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Journal of Bacteriology, August 2005, p. 5486-5492, Vol. 187, No. 15
0021-9193/05/$08.00+0 doi:10.1128/JB.187.15.5486-5492.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Global Regulation of Gene Expression by ArlRS, a Two-Component Signal Transduction Regulatory System of Staphylococcus aureus
Xudong Liang,1
Li Zheng,1
Christina Landwehr,1
Dwayne Lunsford,2
David Holmes,2 and
Yinduo Ji1*
Department of Veterinary and Biomedical Sciences, College of Veterinary Medicine, University of Minnesota, 1971 Commonwealth Ave., St. Paul, Minnesota 55108,1
Anti-Infective Research, GlaxoSmithKline Research and Development, 1250 S. Collegeville Rd., Collegeville, Pennsylvania 194262
Received 3 March 2005/
Accepted 26 April 2005

ABSTRACT
Staphylococcus aureus expresses various cell wall-associated
and extracellular virulence factors, coordinately controlled
by different two-component signal transduction systems and transcriptional
regulators. In this study, we used microarray technology to
identify the genes regulated by ArlR. The microarray data indicate
that ArlR functions as a positive regulator and also as a negative
repressor to directly and/or indirectly mediate the expression
of at least 114 genes involved in different functions, including
autolysis, cell division, growth, and pathogenesis.

TEXT
Staphylococcus aureus is an important human and animal pathogen
that causes a wide range of infections, including life-threatening
endocarditis and toxic shock syndrome (
22,
26). The ability
of this organism to cause a variety of diseases is partly due
to the expression of different cell wall-associated and secreted
virulence factors which enable the bacteria to adhere to and
colonize host cells (
13) or cause toxic shock syndrome (
26).
The expression of virulence factors is coordinately controlled
by two-component signal transduction systems, such as
agr (
1),
sae (
16),
arl (
14), and
srrAB (
30), and global regulators, including
sar (
8,
9),
sigB (
3,
31),
rot (
2,
24), and
mgr (
23). Therefore,
the elucidation of the regulons of these regulatory systems
is important for better understanding molecular mechanisms of
pathogenesis. Recently,
S. aureus regulons of
agr,
sar,
sigB,
and
rot have been revealed by using a microarray-based approach
(
4,
11,
27). In our studies, we identified target genes controlled
by ArlR by a comparison of the transcriptional profile between
an
arlR mutant and the wild-type strain during the mid-exponential
phase of growth by using Affymetrix
S. aureus arrays.
Construction of the arlR deletion mutant.
The arlR deletion mutant (Sa316ko) was constructed by bacteriophage
11-mediated transduction of a cassette containing the tetA gene, flanked by chromosomal fragments upstream and downstream of the alrR, from strain RN4220 into a clinical human isolate strain WCUH29 as described previously (12). Selection for tetracycline resistance and screening for the loss of the erythromycin resistance marker carried by the vector indicated that allelic replacement had occurred and resulted in the arlR mutant strain Sa316ko. The mutation in arlR was verified by PCR and Southern blot analysis (data not shown).
To characterize the arlR mutant strain, we examined the effect of the arlR mutation on the bacterial growth rate, CFU, phenotype, and stress responses to different antibacterial agents, including cell wall synthesis inhibitors such as bacitracin, phosphomycin, and vancomycin. No significant effect of the arlR mutation on stress response to antibacterial agents was observed (data not shown). However, Sa316ko grew slightly slower than WCUH29 between early log and stationary phases (Fig. 1). Growth curves as measured by optical density (OD) were confirmed by determining the effect of arlR mutation on colony size and viable cell counts at intervals during growth. The ArlR mutant strain, Sa316ko, displayed similar-sized colonies on a tryptic soy agar plate and similar morphological features of individual cells under microscopic observation (data not shown) but decreased approximately one log CFU compared to wild-type strain between early log and stationary phases (data not shown). These results suggest that the arlRS regulon may be involved in the modulation of expression of genes associated with growth and cell division. Although it is unlikely that the arlR allelic gene replacement mutation has a polar effect on arlRS downstream gene expression (since arlR and arlS loci are located in a single operon), we examined the transcription level of arlRS downstream gene odhA, encoding 2-oxoglutarate dehydrogenase E1. Our real-time reverse transcriptase (RT)-PCR analyses showed that the arlR mutation had no significant influence on the expression of odhA (see Table 5).
Identification of the arlRS regulon using microarray assay.
To better define the
arlRS regulon, gene expression profiles
of the
arlR mutant and parent cells were analyzed by using Affymetrix
S. aureus arrays as described previously (
11). The
S. aureus array (Affymetrix) contains probe sets to over 3,300
S. aureus open reading frames (ORFs) based on the updated
S. aureus genomic
sequences of N315, Mu50, NCTC 8325, and COL. Total RNA was extracted
from
S. aureus cells grown to mid-log phase (OD at 600 nm, 0.4)
by using the RNAPrep kit (Promega, MI) and treated with a DNA-free
kit (Ambion). The RNA (10 µg) was reverse transcribed
to cDNA by using Superscript II reverse transcriptase and random
primers (Invitrogen). The cDNA was treated with NaOH, purified
by using the QIAquick PCR purification kit (QIAGEN), and digested
with DNase I. The fragmented cDNAs were then directly labeled
with biotin by using a biotin-ddUTP kit (Affymetrix). Biotinylated
cDNA (3 µg) was hybridized to the GeneChips. The GeneChips
were then washed and subjected to a series of staining procedures
as described in the manual for the Affymetrix array. Each GeneChip
was washed and scanned at a 570-nm wavelength and a 3-µm
resolution in an Affymetrix GeneChip scanner. The Affymetrix
Microarray Suite 4.0 algorithms calculated the signal intensities
(average differences) and the present or absent determinations
for each open reading frame. The GeneChips were then normalized,
and their backgrounds were defined by using GeneSpring 4.0 (Silicon
Genetics). The GeneSpring software was used to further analyze
the transcription patterns of genes. To identify genes with
significantly altered expression levels, a series of statistical
analyses (filtering) were performed; cutoff values for ratio
of expression levels of 1.80 and 0.55 were used to filter genes
with expression level changes (
n-fold) greater than ±1.8
in all three independent biological samples. Genes with variations
(
n-fold) of >1.5 across the three samples were excluded.
Furthermore, a statistical group comparison using the Student
t test/analysis of variance was conducted to compare the mean
expression levels of the control and the
arlR mutant samples.
The genes with significant differential expression levels (
P value, <0.05) were selected.
The results of three independent experiments demonstrated that on average, transcripts for 73% of all genes on the arrays were detected by the Affymetrix arrays in the mid-log phase of bacterial cells of WCUH29. A comparison analysis of gene expression levels between wild-type WCUH29 and the arlR mutant revealed that the expression levels of 114 genes were significantly altered in the arlR mutant. Of these, 37 genes showed a decrease (Table 1) and 77 genes showed an increase (Table 2) in expression level after the mutation of arlR.
The array data indicated that ArlR positively regulates a two-component
system,
lytR-lytS, which encodes a response regulator and a
sensor histidine kinase and is involved in autolysis (
6,
14).
The positive regulation of
lytS expression by ArlR was confirmed
by real-time RT-PCR analysis using the Stratagene Mx3000P real-time
PCR system. Gene-specific primers were designed to yield

100
bp of specific products (Table
3), and the housekeeping gene
16S rRNA was used as an endogenous control (
29). All samples
were analyzed in triplicate and normalized against 16S rRNA
gene expression. The results were statistically analyzed for
correlation to the microarray results. Compared to the wild
type, the
arlR mutant possessed low levels of
lytS mRNA in the
early log, mid-log, and stationary phases of growth (Tables
4 and
5). We also found that the mutation in
arlR significantly
down-regulates the expression of
lrgA and
lrgB, encoding different
holin-like proteins involved in murein hydrolase transport and
inhibition of murein hydrolase activity (
7,
17). These enzymes
are involved in the cleavage of specific cell wall components,
which are important for cell division and growth (
7,
17).
To determine whether the growth defect of the mutant is attributable
to increased susceptibility to cell lysis, we examined the effect
of ArlR on autolysis induced by Triton X-100 and detected the
cell wall murein hydrolase activity by using a zymographic assay
as described previously (
19). Consistent with a previous report
(
14), the
arlR mutant cells displayed increased lysis in the
presence of 0.01% Triton and showed enhanced peptidoglycan hydrolase
activity compared to the parent control (unpublished data).
These results indicated that increased autolysis in the
arlR mutant may result from the significant down-regulation of the
lytSR and
lrgAB operons and, in turn, may partially affect bacterial
growth. Although it has been reported that the mutation in
rat,
another autolysis regulator, exhibited a growth defect and enhanced
autolysis partly due to increased murein hydrolase activity
(
19), the reason why the mutation in
arlR led to a slight impact
on growth remains undefined, since the
lrgAB mutation did not
show significant impact on the cell shape and growth rate (
17).
Also, our microarray data showed that ArlR positively regulates virulence factor genes, such as sdrC, sdrD, and sdrE, encoding different Ser-Asp-rich bone sialoprotein-binding proteins (Table 1) (28). The result for sdrD was confirmed by using real-time RT-PCR and demonstrated that the level of sdrD mRNA in the arlR mutant strain is significantly decreased compared with that in the wild-type strain (Table 4). The up-regulation of sdrC by ArlR may indirectly function via up-regulated rot expression, since Rot positively regulates the expression of sdrC (11). In addition, ArlR positively regulates the tcaB gene (which encodes a bicyclomycin-resistant protein), the secretory antigen precursor ssaA, and toxin genes, such as hld and SA0276, encoding delta-hemolysin and diarrheal toxin, respectively (Table 1).
Moreover, the array data showed that ArlR positively regulates the accessory gene regulator (agr) (Table 1). A real-time RT-PCR was employed to validate this result and demonstrated that the mutation in arlR significantly decreased the level of agrA expression (Table 4). The expression of agrA was constantly up-regulated by ArlR at different times of growth (Table 5). These results are inconsistent with the finding that the mutation in arlS leads to the overexpression of Agr (15). This controversy may be due to different sensitivities, culture conditions, and time points of sampling between different assays. In addition, the array results indicated that ArlR also positively regulates the expression of the repressor of toxins (rot). This result was confirmed by real-time RT-PCR and demonstrated that the mutation in arlR causes a decrease in rot expression (Table 4). Our findings are consistent with previous reports that the expression of certain secreted enzymes, toxins, and ureases are repressed by Rot (9, 27). Most virulence factors negatively regulated by ArlR were also repressed by Rot but were up-regulated by Agr (5, 25). Therefore, the down-regulation of these toxins, proteases, and adhesins may be mediated directly by ArlR or indirectly controlled via Rot or Agr (Fig. 2).
To determine the role of the ArlRS regulatory system in pathogenesis,
we examined the effect of the
arlR mutation on virulence by
using a murine hematogenous pyelonephritis model as described
previously (
20). The virulence of the
arlR mutant was significantly
attenuated compared to that of the parent control (unpublished
data). Collectively, these findings suggest that ArlRS is a
global two-component virulence regulatory system which can interact
with other regulators to modulate the expression of virulence
factors.
On the other hand, ArlR functions as a repressor of virulence factors. Our microarray data showed that ArlR negatively regulates some toxin genes, lukD, lukE, phlC (hlb), and hlgC, which encode leukotoxin D, leukotoxin E, beta-hemolysin component C, and gamma-hemolysin component C, respectively (Table 2). Gamma-hemolysin is an S. aureus virulence factor that has been shown to play a role in S. aureus endophthalmitis and corneal pathogenesis (10, 21). The microarray result for lukD was confirmed by real-time RT-PCR. As shown in Table 4, the lukD transcript was present at higher levels in the arlR mutant strain than in the wild-type strain. The array results also demonstrated that ArlR negatively regulates splB (encoding V8 protease), sspC (encoding cysteine protease), and ebhA and ebhB (encoding adhesins) (Table 2). The negative regulation of ebhB expression by ArlR was confirmed by a real-time RT-PCR (Table 4).
Furthermore, the array results showed that ArlR negatively regulates the expression of genes involved in different PTS systems, such as the lacG, lacF, lacD, lacC, lacB, and lacA operons, as well as hypothetical proteins. To validate these results, real-time RT-PCR was performed, and it demonstrated that the mutation of arlR increases the expression of hypothetical proteins (SA0123 and SA0319) and constantly up-regulates lacF expression at different phases of growth (Tables 4 and 5). Also, we found that in the arlR mutant, the levels of kdpD and kdpE transcripts (which encode a two-component sensor kinase and response regulator and involve K+ transport [18]) were obviously higher than those in the wild-type strain. To confirm this result, real-time RT-PCR was performed, and the results demonstrated that the mutation of arlR increases kdpD expression in the mid-log phase of growth but decreases kdpD expression in the early log phase of growth (Tables 4 and 5). These results suggest that ArlR differentially regulates kdpD expression at different times of growth.
Conclusion.
The regulon of ArlRS has been identified by employing transcriptome technology using Affymetrix S. aureus arrays. The results demonstrate that ArlRS is a global transcriptional regulator which directly and/or indirectly interacts with other regulators in regulatory networks and modulates the expression of genes involved in autolysis, cell growth, and pathogenesis (Fig. 2). Some genes/operons mediated by ArlRS may be missed, due to their lack of stability and/or kinetic regulation as well as low detection sensitivity. Further studies to investigate which genes identified using microarray assays are directly regulated by ArlR and are involved in autolysis and/or pathogenesis are in progress.

ACKNOWLEDGMENTS
We thank Junsong Sun for his assistance in the real-time RT-PCR
analysis, Aaron Becker for his assistance in the microarray
analysis, and Doug Weiss for his critical reading of the manuscript
and for his helpful suggestions.
This work was supported by grant O3-O2 from the Academic Health Center at the University of Minnesota. This research was also supported in part by grant AI057451 from the National Institute of Allergy and Infectious Diseases.

FOOTNOTES
* Corresponding author. Mailing address: Department of Veterinary and Biomedical Sciences, College of Veterinary Medicine, University of Minnesota, 1971 Commonwealth Ave., St. Paul, MN 55108. Phone: (612) 624-2757. Fax: (612) 625-5203. E-mail:
jixxx002{at}umn.edu.


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Journal of Bacteriology, August 2005, p. 5486-5492, Vol. 187, No. 15
0021-9193/05/$08.00+0 doi:10.1128/JB.187.15.5486-5492.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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