Investigation of Eumelanin Biosynthesis in Gluconacetobacter tumulisoli FBFS 97: A Novel Insight into a Bacterial Melanin Producer

Acetic acid bacteria (AAB) are a group of bacteria, most of which can produce pigments. However, the mechanism of pigment production by AAB is unclear. A strain of AAB, <i>Gluconacetobacter tumulisoli</i> FBFS 97, which can produce a large amount of brown pigment (BP), was isolated in our previous research. In the current study, it was found that the BP yield of the FBFS 97 strain was enhanced in the presence of tyrosine, and an intermediate of melanin, L-3,4-dihydroxyphenylalanine (L-DOPA), was identified using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS). The structural properties of BP were analyzed by pyrolysis gas chromatography-mass spectrometry (Py-GC-MS). All these analyses suggest that BP may be eumelanin, a type of melanin. Then, the eumelanin biosynthetic pathway was investigated in the FBFS 97 strain, and three related genes with eumelanin including <i>pheA</i>, <i>yfiH</i>, and <i>phhB</i> in its genome were found and knocked out, respectively. The results showed that eumelanin production increased 1.3-fold in the <i>pheA</i> deletion mutant compared to the wild-type FBFS 97 strain, but when either <i>yfiH</i> or <i>phhB</i> was knocked out, the eumelanin production in the mutants was the same as that in the wild-type FBFS 97 strain. Finally, a possible biosynthetic pathway for eumelanin in the FBFS 97 strain is proposed.