# Microbial Cross-Talk: Unlocking the Cytochalasin Diversity from a Termite-Associated Xylaria

## TL;DR

Scientists discovered new cytochalasin compounds from a fungus found in termites by combining genome and chemical analysis with bacterial co-culture experiments.

## Contribution

First demonstration of a fungal nonribosomal peptide synthetase accepting halogenated amino acids and discovery of a new cytochalasin derivative via microbial interaction.

## Key findings

- A cytochalasin-related gene cluster was identified and shown to produce diverse epoxy-cytochalasins.
- Cocultivation with a Streptomyces strain led to a new aspartic acid-containing cytochalasan derivative.
- Isotope labeling confirmed bacterial modification of fungal cytochalasins.

## Abstract

Integrating organismal interaction studies with advanced
genomic
and metabolomic approaches offer great promise for discovering novel
natural products and their derivatives, yet this strategy remains
relatively unexplored. Here, we illustrate its potential by investigating
a newly isolated Xylaria strain from a termite colony
environment through combined genome and metabolome analyses, complemented
by fungal–bacterial coculture experiments. Genome sequencing
of the fungal strain allowed us to pinpoint a cytochalasin-related
biosynthetic gene cluster responsible for the production of a portfolio
of different bioactive epoxy-cytochalasins. Guided by the hypothesis
of biosynthetic promiscuity of the underlying nonribosomal peptide
synthetase (NRPS), we demonstrated for the first time that the NRPS
can accept unnatural ortho- and meta-halogenated phenylalanine derivatives,
leading to the isolation of multiple new chlorinated and brominated
cytochalasin analogs. Second, based on the hypothesis that structural
diversification can arise from interactions with commensal organisms,
cocultivation with a termite-associated Streptomyces strain led to the discovery of a previously undescribed aspartic
acid-containing cytochalasan derivative, designated xylachalasin A.
Isotope labeling experiments revealed that bacterial catabolic activity
is responsible for the modification of the fungal-derived cytochalasin.
Isolated cytochalasins were also amiable for semisynthesis modifications,
which was exemplified by the synthesis of bifunctional probes. Bioassays
of a total of 26 isolated and semisynthesized derivatives demonstrated
structure-dependent cytotoxicity in some cases with up to 3-fold log
differences in potency and generally good plasma stability. Overall,
our integrated approach underscores the vast potential of investigating
fungal strains from underexplored ecological niches and their organismal
interactions, offering new opportunities to discover novel natural
products of potential therapeutic relevance and previously unrecognized
biochemical processes.