You forgot the anchors.

m{
 ^
 (?:
   S-adenosyl-L-methionine-dependent[ ]methyltransferases
   |
   ATPase[ ]domain[ ]of[ ]HSP90[ ]chaperone/DNA[ ]topoisomerase[ ]II/h
+is
+tidine[ ]kinase
   |
   Q8PHW2_XANAC_Q8PHW2;
 )
 \z
}x;
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