A Co-Culturing Approach Enables Discovery and Biosynthesis of a Bioactive Indole Alkaloid Metabolite
Date
2020-01-08
Journal Title
Journal ISSN
Volume Title
Publisher
molecules
Abstract
Whole-genome sequence data of the genus Streptomyces have shown a far greater chemical
diversity of metabolites than what have been discovered under typical laboratory fermentation
conditions. In our previous natural product discovery e orts on Streptomyces sp. MA37, a bacterium
isolated from the rhizosphere soil sample in Legon, Ghana, we discovered a handful of specialised
metabolites from this talented strain. However, analysis of the draft genome of MA37 suggested
that most of the encoded biosynthetic gene clusters (BGCs) remained cryptic or silent, and only a
small fraction of BGCs for the production of specialised metabolites were expressed when cultured in
our laboratory conditions. In order to induce the expression of the seemingly silent BGCs, we have
carried out a co-culture experiment by growing the MA37 strain with the Gram-negative bacterium
Pseudomonas sp. in a co-culture chamber that allows co-fermentation of two microorganisms with
no direct contact but allows exchange of nutrients, metabolites, and other chemical cues. This
co-culture approach led to the upregulation of several metabolites that were not previously observed
in the monocultures of each strain. Moreover, the co-culture induced the expression of the cryptic
indole alkaloid BGC in MA37 and led to the characterization of the known indolocarbazole alkaloid,
BE-13793C 1. Neither bacterium produced compound 1 when cultured alone. The structure of 1 was
elucidated by Nuclear Magnetic Resonance (NMR), mass spectrometry analyses and comparison of
experimental with literature data. A putative biosynthetic pathway of 1 was proposed. Furthermore,
BE-13793C 1 showed strong anti-proliferative activity against HT-29 (ATCC HTB-38) cells but no
toxic e ect to normal lung (ATCC CCL-171) cells. To the best of our knowledge, this is the first report
for the activity of 1 against HT-29. No significant antimicrobial and anti-trypanosomal activities for
1 were observed. This research provides a solid foundation for the fact that a co-culture approach
paves the way for increasing the chemical diversity of strain MA37. Further characterization of other
upregulated metabolites in this strain is currently ongoing in our laboratory.
Description
Research Article
Keywords
silent genes, cryptic genes, co-culture, Streptomyces sp. MA37, Pseudomonas sp, indolocarbazole, alkaloid