 %).

The system was detected in 1112 different species.

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

Structure

Experimentaly determined structure

From () in Escherichia coli:

Septu

Example 1
Example 2

Experimental validation

      
graph LR;
    Doron_2018[Doron et al., 2018] --> Origin_0
    Origin_0[Bacillus thuringiensis 
AMR85048.1, AMR85049.1] --> Expressed_0[Bacillus subtilis]
    Expressed_0[Bacillus subtilis] ----> SBSphiJ & SBSphiC
    Doron_2018[Doron et al., 2018] --> Origin_1
    Origin_1[Bacillus weihenstephanensis 
ABY44616.1, ABY44615.1] --> Expressed_1[Bacillus subtilis]
    Expressed_1[Bacillus subtilis] ----> SBSphiC & SpBeta
    Yi_2024[Yi et al., 2024] --> Origin_2
    Origin_2[Escherichia coli
AIL15948.1, AIL15172.1] --> Expressed_2[Escherichia coli]
    Expressed_2[Escherichia coli] ----> T5 & T7
    subgraph Title1[Reference]
        Doron_2018
        Yi_2024
end
    subgraph Title2[System origin]
        Origin_0
        Origin_1
        Origin_2
end
    subgraph Title3[Expression species]
        Expressed_0
        Expressed_1
        Expressed_2
end
    subgraph Title4[Protects against]
        SBSphiJ
        SBSphiC
        SBSphiC
        SpBeta
        T5
        T7
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

10.1016/j.cell.2020.09.065
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10.1093/nar/gkab883
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10.1126/science.aar4120
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10.1038/s41594-023-01172-8
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10.1111/mmi.15074
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10.1016/j.cell.2014.02.001
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