An unusual galactofuranose lipopolysaccharide that ensures the intracellular survival of toxin-producing bacteria in their fungal host

Article abstract of DOI:10.1002/anie.201003301

Dress code for living in a fungus: Analysis of the carbohydrate coating of the toxin-producing endobacterium of the phytopathogenic fungus Rhizopus microsporus revealed an unprecedented lipopolysaccharide (LPS) structure, which is important for infection and colonization of the fungal host. A mutant lacking the unusual [→2)-β-D-galactofuranose-(1→]_n O antigen (red in the schematic illustration) was incapable of forming a stable symbiosis with the fungus.

Full text of DOI:10.1002/anie.201003301

Communications
DOI: 10.1002/anie.201003301
Natural Products
An Unusual Galactofuranose Lipopolysaccharide That Ensures the
Intracellular Survival of Toxin-Producing Bacteria in Their Fungal
Host**
Maria R. Leone, Gerald Lackner, Alba Silipo, Rosa Lanzetta, Antonio Molinaro,* and
Christian Hertweck*
Bacteria are involved in a plethora of interactions with higher
organisms; these interactions can be beneficial or detrimental
to the host.[1] Irrespective of the type of symbiosis, in all cases
the fine-tuned communication between the organisms is
mediated by biomolecules. Diffusible chemical signals allow
for long-range communication,[2] whereas carbohydrate struc-
tures (antigens) coating the cell surfaces enable cell–cell
recognition.[3] Such antigenic components are highly specific
and particularly important when bacteria have direct physical
contact with the host, or even invade eukaryotic cells.[4,5]
Among the most important and ubiquitously occurring sur-
face determinants of Gram-negative bacteria are lipopoly-
saccharides (LPSs).[3,5,6] LPSs share a common architecture
featuring a hydrophilic heteropolysaccharide moiety that is
There is an evident gap of knowledge on the role of LPSs in
microbe–microbe interactions, such as bacterial–fungal
encounters, which are crucial in the environment.[8]
We have recently unveiled a unique symbiosis of the rice
blight fungus Rhizopus microsporus, and intracellular bac-
teria (Burkholderia rhizoxinica)[9], which serve their fungal
host as toxin factories.[10–12] The bacterial endosymbionts
produce numerous antimitotic macrolides of the rhizoxin
complex that efficiently stall cell division in rice plants and
most other eukaryotes.[13] Notably, the fungal host itself has
become resistant to the toxin as a result of a mutation in the
tubulin sequence.[14] Thus, it is likely that Rhizopus–Burk-
holderia symbiosis underwent a parasitism–mutualism shift
during evolution.[15] Still, it is a mystery how the endobacteria
can survive within fungal host cells and how they interact with
their host by means of chemical recognition and communi-
cation.[12] Herein we disclose the complete structure of an
unusual LPS from B. rhizoxinica, elaborate its molecular
basis, and provide the first evidence that the O antigen is a
critical molecular determinant for the stability of the sym-
biosis.
To elucidate the structure of the LPS of the endofungal
bacteria and its role in the interaction, we used a combination
of GLC–MS, MALDI MS, and a series of 2D NMR experi-
ments. Following hydrolysis of the LPS in an acetate buffer,
the fraction containing lipid Awas recovered as a sediment by
centrifugation. The pure O polysaccharide (OPS), present in
the supernatant, was obtained by gel permeation chromatog-
raphy. Lipid A fatty acid analysis revealed the presence of
(R)-3-hydroxyhexadecanoic acid (16:0 (3-OH)) with an
amide linkage, and (R)-3-hydroxytetradecanoic acid (14:0
(3-OH)) and tetradecanoic acid (14:0) with ester linkages.
According to chemical analysis, MALDI MS, and NMR
spectroscopic data, the B. rhizoxinica LPS lipid A substruc-
ture has a typical Burkholderia lipid A architecture with a
b-1!6 d-GlcN disaccharide and the nonstoichiometric pres-
ence of two l-Ara4N units (Scheme 1).[16]
typically composed of
a core oligosaccharide and an
O-specific polysaccharide. This complex carbohydrate is
covalently linked to the third component, a lipophilic
moiety termed lipid A, which is embedded in the outer
leaflet of the membrane.[3] The ability of a host organism to
recognize LPSs and the consequences of this recognition in
infection and symbiosis have been the subject of many
ground-breaking studies and constitute a major research
area.[7] However, these studies have focused on LPSs in the
context of bacteria–animal or bacteria–plant interactions.[3–5]
[*] Dipl.-Biochem. G. Lackner,[+] Prof. Dr. C. Hertweck
Abteilung fꢀr Biomolekulare Chemie
Leibniz-Institut fꢀr Naturstoff-Forschung und Infektionsbiologie,
HKI, Beutenbergstrasse 11a, 07745 Jena (Germany)
Fax: (+49)3641-532-0804
E-mail: christian.hertweck@hki-jena.de
Prof. Dr. C. Hertweck
Friedrich-Schiller-Universitꢁt, Jena (Germany)
Dr. M. R. Leone,[+] Prof. Dr. A. Silipo, Prof. Dr. R. Lanzetta,
Prof. Dr. A. Molinaro
Dipartimento di Chimica Organica e Biochimica
Universitꢂ di Napoli Federico II
via Cintia 4, 80126 Napoli (Italy)
The OPS was also characterized by a combination of
chemical analysis and 2D NMR spectroscopy, in particular
COSY, TOCSY, NOESY, HSQC (see Figures S1–S3 in the
Supporting Information), and F2-coupled HSQC. The
1H NMR spectrum showed a single anomeric signal indicative
of a single-spin system, which was fully assigned by homo-
nuclear 2D NMR spectroscopy, whereas from HSQC spectra
it was possible to assess the furanosidic nature and anomeric
configuration of the monosaccharide. These data, in con-
jugation with chemical analysis, revealed that the OPS is a
E-mail: molinaro@unina.it
[+] These authors contributed equally.
[**] We thank the JCVI Annotation Service for providing us with
automatic annotation data and the manual annotation tool
Manatee, K. Graupner for PCR screening of mutants, L. Sturiale for
MS spectra, and G.-M. Schwinger for fungal-strain maintenance.
This research was supported by the graduate school of excellence
ILRS/JSMC (C.H.) and by PRIN MIUR 2007/2008 (R.L. and A.M.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003301.
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abundant in filamentous fungi.[18,19] Furthermore, structurally
related galactofuranan bioactive antigens have been identi-
fied in fungi.[20,21] Thus, it is well conceivable that the
O antigen mimics structural components of the host cell.
To test this hypothesis and gain insight into the molecular
basis of LPS formation, we analyzed shotgun-sequence data
of the B. rhizoxinica genome and searched for candidate
genes for LPS biosynthesis. Automated annotation revealed a
gene cluster comprising 29 open reading frames, 22 of which
code for proteins similar to known enzymes involved in LPS
biosynthesis (Figure 1a; see also the Supporting Informa-
tion). The majority of the genes in this locus code for the
biosynthesis and transfer of outer-core and O-antigen build-
ing blocks, including the pathway for dTDP-l-rhamnose
(rmlA-F; dTDP = deoxythymidine diphosphate). More
importantly, the finding of genes that code for a UDP-
galactopyranose mutase (Glf; UDP = uridine diphosphate)[22]
and a UDP-glucose-4-epimerase (GalE) is fully in line with
the elucidated LPS O-antigen structure, since these enzymes
are well known to be involved in the formation of UDP-d-
galactofuranose (Figure 1b).[23] Furthermore, the gene cluster
contains several glycosyl-transferase genes, and two genes
(wzm and wzt) for an ABC transporter system that shuttles
membrane-anchored O-antigen chains from the cytosol to the
periplasm prior to ligation to the core oligosacharide.[24]
Finally, we found a gene coding for an O-antigen ligase
(WaaL). WaaL is essential for the transfer of the O antigen to
the outer core to finish the LPS-assembly process.[24,25]
Consequently, mutants devoid of the O-antigen ligase gene
Scheme 1. Structure of lipid A of endosymbiotic B. rhizoxinica.
Ara4N=4-amino-4-deoxy-l-arabinose, GlcN=2-amino-2-deoxy-d-glu-
cose.
homopolymer of 2-substituted d-galactofuranose, [!2)-b-d-
Galf-(1!]n. This unusual type of polysaccharide is fully
unprecedented for Burkholderia spp. and related bacteria.
Furthermore, the existence of [!2)-b-d-Galf-(1!]n in nature
has only been implicated once, on the basis of a limited data
set.[17] The discovery of an antigenic poly-d-galactofuranose
chain in an endofungal bacterium is particularly intriguing,
since Galf conjugates have been found to be especially
Figure 1. a) Schematic illustration of the LPS biosynthesis gene cluster of B. rhizoxinica. Protein-coding genes are indicated as arrows. Colors
represent deduced functional categories of gene products. See Table S3 in the Supporting Information for details. b) Model for the biosynthesis of
UDP-d-galactofuranose, the O-antigen building block of B. rhizoxinica (GT=glycosyl transferase). c) Attachment of the O antigen to the LOS by
the ligase WaaL.
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(DwaaL mutants) would lack the O-antigenic chain on the
cell surface (rough phenotype). To study the role of the
O antigen in vivo, we created an O-antigen ligase mutant by
replacing the waaL gene with a kanamycin-resistance cassette
(DwaaL:Kanr). Thus, we created a suicide vector containing a
counter-selectable marker: a mutated phenylalanyl-tRNA
synthetase gene pheS.[26] After various selection rounds, we
succeeded in generating the desired mutant with a rough
colony phenotype indicative of the presence of a lipooligo-
saccharide (LOS); that is, with the core oligosaccharide
region attached to lipid A, but with no O antigen.
The full structure of the LOS core region of the mutant
was deduced by a combination of chemical analysis, MALDI
mass spectrometry, and 2D NMR spectroscopy. Seven
1
anomeric signals were identified in the H NMR spectrum.
Scheme 2. Core oligosaccharide structure of the R-LPS (LOS) from the
O-antigen-ligase mutant (DwaaL:Kanr). GalNAc=2-amino-2-deoxy-d-
galactose, Glc=d-glucose, Hep=l-glycero-d-manno-heptose, Rha=l-
rhamnose.
Furthermore, the upfield-shifted signals were identified as the
3-H methylene hydrogen atoms of the 3-deoxy-d-manno-oct-
2-ulosonic (Kdo) residue (see the Supporting Information).
The proton resonances of all spin systems were identified
from DQF-COSY and TOCSY spectra and were used to
assign the carbon resonances in the HSQC spectrum. The
anomeric configuration of each monosaccharide was assigned
sporulation as an indicator for the reestablishment of
symbiosis. Thus, we monitored sporulation behavior over
time both on agar plates and in liquid culture by using 48-well
plates. Cocultivation of the cured fungus with wild-type
bacteria typically resulted in high levels of successful rein-
fection, which is mirrored by about 90% host sporulation. In
stark contrast, the DwaaL mutant showed significantly
reduced reinfection/sporulation rates (Figure 2). In cases in
which reinfection with DwaaL::Kanr mutants gave a positive
response, the intensity of host sporulation was much lower
than with the wild type. When the fungus was reinfected with
DwaaL::Kanr mutants, intracellular bacteria could be
detected by microscopic examination, albeit in greatly
reduced number (< 10%) relative to the wild type. Further-
more, after the subcultivation of reinfected fungi, we
observed persistently low host sporulation or even a complete
lack of spore formation.
Our findings provide strong evidence that the O antigen
plays a crucial role in the bacterial–fungal symbiosis. Since the
carbohydrate coating supports the processes of host infection
and triggering of sporulation, it seems to serve as a key
determinant in chemical-recognition processes during infec-
tion and colonization of the host. Furthermore, it is a
prerequisite for the long-term intracellular survival of the
endosymbiont and for the formation of a stable bacterial–
fungal association. A plausible explanation for these obser-
vations is given by a model in which the polygalactofuranose
O antigen protects endobacteria against as yet unknown
fungal defense mechanisms. Such strategies are only known
from associations of bacteria with higher organisms. For
example, beneficial bacteria inhabiting mammalian intestines
decorate their surface with fucose, which is an abundant
surface molecule of intestinal epithelial cells.[29] It is also well-
known that some other Gram-negative pathogens, such as
Neisseria and Helicobacter, decorate their surface with blood-
group antigens.[30]
3
on the basis of the J1-H,2-H coupling constants observed by
DQF-COSY and the intraresidual NOE contacts observed in
the ROESY and NOESY spectra, whereas the values of the
vicinal 3JH,H coupling constants enabled the determination of
the relative configuration of each sugar residue. The absence
of chemical-shift values above 80 ppm for ring carbon atoms
confirmed that all monosaccharides were in the pyranose
form. The relative intensities of the anomeric signals sug-
gested the existence of a mixture of oligosaccharides with
different carbohydrate-chain lengths owing to the presence of
the Kdo reducing end as multiple ring isomers or as lactone
1
forms. Once H and 13C resonances had been attributed to
each spin system, it was possible to assign the oligosaccharide-
chain resonances from the interglycosidic contacts found in
the NOESY and ROESY spectra (see Table S1 in the
Supporting Information for complete structural assignment).
The full structure was confirmed by mass spectrometry
(see Table S2 and Figures S7 and S8 in the Supporting
Information). The core of the LPS from B. rhizoxinica
consists of a nonasaccharide backbone comprising four
heptose (Hep) residues as well as a GalNAc, a Glc, a
rhamnose (Rha), a 3-deoxy-d-manno-oct-2-ulosonic acid
(Kdo), and a d-glycero-d-talo-oct-2-ulosonic acid (Ko) unit
(Scheme 2). The core oligosaccaride structure of the B. rhi-
zoxinica LOS resembles that of B. multivorans,[27] especially
in terms of the inner core and the presence of a further
heptose residue attached to the b-Glc unit. The outer core is
different from but still coherent with LOS structures from
Burkholderia spp.[16] Finally, the structure of the truncated
LPS clearly showed that the mutant lacks the O antigen.
To test whether this mutant is capable of initiating a stable
symbiosis with the host fungus, we mixed pure cultures of
B. rhizoxinica (wild-type and mutant) with endosymbiont-
free (cured) R. microsporus cultures. Usually, wild-type
bacteria readily reinfect the fungus, reestablish the symbiosis,
and elicit sporulation of the host. Cured fungi, however, are
unable to sporulate.[28] Consequently, we considered host
In summary, we have fully elucidated the first LPS
structure of a bacterium living within a fungus and revealed
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Chemie
galactofuranose units are particularly abundant in filamen-
tous fungi, one may conclude that the galactofuranosyl
O antigen serves as mimicry to put the bacterium into
“stealth mode”. To our knowledge, this study is the first to
shed light on the role of surface carbohydrates in an
interaction between endobacteria and fungi. Our results
disclose the role of a glycoconjugate in a novel biological
context and thus fill an evident gap in the current knowledge
on LPS-mediated communication.
Received: May 31, 2010
Published online: August 16, 2010
Keywords: bacteria · carbohydrates · cell recognition ·
.
lipopolysaccharides · O antigens
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