This domain is found associated with the the catalytic domain of dinoflagellate luciferase . Luciferase is involved in catalysing the light emitting reaction in bioluminescence. The structure of this domain has been solved [1]. This domain has a three helix bundle structure that holds four important histidines that are thought to play a role in the pH regulation of the enzyme.

1: Crystal structure of a pH-regulated luciferase catalyzing the bioluminescent oxidation of an open tetrapyrrole. Schultz LW, Liu L, Cegielski M, Hastings JW; Proc Natl Acad Sci U S A. 2005;102:1378-1383. PMID:15665092