Identification of 1,3,6,8-Tetrahydroxynaphthalene Synthase (ThnA) from sp. CS682.

Type III polyketide synthase (PKS) found in bacteria is known as 1,3,6,8-tetrahydroxynaphthalene synthase (THNS). Microbial type III PKSs synthesize various compounds that possess crucial biological functions and significant pharmaceutical activities. Based on our sequence analysis, we have identified a putative type III polyketide synthase from sp. CS682 was named as ThnA. The role of ThnA, in sp. CS682 during the biosynthesis of 1,3,6,8 tetrahydroxynaphthalene (THN), which is the key intermediate of 1-(α-L-(2--methyl)-6-deoxymannopyranosyloxy)-3,6,8-trimethoxynaphthalene (IBR-3) was characterized. ThnA utilized five molecules of malonyl-CoA as a starter substrate to generate the polyketide 1,3,6,8-tetrahydroxynaphthalene, which could spontaneously be oxidized to the red flaviolin compound 2,5,7-trihydroxy-1,4-naphthoquinone. The amino acid sequence alignment of ThnA revealed similarities with a previously identified type III PKS and identified Cys, Phe, His, and Asn as four highly conserved active site amino acid residues, as found in other known polyketide synthases. In this study, we report the heterologous expression of the type III polyketide synthase in TK24 and the identification of THN production in a mutant strain. We also compared the transcription level of in TK24 and pIBR25-thnA and found that was only transcribed in the mutant.