Update gao_iet.md

content/3.defense-systems/gao_iet.md

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 # Gao_Iet
+
+## Description
+
+Gao_Iet or Iet or IetAS was discovered in 2020 among other defense systems by :ref{doi=10.1126/science.aba0372}.
+
+The Iet system is composed by two protein: IetA (an ATPase) and IetS (a protease) :ref{doi=10.1126/science.aba0372}.
+
+## Molecular mechanism
+
+As far as we are aware, the molecular mechanism is unknown.
+
 ## Example of genomic structure
 
 The Gao_Iet is composed of 2 proteins: IetA and IetS.