Update abia.md

Add description and molecular mechanism of AbiA #309 (closed)

content/3.defense-systems/abia.md

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 # AbiA 
+The AbiA defense system was discovered in 1990 in a lactococcal plasmid :ref{doi=10.1128/aem.56.7.2255-2258.1990}.
 
+AbiA is one of the so-called "Abi" systems for "Abortive infection" discovered in the 90's in research related to the dairy industry :ref{doi=10.1016/j.mib.2005.06.006}. AbiA is classified as a possible abortive infection in :ref{doi=10.1016/j.mib.2023.102312}.
+
+Since it was discovered a similarity in amino acids was found with [AbiK](/defense-systems/abik). 
+
+However, with the discovery of dozens of new systems, it was categorized as one of the UG/Abi defense systems :ref{doi=10.1093/nar/gkac467} along with [DRT](/defense-systems/drt) different subsystems, [Abik](/defense-systems/abik), [AbiP2](/defense-systems/abip2) and [Rst_RT_Nitralase_TM](/defense-systems/rst_rt-nitrilase-tm).
+
+Those systems are characterized by the presence of a reverse transcriptase domain of the "Unknown Group RT".
+
+## Molecular mechanism
+
+To our knowledge, the molecular mechanism is unknown. Similarly, for the other systems of this family, the molecular mechanism remains unknown.
 
 ## Example of genomic structure