The living gills of the fungus Mycena chlorophos spontaneously emit green light. It was previously reported that trans-4-hydroxycinnamic acid and trans-3,4-dihydroxycinnnamic acid are essential for the bright light production in the living gills. However, the chemical mechanisms underlying their bioluminescence are unknown. In the present study, trans-4-aminocinnamic acid was found to inhibit light production in the living gills. The concentrations of trans-4-aminocinnamic acid that inhibited the bioluminescence intensity by 50% of initial values for immature and mature gills were 0.07 mu M and 4 mu M, respectively. Approximately 20% of the bioluminescence intensity of the immature and mature gills was not inhibited by a further increase in the concentration of trans-4-aminocinnamic acid. Moreover, the bioluminescence that was activated by trans-4-hydroxycinnamic acid or trans-3,4-dihydroxycinnamic acid (0.01 mM) was completely inhibited by trans-4-aminocinnamic acid. Therefore, trans-4-hydroxycinnamic acid and trans-3,4-dihydroxycinnamic acid functioned for the bioluminescence that was inhibited by trans-4-aminocinnamic acid. trans-4-Aminocinnamic acid strongly bound to the bioluminescence system(s) and withstood rinsing of the gills with 10 mM phosphate buffer (pH = 7), and high concentrations of trans-4-hydroxycinnamic acid (1 mM) and trans-3,4-dihydroxycinnamic acid (0.1 mM) functioned to displace trans-4-aminocinnamic acid from the bioluminescence system(s) and reactivate bioluminescence. Benzenamine, trans-cinnamic acid, trans-2-aminocinnamic acid, and trans-3-aminocinnamic acid did not inhibit bioluminescence. Therefore, the structure-specific inhibition by trans-4-aminocinnamic acid suggested that the 4-hydroxy group in trans-4-hydroxycinnamic acid and trans-3,4-dihydroxycinnamic acid molecules plays a functional role in the bioluminescence reaction. (C) 2017 Elsevier Inc. All rights reserved.