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Journal of Bacteriology, April 1999, p. 2659-2662, Vol. 181, No. 8
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Sequence Analysis of the Four Plasmid Genes
Required To Produce the Circular Peptide Antibiotic Microcin
J25
José O.
Solbiati,1
Mirella
Ciaccio,1
Ricardo N.
Farías,1
José E.
González-Pastor,2
Felipe
Moreno,2 and
Raúl A.
Salomón1,*
Departamento de Bioquímica de la
Nutrición, Instituto Superior de Investigaciones Biológicas
(Consejo Nacional de Investigaciones Científicas y
Técnicas
Universidad Nacional de Tucumán) and Instituto de
Química Biológica "Dr. Bernabé Bloj," 4000 San Miguel de Tucumán, Argentina,1 and
Unidad de Genética Molecular, Hospital Ramón y
Cajal, 28034 Madrid, Spain2
Received 25 January 1999/Accepted 10 February 1999
 |
ABSTRACT |
A 4.8-kb plasmid region carrying the four genes mcjABCD
necessary for production of and immunity to the cyclic peptide
antibiotic microcin J25 (MccJ25) has been sequenced. mcjA
encodes the primary structure of MccJ25 as a precursor endowed with an
N-terminal extension of 37 amino acids. The products of
mcjB and mcjC are thought to be involved in
microcin maturation, which implies cleavage of McjA and head-tail
linkage of the 21-residue pro-MccJ25. The predicted McjD
polypeptide, which is highly similar to several ABC exporters, was
found to be required for MccJ25 secretion, thus explaining its ability
to confer immunity to MccJ25.
 |
TEXT |
Microcin J25 (MccJ25) is a
plasmid-encoded, hydrophobic cyclic peptide consisting of 21 unmodified
amino acid residues (2) which is synthesized and secreted
into the culture medium by Escherichia coli AY25 isolated
from human feces (15). MccJ25 is active on gram-negative
bacteria, and some pathogenic species of Salmonella and
Shigella are highly sensitive to it. MccJ25 exhibits a
unique mode of action. It induces cell filamentation in a
non-SOS-dependent way, suggesting that its molecular target may be a
factor involved in cell septation (15). Thus, in addition to
its interest as an antibacterial compound, MccJ25 holds promise as a
tool for cell division studies. The production of MccJ25 is induced at the onset of stationary growth phase and is optimal in iron-depleted medium (15, 17). Genetic studies identified three genes,
mcjA, mcjB, and mcjC, which were
essential to microcin production, and one gene, mcjD, that
conferred immunity to exogenous MccJ25. The four genes were located in
a continuous 4.8-kb region of the 50-kb plasmid pTUC100 isolated from
the AY25 strain (21). In the present study, both strands of
this DNA fragment were entirely sequenced by the dideoxy chain
termination method of Sanger et al. (19). We show that the
four genes mcjABCD are required and sufficient to confer on
a bacterial host the ability to produce mature extracellular microcin.
Based upon the results of physiological experiments and features of the
predicted polypeptide gene products, we propose a function for each gene.
E. coli K-12 RYC1000 (F
araD139
lacU169 rib
7 rpsL relA thiA recA56 gyrA) (6) was
used as the host strain for plasmids, except when otherwise indicated.
SBG231 is a MccJ25-resistant spontaneous mutant derived from strain
AB259 (supQ80 
relA1 spoT1 thi-1), kindly
provided by B. Bachmann (E. coli Genetic Stock Center).
Plasmids pTUC203 and pJS200 are described in reference 21. Plasmid pTUC346, which was used for sequencing,
was constructed by subcloning the
HindIII-SalI fragment containing the microcin genes (Fig. 1) into pUC18. A schematic
diagram of all plasmids used in this work is shown in Fig. 1.

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FIG. 1.
(A) Genetic organization of the
HindIII-SalI fragment containing the MccJ25
system. The dotted line below the restriction map shows the sequenced
region. Arrows indicate the extension and transcriptional direction of
the genes identified by nucleotide sequence analysis. Abbreviations for
restriction endonuclease sites: H, HindIII; E,
EcoRI; V, EcoRV; A, AccI; Hc,
HincII; S, SalI. (B) Map of the different
plasmids used in this study. The names of the plasmids are indicated on
the right. The lines correspond to the DNA of the
HindIII-SalI fragment present on each
construct. Boxes represent vector DNA (not drawn to scale).
|
|
The analysis of the nucleotide sequence revealed the presence of four
open reading frames (ORFs), which were preceded by putative ribosome
binding sites at a distance adequate to initiate translation (20). The size and location of these ORFs corresponded to
the four genes previously deduced from genetic complementation
experiments (21). The mcjB, mcjC, and
mcjD genes are transcribed in the same direction, while
mcjA is transcribed in the opposite direction, diverging
from mcjB (Fig. 1). The features of the predicted gene products are summarized in Table 1.
mcjA encodes the MccJ25 precursor.
For
convenience, the nucleotide sequence of mcjA is presented
here (Fig. 2). The deduced amino acid
sequence of the 21-residue C-terminal portion of the McjA polypeptide
(from Gly-38 to Gly-58) is identical to that of purified mature MccJ25
(2). This indicates that MccJ25 derives from McjA by the
elimination of the 37-residue N-peptide and subsequent head-tail
linkage of the 21-residue C-propeptide. Then, as with most
small-peptide antibiotics, MccJ25 is synthesized on ribosomes as a
precursor (the prepropeptide) containing an N-terminal extension (the
leader peptide), which is removed during peptide maturation (4, 7,
10). In the case of McjA, the cleavage should occur between
Lys-37 and Gly-38. This 37-amino-acid leader peptide differs from
typical secretion signal sequences of polypeptides that are secreted
from the cytoplasm using the sec apparatus, suggesting that,
as with many other small peptides, MccJ25 is secreted using a dedicated
export machinery, as demonstrated below.

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FIG. 2.
Nucleotide and deduced amino acid sequences of the 5'
region of the MccJ25 cluster, including the mcjA gene and
the beginning of the mcjB gene. mcjA and
mcjB are transcribed from the opposite DNA strands; thus,
the sequence of mcjA is the noncoding strand, and the
sequence of mcjB presented here is the coding strand. The
stop codon of mcjA is indicated by an asterisk. Putative
ribosome binding sites (S.D.) and promoter sequences are underlined.
The cleavage site in McjA is indicated by a vertical arrow between
amino acids 37 and 38. The amino acid sequence of mature microcin is
shown in boldface.
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|
The mcjB and mcjC gene products are
required for the conversion of McjA to MccJ25.
The end of
mcjB overlaps the Shine-Dalgarno sequence of
mcjC, suggesting that these genes are transcriptionally and
translationally coupled (1, 3). Hydropathy profiles of the
inferred amino acid sequences of McjB and McjC indicated that none of
these proteins has potential membrane-spanning domains or a
characteristic signal peptide, which suggests that they may be
cytosolic proteins. While active MccJ25 may be extracted from cells
expressing the three genes mcjABC, no active peptide was
detected in cells bearing plasmids that expressed only two genes,
mcjA and mcjB or mcjA and
mcjC (21). Therefore, McjB and McjC must take
part in MccJ25 maturation, which would imply the cleavage of pre-MccJ25
and linkage of the Gly-38 residue to the C-terminal Gly-58 residue
resulting in the cyclic MccJ25. Note that no significant similarity was observed between the mcjB and mcjC products and
other known proteins. It is possible that the enzymatic machinery
necessary for microcin biosynthesis represents unknown types of enzymes.
The mcjD gene encodes a putative inner membrane ABC
exporter which is required for MccJ25 secretion.
Computer-aided
analysis of the amino acid sequence deduced from the mcjD
gene showed that it contains all of the typical structural characteristics of known bacterial ABC exporters (5).
Indeed, the amino half of McjD is predicted to span the inner membrane six times, and the carboxy half contains an ATP-binding domain, which
includes the highly conserved Walker A and B motifs (23). McjD was highly similar to polypeptides belonging to the family of ABC
transporter proteins. The strongest similarities were confined to the
C-terminal portion of McjD surrounding the nucleotide-binding fold.
Figure 3 illustrates the alignment of 240 amino acid residues of this domain for McjD and the ABC transporters
that showed the highest levels of homology: HetA from
Anabaena sp. strain PCC 7120, required for the process of
morphological differentiation of heterocysts (35% identity)
(8); YwjA, a Bacillus subtilis hypothetical ABC
transporter (GenBank accession no. P45861) (35% identity); Hmt1, from
the fission yeast Schizosaccharomyces pombe, involved in
heavy metal tolerance (29% identity) (14); and
Rhizobium meliloti NdvA, required for
-(1
2) glucan
production (27% identity) (22).

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FIG. 3.
Alignment of the McjD amino acid sequence flanking the
nucleotide binding site with corresponding regions of other ABC
transporters. The proteins listed are described in the text. Identical
residues are shaded. Dashes indicate gaps introduced in the sequence to
maximize the similarity. Amino acid sequences corresponding to the
Walker A and B motifs (23) are indicated.
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|
Plasmid pTUC203 contains the four genes
mcjABCD (Fig.
1).
When it was mutagenized with transposon Tn
5, many insertion
mutations
impairing microcin production were obtained in genes
mcjABC, but
none was isolated within
mcjD
(
21). This suggested that inactivation
of
mcjD
could be lethal for cells expressing the other three genes;
in other
words, the activity of the latter would result in the
biosynthesis of a
toxic compound (i.e., microcin) which, in the
absence of McjD, would
probably accumulate in the cells and kill
them. Hence, McjD could be
the component or a component of the
pump that extrudes MccJ25 out of
producing cells. This view is
consistent with the findings that McjD
confers resistance (immunity)
to exogenous microcin (
21),
and its predicted sequence displays
all of the typical characteristics
of an ABC exporter, as indicated
above. To confirm this hypothesis, a
HincII fragment was deleted
from pTUC346
(MccJ25
+ Imm
+ Ap
r) to generate
pTUC347 (Fig.
1). This deletion removed the nucleotide
sequence that
encodes the 117 amino acid residues of the C-terminal
end of McjD,
including the Walker B motif, but did not touch
mcjABC,
which remained intact. Since this construct was expected to kill
nonimmune host cells, the digested and religated mixture was
transformed
into immune RYC1000 (pJS200 Tc
r) cells,
selecting for ampicillin resistance. One of the several
clones obtained
was shown to contain both plasmids species, the
residing pJS200 and the
newly constructed pTUC347. This plasmid
mix was then used to transform
RYC1000 cells, selecting for ampicillin
or tetracycline resistance.
Whereas all of the Ap
r transformants were also
Tc
r, only some of the Tc
r clones were
Ap
r. This expected result indicated that pTUC347 could not
be propagated
in the absence of pJS200 and supported the hypothesis
that the
compound synthesized from pTUC347 was lethal in the absence of
McjD.
While the above experiments were in progress, we simultaneously
conducted a search for new types of MccJ25-resistant mutants.
Although
most of the clones isolated belonged to the already-known
classes of
resistant mutants (i.e.,
sbmA,
tonB,
exb, and
fhuA mutants [
16,
18]),
one of them showed a distinct phenotype.
Presuming that this mutant
(called SBG231) was a microcin target
mutant, we transformed it
with a plasmid preparation containing
both pTUC347 and pJS200,
selecting for ampicillin resistance.
This time, clones harboring only
pTUC347 were obtained. These
transformants (Ap
r
Tc
s) grew normally, indicating that the chromosomal
mutation overcame
the inhibitory effect of pTUC347, but they were
unable to give
growth inhibition halos on microcin-sensitive indicator
strains.
When these nonproducing cells were checked for intracellular
antibiotic
activity, we found that they contained as much microcin
as SBG231(pTUC346)
cells. The extracted antibiotic killed
MccJ25-susceptible cells
but had no effect on MccJ25-immune and
MccJ25-resistant mutant
cells (SBG231), indicating that it was true
mature microcin. On
the other hand, when transformed with pJS200, which
contains only
the
mcjD gene, strain SBG231(pTUC347) gained
the ability to produce
normal-sized inhibition halos on
MccJ25-sensitive cells. Together,
these results indicated that McjD is
not involved in MccJ25 synthesis
and maturation but is required for its
export out of the producing
cells.
Concluding remarks.
In this study, we have presented the
molecular characterization of the four plasmid genes,
mcjABCD, involved in microcin production. No other ORF
with potential biological significance was detected in the nucleotide
sequence. Our results indicate that MccJ25 derives from McjA by
elimination of a 37-residue leader peptide and subsequent cyclization
of the 21-residue C-terminal propeptide. The dipeptide Gly-38-Gly-39,
adjacent to the cleavage site (Fig. 2), is reminiscent of the double
glycine motif found in the leader peptides of most nonlantibiotics and
some lantibiotic peptides from gram-positive bacteria and colicin V
from E. coli (4, 10). However, apart from this,
the MccJ25 leader did not share significant homology with consensus
sequences found in the double-glycine-type leader peptides.
Furthermore, in the latter the cleavage occurs at the carboxy side of
the Gly-Gly motif whereas the microcin leader is cleaved just prior to
the glycine residues. Another feature distinguishing the MccJ25 leader
from those of the Gly-Gly class is the net charge; whereas that of the
latter ones is highly negative, the MccJ25 leader peptide is highly positive.
Although the function of McjB and McjC is not well defined, they are
clearly required for microcin biogenesis. With regard
to the
mcjD gene product, we have demonstrated that it is necessary
for endogenously synthesized MccJ25 to be exported out of the
cells.
The predicted location and amino acid sequence of McjD
are consistent
with this role. Thus, the immunity conferred by
McjD could well be
mediated by active efflux of the peptide, which
would keep its
concentration below a critical
level.
Finally, it is interesting to note that posttranslational modification
of ribosomally synthesized peptides by tail-head linkage
is not common.
In fact, MccJ25 appears to be the second example
of this new type of
peptide. The first one is the
Enterococcus faecalis
bacteriocin AS-48, whose genetic system has recently
been characterized
(
12,
13).
Nucleotide sequence accession number.
The nucleotide sequence
of the 4.8-kb region described in this work has been deposited in the
GenBank database under accession no. AF061787.
 |
ACKNOWLEDGMENTS |
This work was supported by Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET) grant PID 3-013800, Fundación Antorchas grant 12576/1-000065, and funds from the
Consejo de Investigaciones de la Universidad Nacional de Tucumán
(CIUNT). J.O.S. was the recipient of a CONICET fellowship, R.N.F. was a career investigator of CONICET, and M.C. was a visiting scientist from
Istituto di Biologia dello Sviluppo (C.N.R., Italy).
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Departamento de
Bioquímica de la Nutrición, INSIBIO, Chacabuco 461, 4000 San Miguel de Tucumán, Argentina. Phone: (54) (381) 4248921. Fax:
(54) (381) 4248025. E-mail: salomon{at}unt.edu.ar.
 |
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Journal of Bacteriology, April 1999, p. 2659-2662, Vol. 181, No. 8
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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