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Applied and Environmental Microbiology, February 2007, p. 1375-1379, Vol. 73, No. 4
0099-2240/07/$08.00+0 doi:10.1128/AEM.01873-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Gateway-Based Destination Vectors for Functional Analyses of Bacterial ORFeomes: Application to the Min System in Brucella abortus
,
Régis Hallez,
Jean-Jacques Letesson,
Jean Vandenhaute, and
Xavier De Bolle*
Unité de Recherche en Biologie Moléculaire, University of Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium
Received 7 August 2006/
Accepted 4 December 2006

ABSTRACT
Twenty Gateway-compatible destination vectors were constructed.
The vectors comprise fluorescent and epitope fusion tags, various
drug markers, and replication origins that should make them
useful for exploring existing microbial ORFeomes. In an attempt
to validate several of these vectors, we observed polar and
oscillating localization of MinD in
Brucella abortus.

INTRODUCTION
ORFeomes are comprehensive collections of predicted coding sequences
or open reading frames (ORFs) of a given organism (
3). The availability
of complete bacterial ORFeomes such as those described for
Brucella melitensis,
Pseudomonas aeruginosa, and
Sinorhizobium meliloti (
6,
14,
18) (other bacterial ORFeomes are available at the TIGR
resources:
http://pfgrc.tigr.org/) require plasmidic tools to
study biological functions of ORFs at proteomic levels, such
as interactomes and localizomes (
13,
15). Since the constructions
of ORFeomes cited above were based on the site-specific integration/excision
reactions used by bacteriophage

and called "Gateway recombinational
cloning" (
8,
23), we designed three sets of Gateway-compatible
destination vectors useful for functional analyses of these
bacterial ORFeomes.
The first set was designed for complementation of loss-of-function mutants and for constitutive overexpression of ORFs. The second set allows translational fusion with fluorescent proteins (cyan fluorescent protein [CFP], yellow fluorescent protein [YFP], and enhanced green fluorescent protein [EGFP]) for determining subcellular localization and colocalization of proteins. The last set of vectors provide translational fusions to tag coding sequences (21, 22), therefore making them useful for in vivo coexpression of ORFs and/or coimmunoprecipitation experiments. Integrative versions of these plasmids were also constructed to allow expression of a tagged ORF under the control of its native promoter. A detailed description of plasmid constructions is provided in the supplemental material. All plasmids reported here are available from the Belgian Coordinated Collections of Microorganisms (BCCM/LMBP plasmid collection) (http://bccm.belspo.be/about/lmbp.php).

Expression vectors.
Vectors described in this section were designed for functional
complementation assays or constitutive overexpression of bacterial
ORFs. Destination vectors pRH001, pRH002, and pRH003 were obtained
by cloning the Gateway cassette reading frame B into pMR10
cat,
pBBR1-MCS1, and pBBR1-MCS4, respectively (Table
1). The orientation
of the Gateway cassette in these three destination vectors was
selected to ensure that ORFs were placed under the control of
the
Escherichia coli lac promoter after
attL-attR (LR) reactions.
pRH001 (previously named pMR10
cat Gateway) was validated in
previous work using complementation assays (
5,
7). The transcriptional
regulators ArsR6 and GntR4 were identified to control the expression
of the
virB operon since no VirB proteins could be detected
in the corresponding
arsR6 and
gntR4 mutants (
7). Expression
of
arsR6 and
gntR4 from pRH001-
arsR6 and pRH001-
gntR4 completely
restored the wild-type level of production of VirB9, one of
the 11 proteins encoded by the
virB operon (
7). pRH001-
rsh was
also used to perform a heterospecific complementation of an
S. meliloti relA mutant with the orthologous
B. melitensis rsh (
5). From these data we can conclude that pRH001 is functional
for performing complementation of loss-of-function mutants,
at least in bacterial species in which the
E. coli lac promoter
is functional.
We used the Y-shape phenotype displayed by
Brucella overexpressing
the response regulator CtrA (
2) to test the functionality of
pRH002 and pRH003 for overexpressing ORFs. As expected,
B. abortus wild-type strain cells expressing
ctrA from pRH002-
ctrA and
pRH003-
ctrA are branched and larger than the wild-type control
(Fig.
1A and B and data not shown), as previously described
(
2). The level of expression of CtrA from these two plasmids
in
B. abortus was also compared to the wild-type CtrA level
by Western blotting (Fig.
1C and data not shown). From these
experiments we can conclude that pRH002 and pRH003 can be used
to overexpress ORFs.

Fluorescence vectors.
An increasing number of bacterial proteins have been reported
in the past decade to be localized at discrete locations in
bacteria (
10,
11,
19). In order to determine the subcellular
localization of bacterial proteins by direct fluorescence microscopy,
we designed the second set of destination vectors (Table
1).
These destination vectors allow the expression of fusions of
ORF-encoded proteins to the N termini of fluorescent proteins.
All fluorescence destination vectors described in this section
are derived from pMR10
cat, and the Gateway cassette in frame
with
cfp,
yfp, or
egfp was placed in the same orientation as
the
E. coli lac promoter.
Selection of the proper division site at midcell in bacteria such as E. coli requires the specific inhibition of septation at poles. This is mediated by the coordinated action of MinC, MinD, and MinE proteins. The division inhibition complex MinC/MinD, restricted to cell poles through the action of the topological specificity factor MinE, avoids the formation of a polar Z ring (4). In this system, MinD supplies polar localization (16, 17). We therefore tested if minDBm (BMEII0926 in the B. melitensis genome; AAL54168 in GenBank) encodes a protein localized at a pole(s) in B. abortus. To this end, we transferred minDBm from the pDONR201-minD entry clone available in the Brucella ORFeome (6) into pRH004, pRH005, and pRH006. The products of the LR reactions, performed as recommended by the manufacturer (Invitrogen), were electroporated into E. coli DH10B and selected on Luria-Bertani plates supplemented with chloramphenicol (20 µg/ml). Plasmids pRH004, pRH005, and pRH006, expressing minDBm, were then transferred by conjugation into B. abortus 544. The mid-exponential-phase cultures of three independent clones expressing minDBm on pRH004, pRH005, and pRH006 were embedded in a 1% agarose pad on microscope slides as previously described (9). Differential interference contrast (DIC) and fluorescence images were then acquired using Nikon E1000 microscope with the corresponding filter cubes.
As illustrated in Fig. 2A, B. abortus producing MinDBm-CFP, MinDBm-YFP, or MinDBm-EGFP showed fluorescent signals in the form of an arc following the line of the pole or polar focus (Fig. 2A, arrows) at either one or both poles in more than 90% of the bacteria examined. This is consistent with the localization of MinD described for other bacterial species such as E. coli (17) or Bacillus subtilis (16). Moreover, images taken at 30-second intervals of the same field indicated that MinDBm in B. abortus is polarly localized in a dynamic fashion by oscillating from one pole to the opposite pole, as was described for E. coli but not for B. subtilis (Fig. 2B). Similar observations were made when MinDBm was localized in the B. melitensis 16 M wild-type strain (data not shown).
We could observe in some bacterial cells a helical pattern of
MinD
Bm localization (data not shown), which has already been
described for
E. coli (
20). Finally, in most cells at a late
stage in the cell division process, a discrete fluorescence
signal at the center of dividing cells was observed (Fig.
2A,
arrowheads). This suggests that MinD
Bm does not localize dynamically
only at old poles, probably to inhibit formation of mislocalized
Z ring (i.e., polar Z ring), but also at division sites before
completion of cell constriction, probably to prevent another
division from taking place near the newly formed poles. It was
previously found that MinD is targeted to old and newly formed
poles in
B. subtilis (
16).
These results demonstrated that (i) pRH004, pRH005, and pRH006 can be used for determining the subcellular localization of bacterial proteins and that (ii) MinDBm is polarly localized and oscillates from one pole to the other in B. abortus, as previously shown for E. coli.

Tagging vectors.
Studying biological functions of a gene product requires understanding
the complex network(s) in which it is directly implicated. In
this context, mapping physical protein-protein interactions
is an important step. Yeast two-hybrid vectors are already available
in a Gateway-compatible format (
15), and here we describe vectors
allowing protein tagging, and therefore high-throughput coimmunoprecipitation
or pull-down tests. The first six tagging destination vectors
are derived from pMR10
cat (pRH013 to pRH015) and pBBR1-MCS1
(pRH016 to pRH018). These vectors allow expression of protein-coding
ORFs in frame with three repetitions of a hemagglutinin (HA)
epitope (3HA in pRH013 and pRH016), glutathione
S-transferase
(GST in pRH014 and pRH017), or 13 repetitions of the MYC epitope
(13MYC in pRH015 and pRH018) under the control of the
E. coli lac promoter. We assayed the tagging destination vectors (pRH013
to pRH018; Table
1) by using
minDBm or
minCBm (genomic and GenBank
accession numbers BMEII0927 and AAL54169, respectively) and
performing Western blot experiments on
E. coli DH10B and
B. abortus 544 crude extracts with anti-HA, anti-GST, or anti-MYC
antibodies. As shown in Fig.
3A and B, hybrid proteins MinD
Bm and MinC
Bm fused to 3HA, GST, or 13MYC were detected at the
expected sizes in crude extracts from
E. coli (Fig.
3A) and
B. abortus (Fig.
3B and data not shown). Neither MinD
Bm nor
MinC
Bm cross-reacted with anti-HA, anti-GST, or anti-MYC antibodies
since we did not detect any protein bands on crude extracts
from
E. coli and
B. abortus strains expressing
minDBm or
minCBm from pRH001 or pRH002 (data not shown).
Moreover, we were also able to detect two different hybrid proteins
(i.e., MinC
Bm-GST and MinD
Bm-13MYC) in a crude extract prepared
from
E. coli DH10B carrying two compatible clones, pRH017-
minCBm-GST and pRH015
-minCBm-13MYC (Fig.
3C). Similar experiments were
performed with
E. coli cells coproducing MinC
Bm-3HA from pRH013
and MinC
Bm-GST from pRH017 (data not shown).
These results validated the use of pRH013 to pRH018 for expression or coexpression of tagged protein-coding ORFs, which will be useful for detecting physical interaction by coaffinity or coimmunopurification of protein complexes.

Integrative reporter vectors.
The last group of destination vectors described here, pRH007
to pRH012, pRH019, and pRH020 (Table
1), are made for monitoring
gene expression. Indeed, they are designed to fuse an ORF to
the coding sequence (CDS) for a fluorescent protein (CFP, YFP,
or EGFP) or to the CDS for a tag (3HA, GST or 13MYC) but differ
from vectors described in previous sections (pRH004 to pRH006
and pRH013 to pRH018) by the absence of a promoter region. Since
these vectors are replicative only in members of the
Enterobacteriaceae family, they are expected, in all other bacteria, to integrate
into the genome at the locus where the ORF is located by homologous
recombination, therefore duplicating the target ORF. This would
result in the fusion between endogenous promoter, CDS, and the
fused tag, allowing the monitoring of gene expression through
fluorescence or quantification of tag abundance. pRH007 to pRH012
destination vectors may also be used to determine the localization
of the corresponding proteins expressed from the endogenous
promoter since after LR reactions ORFs are in frame with the
fluorescent reporter genes. In this way, vectors pRH010 to pRH012
were successfully used for determining the subcellular distribution
of several proteins involved in cell division and differentiation
of
B. abortus (R. Hallez, unpublished results).
In conclusion, we designed 20 Gateway destination vectors divided into three groups for functional analyses of complete or partial bacterial ORFeomes. These vectors open the way to genome-wide investigations such as localizome, i.e., the subcellular localization of all predicted proteins of a given bacterium.

ACKNOWLEDGMENTS
We are very grateful to A. Dricot, J. Mignolet, and R.-M. Genicot
for generous technical assistance during cloning procedures.
We thank M. Vidal, D. Hill, and J.-F. Rual for helpful and stimulating
discussions and L. Van Melderen and B. Nkengfac for comments
on the manuscript. We also thank the "Unité Interfacultaire
de Microscopie Electronique" of the University of Namur for
providing a scanning electron microscopy facility.
This work was supported by Fonds de la Recherche Fondamentale Collective (convention 2.4521.04) and by an "Action de Recherche Concertée" (ARC 04/09-325, Communauté Française de Belgique). At the time of this study, Régis Hallez held a Ph.D. fellowship from the Fonds pour la formation à la Recherche dans l'Industrie et dans l'Agriculture.

FOOTNOTES
* Corresponding author. Mailing address: Unité de Recherche en Biologie Moléculaire, University of Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium. Phone: 32 81 72 44 38. Fax: 32 81 72 42 97. E-mail:
xavier.debolle{at}fundp.ac.be.

Published ahead of print on 15 December 2006. 
Supplemental material for this article may be found at http://aem.asm.org/. 
Present address: Laboratoire de Génétique des Procaryotes, Institut de Biologie et de Médecine Moléculaires, Université Libre de Bruxelles, 12 Rue des Professeurs Jeener et Brachet, B-6041 Gosselies, Belgium. 

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Applied and Environmental Microbiology, February 2007, p. 1375-1379, Vol. 73, No. 4
0099-2240/07/$08.00+0 doi:10.1128/AEM.01873-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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