Submitted to: American Society for Microbiology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 6/10/2005
Publication Date: 7/5/2005
Citation: Wheeler, M.H., Cheng, Q., Puckhaber, L.S., Szaniszlo, P. 2005. Evidence for heptaketide melanin biosynthesis and chain-shortening of the melanin precursor YWA1 to 1,3,6,8-tetrahydroxynaphthalene in Wangiella (Exophiala) dermatitidis [abstract]. American Society for Microbiology. p. 266.
Technical Abstract: It is reported that cell wall melanin in many black fungi, including the human pathogen W. dermatitidis, is synthesized from 1,8-dihydroxynaphthalene (D2HN) via a pentaketide pathway. In these fungi, 1,3,6,8-tetrahydroxynaphthalene (T4HN), an early precursor in the pathway, is believed to be produced directly from malonyl-CoA and acetyl-CoA by type-1 polyketide synthases (PKSs). In contrast, the bluish-gray pigment in conidial walls of Aspergillus fumigatus is produced by a heptaketide pathway. Although A. fumigatus produces most of the melanin precursors made by black fungi, it synthesizes T4HN hydrolytically by chain shortening from an upstream 14-carbon precursor, YWA1, which has not been found in a black fungus. The YWA1 is made by PKS and then converted to T4HN by the enzyme Ayglp. In this study, we investigated whether W. dermatitidis produces T4HN via chain shortening of a heptaketide precursor, such as YWA1. A yellow pigment-producing mutant of W. dermatitidis, WdBrm1, was generated by random molecular genetic insertion of vector pLZ70. After the gene was cloned, it was found to encode an otholog of Ayg1p (amino acid identity, 39%; similarity, 55%), and named WdYG1 (GenBank # AY667610). This suggested that W. dermatitidis produces an enzyme that metabolizes YWA1 to T4HN. Chemical analyses of culture filtrates demonstrated that melanin biosynthesis was blocked in WdBrm1. The disrupted gene prevented the synthesis of T4HN and other downstream precursors, resulting in the accumulation of pathway precursors upstream of T4HN. When the metabolites from WdBrm1, including a compound with a UV-spectrum identical to that of YWA1, were fed to an albino strain of W. dermatitidis, it produced a black pigment. With addition of tricyclazole, an inhibitor of the enzyme that reduces T4HN, the albino failed to produce the black pigment and its cultures accumulated flaviolin, an autoxidative product of T4HN. These results indicate that the albino produced D2HN- melanin, and that W. dermatitidis and possibly other black fungi produce D2HN-melanin via YWA1 and a heptaketide pathway rather than by the pentaketide pathway as reported previously.