Abstract
Background: Indigoids, as popular dyes, can be produced by microbial strains or enzymes catalysis. However, the new valuable products with their transformation mechanisms, especially inter-conversion among the intermediates and products have not been clearly identified yet. Therefore, it is necessary to investigate novel microbial catalytic processes for indigoids production systematically. Findings: A phenol hydroxylase gene cluster (4,606 bp) from Arthrobacter sp. W1 (PH(w1)) was obtained. This cluster contains six components in the order of KLMNOP, which exhibit relatively low sequence identities (37-72%) with known genes. It was suggested that indole and all the tested indole derivatives except for 3-methylindole were transformed to various substituted indigoid pigments, and the predominant color products derived from indoles were identified by spectrum analysis. One new purple product from indole, 2-(7-oxo-1H-indol-6(7H)-ylidene) indolin-3-one, should be proposed as the dimerization of isatin and 7-hydroxylindole at the C-2 and C-6 positions. Tunnel entrance and docking studies were used to predict the important amino acids for indoles biotransformation, which were further proved by site-directed mutagenesis. Conclusions/significance: We showed that the phenol hydroxylase from genus Arthrobacter could transform indoles to indigoids with new chemical compounds being produced. Our work should show high insights into understanding the mechanism of indigoids bio-production.