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Journal of Bacteriology, September 1999, p. 5860-5864, Vol. 181, No. 18
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Synthetic Lethal Phenotypes Caused by Mutations
Affecting Chromosome Partitioning in Bacillus
subtilis
Robert A.
Britton and
Alan D.
Grossman*
Department of Biology, Massachusetts
Institute of Technology, Cambridge, Massachusetts 02139
Received 19 May 1999/Accepted 9 July 1999
 |
ABSTRACT |
We investigated the genetic interactions between mutations
affecting chromosome structure and partitioning in Bacillus
subtilis. Loss-of-function mutations in spoIIIE
(encoding a putative DNA translocase) and smc (involved in
chromosome structure and partitioning) caused a synthetic lethal
phenotype. We constructed a conditional mutation in smc and
found that many of the spoIIIE smc double-mutant cells had
a chromosome bisected by a division septum. The growth defect of the
double mutant was exacerbated by a null mutation in the chromosome
partitioning gene spo0J. These results suggest that mutants
defective in nucleoid structure are unable to move chromosomes out of
the way of the invaginating septum and that SpoIIIE is involved in
repositioning these bisected chromosomes during vegetative growth.
 |
TEXT |
Chromosome partitioning is an
accurate and efficient process in bacteria. Several genes that play
roles in chromosome partitioning in Bacillus subtilis have
been characterized, including smc, spo0J, and
spoIIIE (1, 4, 9, 20, 28). SMC proteins are found in prokaryotes, eukaryotes, and archaea and are involved in a wide
range of processes that affect chromosomes, including partitioning, sister chromatid cohesion, dosage compensation, condensation, supercoiling, and recombination (1, 3, 5, 8, 11, 12, 14, 19, 20,
27) (for reviews, see references 7, 10, and
26). SMC proteins have an N-terminal nucleoside
triphosphate-binding domain, two long coiled-coil regions separated by
a hinge, and a C-terminal signature DA-box motif (13). SMC
proteins were first identified in eukaryotes but have now been found to
be encoded by most prokaryotic genomes sequenced to date (1,
18).
The B. subtilis SMC protein is involved in chromosome
structure and partitioning (1, 20), and its function may be
analogous to that of MukB in Escherichia coli
(21). smc null mutants are temperature sensitive
for growth in rich medium. Under permissive conditions, smc
null mutants have abnormal nucleoids and approximately 10% of the
cells are anucleate (1, 20). A recent biochemical characterization indicates that B. subtilis SMC is an
antiparallel homodimer (18) that has the ability to
aggregate and/or renature single-stranded DNA in ATP-dependent
reactions in vitro (6). The mode of involvement of these SMC
activities in chromosome partitioning and nucleoid structure is unknown.
spo0J of B. subtilis is required for faithful
chromosome partitioning (and sporulation). Spo0J is a member of the
ParB family of partition proteins (22), and spo0J
null mutant cells are ~1 to 2% anucleate, a frequency ~100-fold
higher than that of the wild type (9). Spo0J binds to
multiple sites in the origin region of the chromosome (15)
and forms a large nucleoprotein complex that is visible by the use of
immunofluorescence microscopy or a Spo0J-green fluorescent protein
fusion (4, 16). This complex may play a role in pairing
newly replicated sister origin regions or in the structural
organization of the origin region (1, 15). Consistent with
the notion that both SMC and Spo0J are involved in chromosome
organization and structure, an smc spo0J double mutant has a
synthetic lethal phenotype in rich medium (1).
SpoIIIE is involved in postseptational chromosome segregation during
sporulation (28, 30). spoIIIE mutants are unable to sporulate, and development is arrested with the forespore chromosome bisected by the asymmetric division septum. SpoIIIE localizes to the
sporulation septum (29), has similarity to DNA translocases, and has been proposed to pump the chromosome destined for the forespore
across the polar septum. SpoIIIE is also required for efficient
segregation during vegetative growth when normal cell division or
chromosome partitioning has been perturbed (25). Thus,
during vegetative growth, SpoIIIE may be a backup mechanism for
chromosome partitioning when normal partitioning is defective.
smc spoIIIE double mutants appear to have a synthetic
lethal phenotype.
We attempted to construct an smc
spoIIIE double mutant by combining an smc null mutation
(
smc::kan) with one of two different spoIIIE mutations, spoIIIE36 (29) and
spoIIIE null (23). spoIIIE36 contains
three missense mutations clustered near a region of SpoIIIE that shows
similarity to Tra proteins (30). The spoIIIE null mutation is a deletion-insertion, with a deletion of codons 86 to 667 (of 787) and an insertion of a spectinomycin resistance cassette
(23). Interestingly, the two spoIIIE mutations
cause different phenotypes with respect to cell-type-specific gene
expression (28).
Competent cells of the spoIIIE36 (PL656) and
spoIIIE (PL422) mutants were transformed with chromosomal
DNA from a
smc::kan strain (RB35)
and plated at 24°C on a glucose-supplemented defined minimal medium
(S750) and on Luria-Bertani (LB) medium. We were unable to
isolate stable spoIIIE36
smc transformants on either type
of medium, suggesting that combining
smc and
spoIIIE36 caused a synthetic lethal phenotype. We were able
to isolate a transformant containing both smc and
spoIIIE null mutations on minimal medium at 24°C, but this
strain was stable only in the presence of spectinomycin (used to select
for
spoIIIE::spc). The
smc
spoIIIE double mutant was extremely sick; colonies did not
become visible until after 4 days at 24°C and were very small.
Culturing of the strain without spectinomycin or at higher temperatures
resulted in poorer growth and the accumulation of suppressors.
A conditional allele of smc.
To test smc
spoIIIE double mutants, we constructed a conditional allele of
smc. smc was placed under the control of the
LacI-repressible, isopropyl-
-D-thiogalactopyranoside
(IPTG)-inducible promoter Pspac. The Pspac-smc
spoIIIE+ strain (RB68) was grown in LB liquid medium
at 37°C in both the presence (induced) and the absence (repressed) of
IPTG (Fig. 1 legend). The phenotype in
the absence of IPTG was similar to though less severe than that of an
smc null mutant: the nucleoid structure was abnormal, and
anucleate cells accumulated (Fig. 2B).
However, in contrast to the
smc::kan
mutant, the Pspac-smc mutant (in the absence of IPTG) was
able to form colonies at 37°C on LB medium. Thus, there appears to be
low-level expression of smc in the Pspac-smc mutant in the absence of IPTG.

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FIG. 1.
Growth of Pspac-smc strains. Cells were grown
overnight on solid LB medium with 1 mM IPTG and then inoculated into
prewarmed (37°C) LB medium containing 5 µg of chloramphenicol/ml
with (+) (A and C) or without ( ) (B and D) 1 mM IPTG. Cultures were
diluted once during the experiments (note the drops in optical density
at around 2 h) to keep the cells in the exponential phase of
growth. It takes several generations of growth in the absence of IPTG
to express the Smc mutant phenotype. (A and B) Effects of depleting SMC
from spoIIIE mutant cells. Symbols: Pspac-smc
spoIIIE+ (RB68), squares; Pspac-srb
spoIIIE (RB91), triangles; Pspac-smc spoIIIE36
(RB69), diamonds; Pspac-smc spoIIIE (RB82), circles. (C
and D) Effect of spo0J on the Pspac-smc
spoIIIE mutants. Symbols: Pspac-smc
spoIIIE+ (RB68), squares; Pspac-smc
spo0J (RB74), diamonds; Pspac-smc spo0J spoIIIE36
(RB75), circles. Note that the scales on the x axes in the
top and bottom panels are different. To place the smc gene
under the control of the Pspac promoter, a PCR fragment including the
ribosome binding site and the 5' end of smc was generated
and directionally cloned into the HindIII and
SphI sites of pAG58 (primer sequences are available on
request). The resulting plasmid, pRB21, was integrated into the
chromosome by a single crossover. pRB21 (Pspac-smc) was
transformed into AG174 (wild type) to generate strain RB68
(Pspac-smc), into PL656 (spoIIIE36) to generate
RB69 (Pspac-smc spoIIIE36), and into strain PL422
( spoIIIE) to generate strain RB82 (Pspac-smc
spoIIIE). RB74 (Pspac-smc spo0J) was constructed
by integrating pRB21 by a single crossover into strain AG1468
( spo0J). RB75 (Pspac-smc spo0J spoIIIE36)
was constructed by integrating pRB21 by a single crossover into strain
RB1 ( spo0J spoIIIE36). The srb gene was placed
under the control of the Pspac promoter by PCR amplifying the ribosome
binding site and 5' end of srb and cloning the fragment into
pAG58. This plasmid was integrated into the chromosome by a single
crossover into strain PL422 to generate RB91 (Pspac-srb
spoIIIE).
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FIG. 2.
Chromosome partitioning phenotypes of
Pspac-smc strains. Cells were grown as described in the
legend to Fig. 1 and in the text. Cells were fixed with methanol and
visualized by a combination of fluorescence and Nomarski microscopy as
described elsewhere (1). The DNA was stained with the
DNA-specific dye 4',6-diamidino-2-phenylindole (DAPI). (A) RB82
(Pspac-smc spoIIIE) grown in the presence of IPTG; (B)
Pspac-smc spoIIIE+ (RB68) grown without IPTG for
six generations; (C) Pspac-smc spoIIIE+ (RB68)
grown without IPTG for nine generations; (D) Pspac-smc
spoIIIE36 (RB69) grown without IPTG (cells taken approximately one
generation before the cessation of growth); (E) Pspac-smc
spoIIIE (RB82) grown without IPTG (cells taken approximately
one generation before the cessation of growth); (F to I) Examples of
the CUT phenotype: combination Nomarski and fluorescence microscopy of
smc spoIIIE double-mutant cells (F and H), and Nomarski
images only of the same cells (G and I). Arrows indicate examples of a
nucleoid bisected by a septum.
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Because
smc is the second gene of a three-gene operon, we
also depleted Srb, which is encoded by the gene immediately downstream
of
smc, and found that Srb depletion was not involved in the
phenotypes
associated with
smc depletion (data not shown). A
previous study
investigating the effects of depleting SMC and Srb
obtained similar
results (
20).
Depletion of SMC from cells containing spoIIIE
mutations.
We constructed Pspac-smc spoIIIE double
mutants and characterized the phenotypes caused by depletion of SMC
following removal of IPTG. When strains harboring Pspac-smc
and either spoIIIE36 (RB69) or the spoIIIE null
mutation (RB82) were depleted of SMC (following removal of IPTG),
growth slowed and eventually ceased (Fig. 1B). When depleted of SMC,
the two spoIIIE mutants behaved somewhat differently; the
spoIIIE36 mutant had a more severe phenotype than the
spoIIIE null mutant. For approximately four generations after removal of IPTG, the Pspac-smc spoIIIE36 strain (RB69)
grew similarly to the Pspac-smc spoIIIE+ strain
(RB68). After this time, growth of the double mutant slowed and then
ceased after approximately six or seven generations (Fig. 1B). The
Pspac-smc
spoIIIE strain (RB82) had a longer period of
slow growth before all growth ceased at approximately 9 to 11 generations after removal of the IPTG. A similar stoppage of growth was
not observed when SMC was depleted from spo0J mutant cells
(see below) or when Srb was depleted in either of the
spoIIIE mutant backgrounds (Fig. 1B and data not shown).
Also, no difference in growth was observed when these strains were
grown in the presence of IPTG (smc+) (Fig. 1A),
indicating that neither spoIIIE mutation significantly affects growth. These results demonstrate that spoIIIE is
required for growth of smc mutant cells.
We measured the viability (plating efficiency) of the Pspac-
smc
spoIIIE mutant cells after growth in the absence of IPTG.
Cells
were grown in LB medium, and samples were taken at the time
growth
ceased (approximately six and nine generations after removal
of IPTG
for Pspac-
smc spoIIIE36 and Pspac-
smc
spoIIIE,
respectively).
Samples were plated under permissive conditions (LB
medium in
the presence of 1 mM IPTG). Depleting SMC in an otherwise
wild-type
background (RB68) resulted in ~50% of the cells still
being viable
compared to strains grown in the presence of IPTG.
Pspac-
smc spoIIIE36 (RB69) and Pspac-
smc
spoIIIE (RB82) exhibited ~97 and ~85% loss
of viability,
respectively. Neither
spoIIIE mutation alone had
any
detectable effect on cell viability. The difference in the
viabilities
of Pspac-
smc spoIIIE36 (RB69) and Pspac-
smc
spoIIIE (RB82) correlates with the growth phenotypes (Fig.
1B).
smc spoIIIE double mutants have a CUT phenotype.
We analyzed the Pspac-smc spoIIIE double mutants for
chromosome partitioning defects. Samples were taken for analysis
approximately one generation before cessation of growth (approximately
five and eight generations without IPTG for Pspac-smc
spoIIIE36 [RB69] and Pspac-smc
spoIIIE [RB82],
respectively). Pspac-smc spoIIIE+ (RB68) was
grown for six and nine generations without IPTG for comparison. In all
three cases, cells displayed a typical smc phenotype (Fig.
2). Nucleoids had a decondensed appearance, and anucleate cells were
present in all strains. The frequency of anucleate cells did not differ
significantly between the three strains and was ~10%.
In addition to anucleate cells, the Pspac-
smc spoIIIE
mutants had many cells with a chromosome bisected by a division septum
(Fig.
2F to I). This phenotype is similar to that of the CUT mutants
of
Schizosaccharomyces pombe (interestingly, two of these
S. pombe CUT mutants turned out to have mutations affecting
smc genes)
(
24).
Examples of CUT chromosomes are shown in Fig.
2F to I. In the
Pspac-
smc single mutant, ~10% of septa were observed to
bisect
a chromosome (Table
1). In
contrast, in the Pspac-
smc spoIIIE36 double mutant, ~38%
of septa bisected a chromosome after five
generations in the absence of
IPTG (Table
1). Similarly, after
eight generations without
smc expression, the Pspac-
smc
spoIIIE double
mutant (RB82) had an increase in CUT chromosomes over the
number
exhibited by the Pspac-
smc spoIIIE+ single
mutant (RB68) (~30% versus ~20%), but the overall difference
was
less pronounced. These results are consistent with the
spoIIIE36 mutation causing a more severe phenotype than the
spoIIIE mutation
in combination with
Pspac-
smc.
We suspect that the CUT phenotype might be contributing to the loss of
viability of the Pspac-
smc spoIIIE double mutants.
Presumably, in the
spoIIIE+ cells, some of these
CUT chromosomes are resolved by the action
of SpoIIIE pumping the
chromosome through the septum, whereas
in the
spoIIIE
mutants a CUT chromosome is likely to be a terminal
event. We have not
detected induction of the SOS response, as
measured by induction of a
dinC-lacZ fusion (
2), in the Pspac-
smc spoIIIE double mutants (data not shown). The SOS response has
been
detected in
ftsK mutants of
E. coli
(
17); the C-terminal
domain of FtsK is similar to that of
SpoIIIE.
Why does
spoIIIE36 cause a more severe defect? The major
distinction between the two mutations is that
spoIIIE36
encodes a
protein that correctly localizes to the septum
(
29) whereas
the null mutation likely results in a complete
lack of protein
(
23). One possibility is that when a
chromosome is bisected
by a septum, the chromosome is then grabbed by
SpoIIIE and pumped
through the septum. We suspect that the defective
spoIIIE36 gene
product still makes contact with the
chromosome but cannot complete
translocation, in effect holding the
chromosome in place. Any
chromosome caught in the way of the septum in
spoIIIE36 cells
would be trapped, and a full complement of
the genome would not
be received by the daughter cells. In a
spoIIIE null mutant, the
chromosome would not be held in
place and might be able to move
out of the way of the invaginating
septum, causing an increase
in the number of viable cells compared to
spoIIIE36 mutants. However,
the outcome is eventually the
same: in the absence of SMC, the
spoIIIE mutant cells do
not survive. The loss of viability is
likely due to a combination of
the CUT phenotype and other, uncharacterized
effects on the
chromosome.
Exacerbation of the smc spoIIIE mutant phenotype by a
spo0J null mutation.
A
spo0J
smc
double mutation gives rise to more anucleate cells and causes a greater
disruption of nucleoid structure than the single
smc
mutation (1). In addition, the double mutation results in
synthetic lethality on LB medium. We hypothesized that the more severe
the segregation defect, the more important the backup partitioning
function of SpoIIIE becomes. Therefore, we depleted SMC from a
spo0J spoIIIE36 mutant.
Depleting SMC from cells harboring both
spo0J and
spoIIIE36 mutations resulted in cessation of growth earlier
than for cells
with
spoIIIE36 alone. Strains containing
Pspac-
smc spo0J+ spoIIIE+ (RB68),
Pspac-
smc
spo0J spoIIIE+ (RB74), or
Pspac-
smc
spo0J spoIIIE36 (RB75) were grown in LB
medium
at 37°C with (Fig.
1C) or without (Fig.
1D) IPTG. Although
the
smc
spo0J double mutant is not viable on LB medium,
depletion
of SMC from
spo0J cells (RB74) did not result in
cessation of
growth, even after 10 generations (~20% of the cells
were anucleate,
versus ~10% for RB68, consistent with previous
results [
1]).
In contrast, the triple-mutant
(Pspac-
smc
spo0J spoIIIE36) strain
ceased growth
approximately four or five generations after removal
of IPTG, a full
two generations before the Pspac-
smc spoIIIE36 (RB69)
mutant. The
spo0J spoIIIE36 double mutant had no growth
defect.
Summary.
We have demonstrated that SpoIIIE, a putative DNA
translocase capable of pumping DNA through a septum, is required for
the viability of smc mutant cells. Our results suggest that
in smc mutants the bulk of the chromosome is not being
properly partitioned out of the way of the invaginating septum and that
the SpoIIIE protein provides a critical backup partitioning mechanism
to pump DNA through the septum when normal partitioning is disrupted. These results support and extend earlier work demonstrating a postseptational partitioning role for SpoIIIE during vegetative growth
(25). Our results suggest that nucleoid structure is extremely important, if not essential, for proper nucleoid partitioning and that the postseptational partitioning provided by SpoIIIE helps to
resolve defects. Clearly, spoIIIE does not substitute completely for the lack of smc function, but it can help the
cell overcome the partitioning defect in enough cases to yield viable cells.
Mutations in
B. subtilis smc and
E. coli mukB
cause strikingly similar phenotypes (
1,
20,
21).
Interestingly, it was
recently reported that an
E. coli mukB
and
ftsK double mutant,
which has a postseptational
partitioning function, could not be
isolated (
31). This
further suggests that SMC and MukB play
similar roles in nucleoid
structure and chromosome partitioning.
We suspect that the disruption
in nucleoid structure causes the
partitioning defect, although a more
direct role for SMC in chromosome
partitioning is
possible.
 |
ACKNOWLEDGMENTS |
We thank members of our lab for useful discussions and Katherine
Lemon and Petra Levin for comments on the manuscript.
R.A.B. was supported, in part, by postdoctoral fellowship GM19302 from
NIH. This work was also supported in part by Public Health Service
grant GM41934 from NIH to A.D.G.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Biology, Building 68-530, Massachusetts Institute of Technology,
Cambridge, MA 02139. Phone: (617) 253-1515. Fax: (617) 253-2643. E-mail: adg{at}mit.edu.
 |
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Journal of Bacteriology, September 1999, p. 5860-5864, Vol. 181, No. 18
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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