In plants, two-component 
In plants, two-component systems rely on a kinase that phosphorylates this amino acid. A polymer of this amino acid is an epitope tag used to purify recombinant proteins on nickel columns. A strain of salmonella that is unable to synthesize this amino acid is used to detect carcinogens in the Ames test. This amino acid's side chain is partially deprotonated at biological pH and consists of an imidazole ring. Like cysteine, metal prosthetic groups on enzymes are often coordinated to this amino acid. A decarboxylated form of this amino acid is stored in (*) mast cells. That decarboxylated form of it is released during allergic reactions. For 10 points, name this essential amino acid with a basic aromatic side chain, symbolized H, which is the precursor of histamine.
In plants, two-component systems rely on a kinase that phosphorylates this amino acid.
histidine [or His; accept H before mention; do NOT accept "histamine"]