How Does Curtobacterium Produce a Bright Flash-Yellow Color?

Curtobacterium, a bright flash-yellow color-producing bacterium, has been isolated from the body fluid of the pupae of Graphium sarpedon. Several experiments were conducted to identify three mechanisms by which this bacterium expresses a vibrant color. The first is caused by yellow pigmentation; the second is due to a substance that absorbs blue light and emits green fluorescence; and the third is a process that excludes brownish colors by inhibiting melanin synthesis. In particular, we focused on melanin synthesis inhibition and used the water-soluble fraction of Curtobacterium and the melanin synthase in vitro human tyrosinase activity assay to examine its inhibitory effects on monophenolase activity, which converts Tyr to L-3,4-dihydroxyphenylalanine (L-DOPA), and diphenolase activity, which uses L-DOPA as a substrate to produce L-dopaquinone. The results demonstrated that the water-soluble fraction of bacterial secretion inhibited monophenolase activity but not diphenolase activity. Additionally, we discovered a compound with the molecular weight of approximately 379.2 Da and the tyrosinase inhibitory effect in which two protons were bound in the absence of copper ions and copper ions were bound instead when they were present in the water-soluble fraction of the bacterial secretion. Since the enzymatic activity of tyrosinase is copper ion-dependent, Curtobacterium may be able to suppress the browning caused by the copper-binding compound-induced oxidation of phenols, such as Tyr, allowing it to express a bright flash-yellow color.