Update butters_gp57r.md

added description and molecular mechanism 

content/3.defense-systems/butters_gp57r.md

@@ -12,7 +12,13 @@ relevantAbstracts:
 
 # Butters_gp57r
 
-## To do 
+## Description
+
+The Butters_gp57r system is encoded in the Butters mycobacterium prophage and has been shown to inhibit rival phage replication <a href='https://ncbi.nlm.nih.gov/protein/YP_009304936.1'>.
+
+## Molecular Mechanism
+
+Butters_gp57r is activated post-DNA injeciton and inhibits island3 and other phage DNA amplification. This system may inhibit heterotypic phage infeciton with the HEPN domain <a href='https://ncbi.nlm.nih.gov/protein/YP_009304936.1'>. 
 
 ## Example of genomic structure