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Applied and Environmental Microbiology, July 2003, p. 4285-4290, Vol. 69, No. 7
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.7.4285-4290.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Improved Adaptation to Cold-Shock, Stationary-Phase, and Freezing Stresses in Lactobacillus plantarum Overproducing Cold-Shock Proteins
Sylviane Derzelle,
Bernard Hallet, Thierry Ferain,
Jean Delcour, and Pascal Hols*
Institut des Sciences de la Vie/Unité de Génétique, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium
Received 9 December 2002/
Accepted 23 April 2003

ABSTRACT
We have investigated the effect of overproducing each of the
three cold shock proteins (CspL, CspP, and CspC) in the mesophilic
lactic acid bacterium
Lactobacillus plantarum NC8. CspL overproduction
transiently alleviated the reduction in growth rate triggered
by exposing exponentially growing cells to cold shock (8°C),
suggesting that CspL is involved in cold adaptation. The strain
overproducing CspC resumed growth more rapidly when stationary-phase
cultures were diluted into fresh medium, indicating a role in
the adaptation and recovery of nutritionally deprived cells.
Overproduction of CspP led to an enhanced capacity to survive
freezing.

INTRODUCTION
It is well established that after a quick downshift in temperature
(cold shock), a set of proteins are preferentially expressed,
among which a set of small ß-barrel proteins referred
to as the major cold shock proteins (CSPs) show the highest
induction level (
14,
20). Members of the CSP family, which includes
cold-inducible and non-cold-inducible representatives, seem
to play key roles not only in the adaptation to various stresses
such as cold temperature, stationary phase, or nutritional deprivation,
but also during growth under optimal conditions (
15,
16,
26,
32,
38,
40).
Escherichia coli contains nine CSPs (CspA to CspI), of which four (CspA, -B, -G, and -I) are cold shock inducible (31, 38). CspA is a cold shock and nutritional-upshift stress protein (39). CspA, CspC, and CspE are RNA-binding proteins which function as transcriptional antiterminators by preventing the formation of secondary structures in the nascent RNA. Csp-induced transcriptional antitermination is responsible for the increased expression of several genes (4, 27). CspC and CspE, which are constitutively produced at 37°C, likely function as regulators of the expression of some stress response proteins, playing important roles in the global control of carbon flow in cells (26). They also seem to be involved in the acclimation of nutritionally starved cells to fresh medium (3) and chromosome condensation at 37°C (18). CspD is a nutritional-downshift and stationary phase-induced stress response protein which may function as an inhibitor of DNA replication and which plays a regulatory role in chromosomal replication of nutrient-depleted cells (40). CspF and CspH have not yet been characterized. Bacillus subtilis contains three CSPs (CspB, -C, and -D); CspB is essential for cellular growth in a strain lacking CspC and CspD and plays an important role for efficient protein synthesis at optimal and low temperatures (15), while CspB and CspC are major stationary phase-induced proteins (16). CSPs are also found in lactic acid bacteria (22, 23, 34), but their functions are still largely unknown.
Interestingly, it was noted that many bacteria, including lactic acid bacteria, develop an increased ability to survive freezing after cold shock pretreatment (13, 21, 34). In B. subtilis, CspB appeared to be involved in this phenomenon, since cspB knockout reduced cryotolerance (33). In Lactococcus lactis, overexpression of three cold-induced csp genes (cspB, cspD, and cspE) enhanced the survival capacity through successive freeze-thaw cycles, although to a lesser extent than a cold shock pretreatment (35, 36).
Lactobacillus plantarum plays a significant role in a wide range of spontaneous and controlled lactic fermentations in food processing. Nowadays, it is commercially prepared as a starter for the production of dry and semidry sausages, vegetables (sauerkraut, cucumbers, olives, and pickled vegetables), and silage (2, 7, 12, 17, 24). For L. plantarum NC8, we have previously described three cold-inducible csp genes, cspL, cspP, and cspC, encoding highly similar proteins (9, 10). In this paper, we report our investigations on the effects of CspL, CspP, and CspC overproduction on adaptation to cold-shock, stationary-phase, and freezing stresses.

Growth of L. plantarum at various temperatures.
In order to set the optimal and cold-shock temperatures, respectively,
an Arrhenius plot of the growth of
L. plantarum NC8 (
1) was
established (Fig.
1) by expressing the log of the growth rate
(
k) against the reciprocal of the temperature (in kelvins) (
30).
k (expressed in generations per hour) was determined as the
slope of a semilogarithmic plot of the optical density (OD)
at 600 nm versus time. The temperatures over which
L. plantarum can grow were found to split into the following three ranges:
an Arrhenius zone from 13°C (critical temperature,
Tcrit)
to 27°C (optimal temperature,
Topt), within which the activation
energy of growth is constant; a cold shock subrange (below 13°C);
and a heat shock subrange (above 27°C). According to this
Arrhenius plot, a temperature of 27°C corresponds to the
optimal temperature (
Topt) for
L. plantarum. A temperature of
8°C, below the critical temperature (
Tcrit), was chosen
for the cold-shock experiments.

Overproduction of CspL, CspP, and CspC using the nisin controlled expression system.
In order to study the function played by the three CSPs, we
overproduced each of them in turn using the nisin controlled
expression system, initially developed for protein overproduction
in
L. lactis (
8) and recently adapted for
L. plantarum (
25).
All DNA manipulations were performed according to established
procedures (
28). The different
csp genes were PCR amplified
with Dynazyme DNA polymerase (Finnzymes Oy, Espoo, Finland)
in a DNA thermal cycler (Perkin-Elmer, Norwalk, Conn.) with
the following settings: denaturation at 92°C for 1 min,
annealing at 50°C for 1 min, and extension at 72°C for
1 min for a total of 30 cycles. The pairs of oligonucleotides
used were as follows: 5'-CG
GAATTCTTTATAGGTGTTAATAACAT-3' (
EcoRI
site, underlined) and 5'-GCA
CTGCAGATTGGTGGCGCTTACTCG-3' (
PstI
site, underlined) for
cspP, 5'-CC
GAATTCAATAAACTATCCCATTTGTAC-3'
(
EcoRI site, underlined) and 5'-GCCTTCAAGCAAGTCGCAAT-3' for
cspC, and 5'-ACATG
CCATGGAGAATGGTACAGTAAAATGGTTCAA-3' (
NcoI site,
underlined) and 5'-TTACAATGCTAACTAATCCCG-3' for
cspL. The
cspP PCR fragment was digested with
EcoRI-
PstI and cloned in the
intermediate plasmid pMTL23P (
6). An
EcoRI-
SalI fragment from
the resulting plasmid was subsequently cloned between
EcoRI
and
XhoI in pNZ8008 (
8). The resulting plasmid, pGIS410, harbored
a transcriptional fusion between the
nisA promoter and the
cspP gene. The
cspC PCR fragment was digested with
EcoRI-
HindIII
and subsequently cloned at the respective sites of pNZ8008,
leading to pGIS411, which also resulted in a transcriptional
fusion with the
nisA promoter. Previous Northern blot analysis
has revealed that the
cspL promoter drives the transcription
of two cold-inducible transcripts likely to result from terminator
readthrough and/or RNase processing: the shorter one covers
the
cspL open reading frame (ORF), whereas the longer one extends
further downstream into a region containing a putative 77-amino
acid (aa) ORF (named
orfX) (
9). A
cspL PCR fragment covering
both ORFs was digested with
NcoI-
HindIII and directly cloned
in pNZ8032 (
8) to construct an ATG translational fusion (
NcoI)
with the
nisA expression module; the resulting plasmid was named
pGIS412. In order to assess a possible role for
orfX, plasmid
pGIS412 was
BamHI digested, filled in, and self-ligated, resulting
in an inactivation of
orfX (35-aa truncation at the C terminus);
this plasmid was named pGIS414. The positioning of an
NcoI site
at the fusion point mutated the second codon (AAG to GAG) of
the
cspL ORF, resulting in the replacement of lysine with glutamate.
This mutation is unlikely to modify the function of the protein
since glutamate is often found at this position in several CSPs.
The absence of other mutations in the three fusions was confirmed
by DNA sequencing. Plasmids pGIS410, pGIS411, pGIS412, and pGIS414
were next transformed by electroporation (
1) into
L. plantarum SD1, an NC8 derivative obtained by site-specific chromosomal
integration (
11) of the regulatory genes
nisRK necessary for
nisA promoter induction. Overproduction of CSPs was performed
by the addition of 25 ng of nisin ml
-1 to cultures in the early
exponential growth phase (3% inoculum; OD at 600 nm [OD
600]
= 0.15) as described previously (
25). Nisin-induced cells were
collected after 5 h at an OD
600 of 0.8, and total cellular proteins
were extracted from 20-ml cultures by homogenizing cells with
glass beads.
Initially, the three genes, cspL, cspP, and cspC, were transcriptionally fused to the nisA promoter, but only the transcriptional fusions with cspP (pGIS410) and cspC (pGIS411) responded to nisin induction in L. plantarum SD1, each yielding an overproduced band of approximately 7 kDa on Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels (29; data not shown). As an alternative, the cspL ORF in combination with orfX or the cspL ORF alone were fused at ATG to the nisA expression module as described above, and the resulting plasmids, pGIS412 and pGIS414, allowed overproduction of CspL (data not shown). Both constructions displayed a similar protein profile. This suggests that if orfX is coexpressed with cspL in pGIS412, either its expression level is extremely low or both proteins migrate at the same position (1.5 kDa difference in molecular mass).

CspL overproduction transiently alleviates cold shock impairment of growth.
We first monitored the growth in MRS medium (Difco) of mid-exponential-phase
cultures (OD
600 = 0.8) of CSP-overproducing strains shifted
from 27 to 8°C in a precooled water bath (
9). The recipient
strain SD1 containing the basal cloning vector pNZ8020 was used
as a control. Cold shock drastically slowed down the growth
of the control and of the overproducing strains (for a comparison,
see Fig.
2A and Fig.
3A). Interestingly, the growth of the CspL-overproducing
strains (SD1 harboring pGIS412 or pGIS414) was less affected
after cold shock than that of the control, while CspP and CspC
overproduction had no effect on growth during at least 7 h following
cold shock. This observation suggests that preloading with CspL
preadapts
L. plantarum to cold shock and that OrfX does not
contribute to this effect since similar growth curves were obtained
for both CspL-overproducing strains (pGIS414 or pGIS412). This
preadaptation effect was transient, as the growth rate of the
CspL-overproducing strains became similar to that of the control.
In contrast, no preadaptation to growth at a low temperature
was observed for overproduction of CspC or CspP (Fig.
2A). We
noticed that upon prolonged cold shock (over 24 h), the growth
rate of the CspC-overproducing strain (SD1 harboring pGIS411)
was reproducibly higher than that of the other strains (Fig.
2A). This observation suggests that CspC, although not primarily
implicated in cold shock adaptation, may play a positive role
during the acclimation phase of growth at cold temperature.
In order to further characterize the effect of CspL overproduction
upon cold shock adaptation, a nisin dose-response experiment
was performed (Fig.
2B and C). As expected, it was found that
CspL production visualized on a Tricine-SDS-PAGE gel was dose
dependent (Fig.
2C). The amount of CspL present in the overproducing
cells after a cold shock of 4 h at 8°C was similar to those
observed before the temperature downshift (27°C) for 7 and
25 ng of nisin ml
-1. The noninduced culture contained more CspL
after 4 h at 8°C than before the downshift, likely resulting
from the chromosomal
cspL cold shock induction. Higher nisin
concentrations could not be used due to strong growth inhibition
(data not shown). At low nisin doses (0 to 25 ng ml
-1), all
cultures of the CspL-overproducing strain were preadapted to
cold shock in comparison to their respective controls grown
under the same conditions (Fig.
2B). This preadaptation in the
absence of nisin shows that the
nisA promoter displays a basal
level of activity, as previously shown in
L. plantarum (
25),
and that a low amount of CspL is sufficient for this preadaptation
effect.
We previously demonstrated that cspL mRNA was the most cold shock induced of the three csp transcripts (9). This suggests that CspL could be a key protein needed for cellular adaptation to lower temperatures. The major cold-inducible protein CspA of E. coli is an RNA chaperone playing a crucial role in efficient translation of mRNAs at low temperatures (19). It is also a transcriptional antiterminator playing an important role in cold shock adaptation by reprogramming gene expression to induce several other cold-induced proteins (4). We speculate that CspL plays a similar role and that its overproduction directly or indirectly alleviates the negative effects of cold shock on protein synthesis.

CspC overproduction improves growth resumption at 27°C.
Stationary-phase cultures (OD
600 = 6) grown without nisin were
used to inoculate fresh MRS medium prewarmed at 27°C, and
nisin was added to these cultures 1 h later. The growth curves
obtained clearly indicated that the CspC-overproducing strain
(SD1 harboring pGIS411) resumed growth more rapidly than the
control while the growth of the two other overproducing strains
was not significantly improved (Fig.
3A). Analysis of the nisin
dose response on growth at 27°C showed that the CspC-overproducing
strain induced with a nisin concentration range of 0 to 25 ng
ml
-1 displayed a higher growth rate and resumed growth more
rapidly than the respective controls (Fig.
3B). Increasing nisin
concentration resulted in a progressive growth inhibition of
the control strain without affecting the growth rate of the
CspC-overproducing strain. This result suggests that increasing
the CspC amount counteracts the growth inhibition resulting
from nisin addition to the growth medium.
During the lag phase following inoculation of stationary-phase bacteria into fresh medium, many physiological functions must be restored, and an adaptation process must therefore take place. Our results indicate that CspC could play an important role in this adaptation. We previously showed that L. plantarum cells contain a substantial amount of the cspC transcript during early exponential growth at 27°C and that cspC mRNA abundance drastically declines in stationary phase (9). The presence of CspC in exponentially growing cells at 27°C was recently established using two-dimensional gel electrophoresis followed by microsequencing (data not shown). In E. coli, the cspA mRNA and CspA protein amounts are also very high during early exponential growth at 37°C. Furthermore, it has been shown that a double cspA cspE mutant lagged longer than the single cspE mutant after dilution into a fresh medium (3).

CspP overproduction enhances cryotolerance.
The potential role of CSPs in cryotolerance was investigated
by performing freeze-thaw experiments with the three CSP-overproducing
strains and their respective controls (NC8 and SD1 harboring
pNZ8020). The recombinant
L. plantarum SD1 strains were subjected
to the nisin induction procedure and frozen at -80°C in
their growth medium without cryoprotective agents. For each
sample, CFU were measured before freezing by plating serial
dilutions in the same medium, and the colonies were counted
after incubation at 27°C for 48 h. Every 24 h (referred
to here as a cycle), the samples were thawed at room temperature
to allow CFU counting as described above and were immediately
refrozen. The freezing experiments were performed in triplicate
with samples from individual cultures. An increased cryotolerance
was observed for the CspP-overproducing strain (SD1 harboring
pGIS410) after four to six challenges (Fig.
4), while the CspL-
and CspC-overproducing strains did not differ from the control
(data not shown). The implication of CSPs in cryotolerance has
already been reported for other bacteria. For
B. subtilis, knockout
of
cspB led to an increased sensitivity to freezing stress (
33).
In
L. lactis, overproduction of CspB, CspD, and CspE also enhances
cryotolerance (5- to 10-fold), although to a lesser extent than
cold shock preadaptation (
35). How could CspP play a role in
cryoprotection? A direct cryoprotective role for CspP as an
antifreeze protein cannot be rule out. Yet CSPs are thought
to be general RNA-binding proteins involved in the maintenance
of active growth conditions by favoring transcription, translation,
and/or ribosome assembly. In this way, they may indirectly stimulate
the production of critical factors, ensuring cryotolerance through
the maintenance of membrane, DNA, RNA, or protein integrity
upon freezing. A direct cryoprotective effect may also be invoked.
Indeed, CspE of
E. coli is quite abundant at optimal temperature
and seems to be involved in chromosomal condensation (
18), a
function which could be important to help DNA face off freezing.
In addition, as RNA-binding proteins, CSPs may protect some
mRNAs from degradation by direct binding (
26).

Concluding remarks.
Our studies support the claim that different members of the
CSP family perform specific functions in adapting
L. plantarum to specific environmental stresses. In
B. subtilis, CspB is
important for cellular growth at an optimal temperature and
for adaptation to cold shock and freezing and survival during
stationary phase (
15,
16,
33). Its absence in knockout mutants
is compensated for by CspC at low temperature and during stationary
phase and by CspD at 37°C (
15,
16). In
E. coli, CspA is
important for cold shock adaptation and growth resumption after
dilution at optimal temperature. Its absence in knockout mutants
is compensated for at low temperature by CspB, CspG, and CspI
and after the dilution effect by CspE. No growth defect was
observed until four
csp genes (
cspABEG) were deleted (
37). As
for CspD, it is important during nutritional deprivation (
3,
5,
31,
38,
40). In contrast to these functional redundancies,
we show that overproduction of each CSP causes distinct phenotypic
effects in
L. plantarum. It is attractive to speculate that
the different CSP proteins may antiterminate transcription of
different sets of genes, thus allowing the cell to adequately
respond to different environmental challenges. However, we cannot
exclude the idea that the overproduction of a single CSP could
modulate a regulation network in a complex manner involving
more than one CSP such as that observed through the overproduction
of CSPs in
L. lactis (
35).
Although the new properties of the recombinant strains have not been validated in industrial processes, an increased cryotolerance could be relevant for improving starter production, and a faster growth rate during the first hours following cold shock or inoculation would allow for the development of cultures showing improved performance in fermentations carried out at cold or optimal temperature. The observations reported in this study could be the starting point for the construction of food-grade recombinant starter cultures overproducing CSPs.

ACKNOWLEDGMENTS
We thank M. Kleerebezem for providing plasmids pNZ8020, pNZ8008,
and pNZ8032.
S.D. holds a fellowship of the "Fonds pour la Formation à la Recherche dans l'Industrie et dans l'Agriculture" (FRIA). B.H. is a research associate at the FNRS.

FOOTNOTES
* Corresponding author. Mailing address: Unité de Génétique, Université catholique de Louvain, 5 Place Croix du Sud, B-1348 Louvain-la-Neuve, Belgium. Phone: 32-10-478896. Fax: 32-10-473109. E-mail:
hols{at}gene.ucl.ac.be.

Present address: Génétique des Génomes Bactériens, Institut Pasteur, F-75724 Paris cedex 15, France. 
Present address: Euroscreen S.A., B-1070 Brussels, Belgium. 

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Applied and Environmental Microbiology, July 2003, p. 4285-4290, Vol. 69, No. 7
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.7.4285-4290.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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