Organic Letters
Letter
Notes
We then investigated whether the production of ralstonins is
controlled by QS in R. solanacearum. The deletion of phcB, a
gene encoding 3-OH MAME synthase, resulted in the loss of
ralstonin production (Figure 4C). This ablation was rescued
by 3-OH MAME in a dose-dependent manner. Therefore,
we concluded that QS controls the production of ralstonins in
strain OE1-1.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We are grateful to Seiya Kato (Yamagata University) for his
technical assistance. This work was supported by JSPS
KAKENHI Grant Number: 17K19244.
RmyA contains an initiating fatty acyl-AMP ligase domain,
followed by a keto-synthase domain and acetyl ornithine
aminotransferase domain coupled to luciferase-like monoox-
ygenase (module 1) (Figure 1A). The remainder of the rmyA
gene product contains four NRPS modules (modules 2−5).
RmyB is an NRPS enzyme composed of five NRPS modules
(modules 6−10). The architecture of module 1 is similar to that
of MycA responsible for the production of the lipopeptide
mycosubtilin generated by Bacillus subtilis.9 The Ahod formed
appears to be loaded to those NPRS modules and is then con-
densed with 11 amino acids to form undecapeptide ralstonins.
However, RmyAB only has nine NRPS modules. The substrate
predictions by NRPSpredictor2 for each NRPS module were
consistent with the sequences of ralstonins except for module 5
This implies that module 5 may utilize the tripeptide Hse-Gly-
Hse as a substrate.
The structures of ralstonins are unique in several aspects.
They contain unusual amino acids such as β-OH-Tyr and Dha
and the novel β-amino fatty acid Ahod. Several lipopeptides
with a β-amino fatty acid have been reported.9,10 In these cases,
the amino groups of N-terminal β-amino fatty acids were
connected to the carboxyl groups of the C-terminal residues,
forming large cyclic peptides. However, the amino groups of
Ahod in ralstonins are of the free form and the carboxyl groups
in the C-terminal β-Ala were connected to the hydroxy
group of Hse′, forming small cyclic depsipeptide moieties.
Thus, to the best of our knowledge, ralstonins are a previously
undescribed type of lipodepsipeptide in nature.
In conclusion, we herein elucidated the structures and absolute
configurations of ralstonins produced by R. solanacearum. They
are unique lipodepsipeptides containing the new β-amino fatty
acid Ahod. This is the first study to examine the absolute
configuration of β-OH-Tyr in lipopeptides. Ralstonins exhibited
chlamydospore-inducing activity and phytotoxicity, suggesting
that these compounds contribute to the entry of R. solanacearum
into their hosts (plants and fungi). Since limited information
is currently available on the endosymbiotic interactions of
R. solanacearum with fungi, we are interested in how to organize
this unique symbiosis in nature. A detailed analysis of ralstonin-
deficient mutants will help to answer these questions.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
All experimental details, additional figures and tables, and
copies of NMR spectra (PDF)
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