Identification and Biosynthesis of Tropodithietic Acid by Janthinobacterium sp.

Tropodithietic acid (TDA) is a sulfur-containing secondary metabolite with pronounced antimicrobial activity that has been primarily described in marine Alphaproteobacteria of the Roseobacter clade. Despite extensive studies of these bacteria, the occurrence and genetic organization of the TDA biosynthetic pathway in other bacterial groups remain poorly understood. In this study, we report the production of TDA by the freshwater bacterium Janthinobacterium sp. PLB04 isolated from diseased cell cultures of the primmorphs from the Baikal sponge Lubomirskia baikalensis. The presence of a TDA biosynthetic gene cluster homologous to the canonical tda operon previously described in the marine Roseobacter clade was found in Janthinobacterium sp. PLB04 by genome mining with bioinformatic analysis. However, comparative analysis of the cluster architecture demonstrated the absence of the gene tdaC in the Janthinobacterium sp. PLB04 genome. Despite this difference, the strain retained the ability to synthesize TDA. TDA was extracted from the culture medium and identified using chromatographic and MALDI-TOF mass spectrometric analysis. These results suggest that tdaC may not be strictly required for TDA biosynthesis in this strain and may be functionally replaced with alternative enzymatic steps or functional redundancy within the pathway. The discovery of TDA production in a freshwater Janthinobacterium strain expands the known phylogenetic and ecological diversity of TDA-producing bacteria and provides new insights into the plasticity of the TDA biosynthetic gene cluster.