Update shedu.md

Issue #183 (closed) Additional experimental evidence to add E coli in E. coli / T6 in 10.1101/2023.08.10.552762

content/3.defense-systems/shedu.md

@@ -10,9 +10,28 @@ tableColumns:
     Activator: Unknown
     Effector: Unknown
     PFAM: PF14082
+contributors: 
+  - Aude Bernheim
+relevantAbstracts:
+  - doi: 10.1126/science.aar4120
+  - doi: 10.1101/2023.08.10.552762
+  - doi: 10.1101/2023.08.10.552793
 ---
 
 # Shedu
+
+## Description
+The Shedu antiphage system consists of a single protein, SduA, which acts as a nuclease with a conserved DUF4263 domain belonging to the PD-(D/E)XK nuclease superfamily. The system was named after an Assyrian Mythical Deity. The N-terminal domain is very diverse including diverse nucleic acid binding, enzymatic, and other domains. 
+
+
+## Molecular Mechanism
+The Shedu protein is proposed to act as a nuclease, and its N-terminal domain inhibit its activation until triggered by phage infection.
+The activation of the protein was described in :ref{doi=10.1101/2023.08.10.552793}.
+In B. cereus Shedu, *"a key catalytic residue in Shedu’s nuclease domain is sequestered away from the catalytic site. Activation involves a conformational change that completes the active site and promotes assembly of a homo-octamer for coordinated double-strand DNA cleavage. Removal of Shedu’s N-terminal domain ectopically activates the enzyme, suggesting that this domain allosterically inhibits Shedu in the absence of infection."* 
+The nuclease activity and specific sensing of an E. coli Shedu was described in :ref{doi=10.1101/2023.08.10.552762}
+*"The N-terminal domains of SduA form a clamp that recognizes free DNA ends. End binding positions the DNA over the PD/ExK nuclease domain, resulting in dsDNA nicking at a fixed distance from the 5’ end. The end-directed DNA nicking activity of Shedu prevents propagation of linear DNA in vivo"*. In E. coli, T6 phages can escape Shedu immunity by suppressing their recombination-dependent DNA replication pathway.
+
+
 ## Example of genomic structure
 
 The Shedu system is composed of one protein: SduA.
@@ -71,13 +90,4 @@ end
     style Title3 fill:none,stroke:none,stroke-width:none
     style Title4 fill:none,stroke:none,stroke-width:none
 </mermaid>
-## Relevant abstracts
-
-::relevant-abstracts
----
-items:
-    - doi: 10.1126/science.aar4120
-
----
-::