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Journal of Bacteriology, September 2001, p. 5426-5430, Vol. 183, No. 18
Unité de Génétique des
Génomes Bactériens, Institut Pasteur, 75724 Paris Cedex 15, France
Received 31 October 2000/Accepted 13 June 2001
The spore coat protein CotA of Bacillus subtilis
displays similarities with multicopper oxidases, including manganese
oxidases and laccases. B. subtilis is able to oxidize
manganese, but neither CotA nor other sporulation proteins are
involved. We demonstrate that CotA is a laccase. Syringaldazine, a
specific substrate of laccases, reacted with wild-type spores but not
with The spore-forming bacterium
Bacillus subtilis synthesizes and deposits a protein coat
around the developing endospore during differentiation
(8). The spore coat consists of at least 25 different
polypeptides of 5 to 65 kDa, and some of them are highly cross-linked.
These proteins are assembled into a lamella-like inner coat and an
electron-dense outer coat, which protects the spore from a diverse
range of stresses (8).
The cotA gene codes for a 65-kDa protein belonging to the
outer spore coat of B. subtilis. It corresponds to the
former pig locus (7, 21) and was one of the
first cot genes to be cloned (7). The
cotA gene is expressed under the control of sigma K, with
GerE acting as a transcriptional repressor instead of its more
usual function of transcriptional activator (24). The absence of CotA has no apparent effect on spore resistance, but it
results in the loss of the usual brownish pigmentation of the colonies.
Moreover, wild-type spore-forming colonies are darker at higher
manganese concentrations (13). This is similar to marine
Bacillus sp. strain SG1, which has spores that express manganese oxidase (30).
The CotA protein displays similarities with multicopper oxidases (for a
review, see reference 28). In particular, it contains the
four copper-binding sites, the type I blue copper center (T1) and the
T2/T3 trinuclear cluster, which differ in their spectroscopic features (Fig. 1). The multicopper
oxidase family includes ascorbate oxidase (EC 1.10.3.3), ceruloplasmin
(EC 1.16.3.1), laccase (EC 1.10.3.2), various manganese oxidases, and
other enzymes. Among the proteins most closely related to CotA are the
manganese oxidase of Leptothrix discophora (4)
and the CumA protein of Pseudomonas putida (1),
which is essential for manganese oxidation. CotA also shares
similarities with a polyphenoloxidase of Acremonium murorum
and, with much lower similarity scores, laccases including those of
Neurospora crassa (9), Agaricus
bisporus (18), and Marinomonas
mediterranea
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.18.5426-5430.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
CotA of Bacillus subtilis Is a
Copper-Dependent Laccase
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ABSTRACT
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Abstract
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cotA spores. CotA may participate in the
biosynthesis of the brown spore pigment, which appears to be a
melanin-like product and to protect against UV light.
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TEXT
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Abstract
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formerly an Alteromonas species (MMB1) (23). Laccase was first found in the sap of the Japanese
lacquer tree (hence its name). Laccases are widespread in plants, where they are involved in lignin biosynthesis, and in fungi, where they are
involved in ligninolysis, development-associated pigmentation, detoxification, and pathogenesis (28, 29). Laccases are
polyphenoloxidases which oxidize a wide range of polyphenols,
methoxy-substituted phenols, and diamines. Substrate specificity
differs from one laccase to another, but most laccases do not oxidize
tyrosine, in contrast to the classic tyrosinase (EC 1.14.18.1) (for a review, see reference 29). Tyrosinase is also a
polyphenoloxidase, but it contains only one coupled binuclear copper
center (type III; T3).

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FIG. 1.
The four copper-binding sites in CotA and in some fungal
laccases. Abbreviations: AgaBi, A. bisporus; TraVe,
Trametes versicolor; ThaCu, Thanatephorus
cucumeris; NeuCr, N. crassa; PycCi,
Pycnoporus cinnabarinus; MarMe, M.
mediterranea. There are two laccases (laccases 1 and 2) which
have identical copper-binding sites in A. bisporus,
T. versicolor, and N. crassa. Conserved
amino acids are shaded in black (90% conservation or more) or in grey
(70 to 90% conservation). T1, T2,
T3A, and T3B are the type 1, 2, and 3 copper
centers, which differ by their spectroscopic features (T3 center
contains two copper atoms called A and B). For each copper center,
arrows point to the copper-binding residues, which upon folding of the
protein come into close proximity and coordinate copper.
Despite similarities with the multicopper oxidases, it is not known if CotA has any oxidase activity in B. subtilis. We tested whether CotA is a manganese oxidase or a laccase with or without tyrosinase activity. We report that CotA is a classical laccase.
To identify the function of CotA, a
cotA::aphA3 mutant was constructed. A
DNA fragment including the cotA gene was amplified by PCR
using chromosomal DNA as the template. Oligonucleotides allowing the
creation of a HindIII site 294 bp upstream from the start codon of cotA and an EcoRI site 114 bp
downstream from the stop codon were used. The resulting fragment was
ligated between the EcoRI and HindIII sites
of pUC18. A kanamycin cassette was then inserted between a
PstI site (95 bp upstream of the translation start) and an
XbaI site (54 bp upstream from the stop codon), deleting the
whole coding sequence except for the last 18 amino acids. The resulting
plasmid was linearized and used to transform B. subtilis
strain 168 to give the cotA::aphA3
mutant (
cotA).
All experiments were performed in accordance with European regulation requirements concerning the contained use of genetically modified organisms from group 1 (regulation no. 2735).
To define the relationship between spore pigmentation and manganese,
the formation of the brownish color was studied in several culture
conditions and in various mutants. Precultures were diluted to an
optical density at 600 nm of 0.1. Using a home-made replicator (for
details, see
http://locus.jouy.inra.fr/genmic/madbase/mutant/home.gif), the bacteria were streaked on SP agar medium (pH 7.0)
(26) in square Falcon plates (Fig.
2A). The bacteria were grown for 48 h at 37°C. To modify the pH, a Whatman paper (8 by 8 cm) was then laid in the plate lid and soaked with 1.5 ml of either distilled water
(Fig. 2A, plate I) or 3 M sodium acetate, pH 4.8 (Fig. 2A, plate II).
After 24 h, the pH was checked and found to be 8 in plate I and
3.5 in plate II. Small pieces of Whatman paper containing 10 µl of 5 M MnCl2 were then placed at one end of the
streaks. After 48 h, a brown pigmentation appeared near the site
of MnCl2 application in plate I, but not in plate
II (Fig. 2A). There was no pigmentation on plates similarly treated
with manganese in the absence of bacteria (data not shown). Thus, the
appearance of brown pigmentation in the agar around the colonies
depends on the presence of B. subtilis and manganese and
also on the pH of the plates. Similar results were obtained with strain
168, a
sigF mutant which is blocked early in the
sporulation process, and the
cotA mutant (Fig. 2A). This
pigmentation found at high pH is therefore independent of sporulation
and particularly of CotA.
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To determine whether the observed brown color was due to manganese oxide, leucoberbelin blue, which specifically reacts with MnIII to MnVII, was tested with the browned agar (14). For each strain, dark blue (tube I, corresponding to plate I) and pale blue (tube II, corresponding to plate II) staining revealed the presence of manganese oxide on plate I but not on plate II (Fig. 2B). Therefore, B. subtilis is able to oxidize manganese, but neither CotA nor sporulation is involved. Colonies tranferred to nylon filters did not react with the leucoberbelin blue (data not shown). This indicates that the bacterial spore pigment is not manganese oxide, which agrees with the report of van Waasbergen and coworkers (31). Manganese oxide in the agar was also detected when the plates were autoclaved before the addition of manganese (data not shown) and is therefore not produced by enzymatic activity. It is probably formed by the alkalinization of the medium, resulting from bacterial growth (5).
So, the CotA-dependent pigmentation remains to be explained. Four
copper-binding sites are present in CotA and in laccases (Fig. 1). In
spite of the overall low similarities, we then tested whether CotA
could be a laccase. A substrate specific for laccase, syringaldazine
[N,N'-bis(3,5-dimethoxyhydroxybenzylidenehydrazine)], was used (10). Spore suspensions were incubated with 50 µM syringaldazine and 10 µM CuSO4 in 100 mM phosphate buffer. A
purple color developed with 168 spores, but not with
cotA
spores, indicating that CotA may be a laccase (Fig. 2C). The optimal pH
and temperature for this reaction were pH 7 and 45°C (data not
shown). To confirm the laccase activity, the CotA protein was
overproduced in Escherichia coli. The cotA gene
(nucleotides
1 to +1656 from the start codon) was cloned under the
control of the strong inducible T7 promoter in the pET20b+ vector
(Novagen). The pET20b+-cotA plasmid was transformed into
E. coli strain BL21(DE3) (Novagen). Cells were grown at
37°C in Luria-Bertani medium until they reached an optical density at 600 nm of 3. IPTG
(isopropyl-
-D-thiogalactopyranoside) (1 mM)
was then added to induce cotA expression. Cells were
harvested 2 h later, resuspended in 100 mM phosphate buffer (pH
7), and treated with toluene. After the addition of 50 µM
syringaldazine and 10 µM CuSO4, the purple color appeared at 45°C.
No color appeared with pET20b+ alone. This demonstrates that
CotA is a laccase.
Since fungal laccase synthesis is often induced by the copper cofactor or by their substrates, the expression of a transcriptional cotA-lacZ fusion (19) was tested on Luria-Bertani plates enriched with various concentrations of copper or with putative substrates. Neither copper, p-anisidine, veratric acid, resorcin, p-toluidine, p-coumaric acid, lignosulfonic acid, orcinol, ferulic acid, p-xylidine, nor lignosulfonic acid elicited any induction. In addition, no ligninolytic activity was found with lignosulfonic acid (data not shown).
As some Bacillus strains produce pigment on agar media
supplemented with tyrosine and copper salts (27), the CotA
protein could be a laccase with tyrosinase activity, like the
multipotent polyphenoloxidase of M. mediterranea
(22). The addition of 1 mM tyrosine to SP medium did not
modify the color of the colonies. In contrast, addition of 10 µM
CuSO4 to SP medium enhanced the difference in the
spore pigmentation between strain 168 and the
cotA mutant
(Fig. 2D, plate III). This substantiates the role of copper in the
enzymatic activity of CotA. Further addition of 1 mM tyrosine did not
modify this result, suggesting that CotA has no tyrosinase activity
(data not shown). The color of the CotA-dependent pigment and that of
manganese oxide are quite similar. The effect of Cu and Mn on the
pigmentation of 168 and
cotA strains was then tested.
MnCl2 (1 mM) completely masked the difference between both strains on SP plates and reduced it on SP plates with 10 µM CuSO4 (Fig. 2D, plate IV). This may explain
why the spore pigmentation has been thought to be manganese dependent (13).
Laccase activity has been detected in Bacillus sphaericus. It correlates closely with spore formation and the appearance of dipicolinic acid (3). It has also been found in Streptomyces galbus (15), in Azospirillum lipoferum (6), and in M. mediterranea (22), all of which produce a melanic pigment. This suggests a possible link between laccase activity and melanin production, at least in S. galbus and A. lipoferum, as tyrosinase is responsible for pigmentation in M. mediterranea (23). This is reminiscent of some fungal laccases involved in melanization (2). CotA participates in the biosynthesis of the brown spore pigment, which is also thought to be a melanin-like product (25). A few bacilli, among them a strain of B. subtilis, produce a black melanic pigment (11, 12, 17, 25).
To test whether the pigment produced by B. subtilis has some
properties of melanin, spores of strains 168 and
cotA
were purified following Riesenman and Nicholson (20) after
growth on SP plates with and without 10 µM
CuSO4. The effect of 15%
H2O2 was tested on the
brownish spores of strain 168. The spore pigment was immediately bleached by this treatment, as would be expected for melanin. The spore
coat has been previously shown to protect against hydrogen peroxide
(20). The role of CotA in the resistance of spores to
H2O2 was therefore tested.
The 168 and the
cotA strains were treated 1 h in 5%
H2O2 as described by
Riesenman and Nicholson (20). A total of 2.2 × 10
3 cells survived in the 168 strain versus
10
8 cells in the
cotA
strain. Weakly pigmented 168 spores obtained on SP plates were as
resistant as brownish 168 spores obtained on SP plates supplemented
with 10 µM CuSO4. This shows that the CotA
protein and/or the pigment is involved in protection against hydrogen peroxide.
Melanin shields against radiation (16), and the spore coat
confers resistance to UV light (20). The effects of UVB,
UVA, and simulated solar light were therefore tested on spore survival following Riesenman and Nicholson (20). At the highest
level of irradiation energy applied, the brownish spores of strain 168 were 177-, 18-, and 19-fold more resistant than the
cotA
spores to UVB, UVA, and simulated solar light, respectively (Fig.
3). The weakly pigmented spores of strain
168 obtained in the absence of copper display an intermediate
protection (Fig. 3). This suggests a possible correlation between the
protective effect and the amount of pigment.
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To our knowledge, this is the first time that an enzymatic function is attributed to a spore coat protein. CotA seems to be responsible for most of the protection afforded by the spore coat against UV light and hydrogen peroxide. This effect is probably at least partly mediated by the spore pigment. However, identification of this pigment as a melanin would require further study.
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ACKNOWLEDGMENTS |
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We are deeply indebted to Evelyne Sage for helping with the UV work. We are grateful to W. Krumbein for kindly providing leucoberbelin blue, to G. Rapoport for helpful discussion, to O. Soutourina for translation of reference 27, to Cécilia Fabry for her bibliographic work, and to P. Ollivon, mechanic at the Pasteur Institute, for the construction of the replicator.
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FOOTNOTES |
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* Corresponding author. Mailing address: Unité de Génétique des Génomes Bactériens, 28 rue du Docteur Roux, 75724 Paris Cedex 15, France. Phone: 33 1 45 68 72 95. Fax: 33 1 45 68 89 48. E-mail: iverstra{at}pasteur.fr.
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