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In most bacteria, nutrient limitations provoke the stringent control through the rapid synthesis of the alarmones pppGpp and ppGpp. Little is known about the stringent control in the human pathogen Staphylococcus aureus, partly due to the essentiality of the major (p)ppGpp synthase/hydrolase enzyme RSH (RelA/SpoT homolog). Here, we show that mutants defective only in the synthase domain of RSH (rshsyn) are not impaired in growth under nutrient-rich conditions. However, these mutants were more sensitive toward mupirocin and were impaired in survival when essential amino acids were depleted from the medium. RSH is the major enzyme responsible for (p)ppGpp synthesis in response to amino acid deprivation (lack of Leu/Val) or mupirocin treatment. Transcriptional analysis showed that the RSH-dependent stringent control in S. aureus is characterized by repression of genes whose products are predicted to be involved in the translation machinery and by upregulation of genes coding for enzymes involved in amino acid metabolism and transport which are controlled by the repressor CodY. Amino acid starvation also provoked stabilization of the RNAs coding for major virulence regulators, such as SaeRS and SarA, independently of RSH. In an animal model, the rshsyn mutant was shown to be less virulent than the wild type. Virulence could be restored by the introduction of a codY mutation into the rshsyn mutant. These results indicate that stringent conditions are present during infection and that RSH-dependent derepression of CodY-regulated genes is essential for virulence in S. aureus.
Staphylococcus aureus causes a variety of infections in humans but also asymptomatically colonizes the nose of healthy individuals. Gene expression must be closely coordinated to allow the pathogen to survive and/or multiply in different compartments during infection and colonization. However, knowledge about growth conditions in vivo is still limited, and the interaction of the regulatory circuits leading to metabolic adaptation and to differential expression of virulence factors is not yet understood completely.
The stringent control is one of the first known and most intensively studied global systems of gene regulation in bacteria (for previous reviews, see references 6, 8, 10, 18, 19, 25, 35, 46, 48, 52, 56, and 57). It is provoked by the rapid synthesis of the alarmones pppGpp and ppGpp upon nutrient limitation. Alarmone synthesis is linked to many physiological processes involving rRNA degradation, gene activation/repression, protein translation, enzyme activation, and replication. In many pathogenic bacteria, virulence, persistence, and host interaction are also influenced through (p)ppGpp (18). (p)ppGpp is synthesized by cytoplasmic enzymes containing a conserved synthase domain first described in RelA and SpoT enzymes from proteobacteria. In most firmicutes, bifunctional Rel/SpoT homologs (RSHs), which constitute a distinct class of (p)ppGpp synthases (40, 56), are present. They are composed of a C-terminal regulatory and an N-terminal enzymatic domain. Structural data indicate two conformations of the enzyme corresponding to the known reciprocal activity states, (p)ppGpp-hydrolase-OFF/(p)ppGpp-synthase-ON and hydrolase-ON/synthase-OFF (23). The C-terminal domain is probably involved in the reciprocal regulation of the two catalytic activities, since truncation of the C-terminal domain enhances the synthase activity of the enzyme (39). Recently, genes coding for additional small (p)ppGpp synthases were identified in most of the firmicutes (41).
Overall, there is now growing evidence that there are fundamental differences in (p)ppGpp synthesis, regulation, and molecular function between firmicutes and proteobacteria (56). For instance, in Escherichia coli, binding of (p)ppGpp to the RNA polymerase in concert with the cofactor protein DksA results in the inhibition of the rrn promoters. In firmicutes, DksA is not present and (p)ppGpp does not appear to interact with RNA polymerase (RNAP) directly (27). (p)ppGpp synthesis reduces intracellular GTP levels, and the GTP level in turn seems to directly influence promoter activity in Bacillus subtilis. It was proposed that the nature of the nucleoside triphosphate (NTP) initiating transcription determines whether genes are under positive or negative stringent control (27, 28, 42, 50). For instance, some rrn promoters of B. subtilis initiate with GTP, and a change in the identity of the base at position +1 results in a loss of regulation by (p)ppGpp and GTP.
The GTP level is also crucial for the repressive function of the metabolic regulator CodY, at least in some firmicutes (reviewed in reference 47). In B. subtilis, the repressor function of CodY is activated by GTP, which enhances the affinity of CodY to a conserved binding motif in the promoter region of target genes (21). Thus, there is a direct link between stringent control and CodY-mediated gene regulation: lowering of the GTP pool through the induction of stringent controls leads to derepression of CodY target genes (4, 24). However, GTP is an active ligand for CodY in only some of the firmicutes. For instance, CodY from streptococci or lactococci appears not to interact with GTP. Here, branched-chain amino acids are bound by CodY and constitute the primary signals for CodY-mediated gene repression (12, 20, 22, 33).
In S. aureus, CodY is a potent repressor of genes mainly involved in nitrogen metabolism and transport. Virulence factors are also regulated by CodY, mainly through CodY-dependent repression of the agr system (37, 45). There is strong evidence that isoleucine is the major ligand of CodY in S. aureus, since CodY target genes were derepressed in medium lacking isoleucine (45).
Little information is available on stringent control in staphylococci. Interestingly, it could be shown that the RSH homolog is essential for survival in S. aureus (16). Thus, the lack of viable rsh mutants has hampered an in-depth analysis of the stringent response. However, staphylococci mount a stringent response that is characterized by the generation of (p)ppGpp when treated with mupirocin to mimic isoleucine starvation (9). Mupirocin is a potent antimicrobial agent which inhibits the bacterial isoleucyl tRNA synthetase. RSH contains a typical C-terminal domain which may sense the accumulation of uncharged tRNA after amino acid deprivation, leading to a shift toward the synthase-ON/hydrolase-OFF conformation of the bifunctional enzyme (23). Microarray analysis revealed that in S. aureus, mupirocin induces transcriptional changes which are similar to those observed in other bacteria, e.g., the upregulation of several classes of gene products, including transport proteins, virulence factors, regulatory molecules, and peptidases (2). However, it is not clear whether all of these changes are due to (p)ppGpp synthesis.
Here, we characterize the stringent control elicited by the (p)ppGpp synthase RSH of S. aureus, which is responsible for (p)ppGpp synthesis upon amino acid deprivation. We show that RSH synthase activity is required for derepression of genes of the CodY regulon and for the repression of genes involved in the translation machinery after amino acid limitation. A mutant defective in RSH synthase activity was less virulent in a murine model of kidney infection than the wild type (WT) or the codY rsh double mutant.
Strains and plasmids are listed in Table Table1.1. S. aureus strains were grown in CYPG medium (10 g/liter Casamino acids, 10 g/liter yeast extract, 5 g/liter NaCl, 0.5% glucose, and 0.06 M phosphoglycerate) (43), in B medium (7), or in a chemically defined medium (CDM) (45). For strains carrying resistance genes, antibiotics were used only in precultures at the following concentrations: erythromycin, 10 μg/ml, and tetracycline, 5 μg/ml. Bacteria from an overnight culture were diluted to an initial optical density at 600 nm (OD600) of 0.05 in fresh medium and grown with shaking (220 rpm) at 37°C to the desired growth phase. For downshift experiments, strains were grown in complete CDM, including leucine/valine (Leu/Val), to an OD600 of 0.5. The cultures were filtered over a 0.22-μm filter with vacuum and washed twice with sterile phosphate-buffered saline (PBS), and bacteria were resuspended in an equal volume of CDM medium with or without Leu/Val and grown for another 30 min.
Bacteria were grown in Mueller-Hinton broth to the exponential growth phase (OD600 of 0.5, corresponding to 5 × 108 bacteria/ml). Ten microliters of a 1:100 dilution of the culture were mixed with 80 μl of Mueller-Hinton broth and 10 μl of mupirocin (0.001 to 10 μg/ml) in microtiter plates. Plates were incubated at 37°C for 24 h and 48 h. The MIC was defined as the lowest concentration where no turbidity could be monitored. Bactericidal activity was determined by plating the dilutions at which no turbidity was detected. The minimal bactericidal concentration (MBC) was defined as the minimal concentration at which no colonies could be recovered. MICs toward vancomycin and ciprofloxacin were determined using an Etest as described by the manufacturer (AB-Biodisk, Solna, Sweden).
A conditional rsh mutant was generated using the pMUTIN4 vector (51). With this system, conditional mutants can be obtained by integrating the vector upstream of the target gene, which falls under the control of the isopropyl-β-d-thiogalactopyranoside (IPTG)-inducible promoter Pspac carried by pMUTIN4. For this purpose, a 942-bp internal fragment of the rsh gene containing the native ribosomal binding site was amplified from strain Newman by employing the primers listed in Table Table2.2. The amplicon was cloned into the EcoRI/BamHI restriction sites of pMUTIN4 to yield pCG55. The vector was transferred into the restriction-deficient strain RN4220 under IPTG induction. Since the vector cannot replicate in Gram-positive bacteria, pMUTIN4 inserts into the chromosome via a single crossover event by erythromycin selection. The conditional rsh mutant strain (RN4220-55) obtained was verified by PCR and pulsed-field gel electrophoresis (PFGE). The mutation was transduced into S. aureus strain Newman or HG001 using Φ11 lysates of strain RN4220-55 (Table (Table1).1). Transductants were verified by PCR and PFGE.
The markerless rshsyn mutant was obtained using the pKOR1 system (3). A deletion in the synthase domain of rsh was introduced by overlapping PCR employing the primers listed in Table Table2.2. The amplicon obtained was moved into pKOR1 using the Gateway cloning system (Invitrogen). The resulting plasmid (pCG86) was verified and transformed into RN4220, from which it was transduced into the different S. aureus strains. Mutagenesis was performed as described previously (3). Mutation of the synthase domain of RSH in strains HG001 and Newman was verified by sequence analysis, and the mutants were designated HG001-86 and Newman-86, respectively (Table (Table11).
For complementation, the rsh gene along with a 960-bp upstream region was amplified with the oligonucleotides listed in Table Table22 and cloned into the pCR2.1 cloning vector (Invitrogen). The insert was subcloned into the EcoRI site of the integration vector pCL84, yielding plasmid pCG199. The plasmid was used to transform strain CYL316, from which it was transduced into the rshsyn mutant strains (Table (Table11).
RNA isolation and Northern blot analysis were performed as described previously (20). Briefly, bacteria were lysed in 1 ml of Trizol reagent (Invitrogen Life Technologies, Karlsruhe, Germany) with 0.5 ml of zirconia-silica beads (0.1-mm diameter) in a high-speed homogenizer (Savant Instruments, Farmingdale, NY). RNA was isolated as described in the instructions provided by the manufacturer of Trizol. Digoxigenin (DIG)-labeled probes for the detection of specific transcripts were generated using a DIG-labeling PCR kit following the manufacturer's instructions (Roche Biochemicals). Oligonucleotides used for probe generation were as described previously (36, 45) or are listed in Table Table22.
For determination of transcript stabilities, strains were grown and shifted in CDM with and without Leu/Val as described above. Rifampin (500 μg/ml) was added to the cultures for the times indicated in Fig. Fig.8B.8B. At the different time points, RNAs were isolated and Northern blot analyses were performed as described above.
S. aureus wild-type strain HG001 and its isogenic mutants were grown in CDM to an OD600 of 0.5. Cells were shifted to CDM containing 0.5 μg/ml mupirocin or to CDM without Leu/Val as described above. Samples for intracellular metabolite analysis were harvested directly before and 30 min after shift by fast filtration over a 0.22-μm sterile filter with vacuum. Cells were washed and quenched, and metabolites were extracted as described recently (13, 39a). Detection of nucleotides was performed by ion-pairing liquid chromatography-mass spectrometry (IP-LC-MS) (13). Quantification of compounds was done by referring to the internal standard Br-ATP in all samples. Identification and calibration curves for all metabolites were achieved by measuring pure standard compounds, with the exception of pppGpp, for which no chemical standard was commercially available. For this, the precise mass of pppGpp was calculated and compared with the mass found ([M − H+]calculated = 681.911 versus [M − H+]detected = 681.910). Quantification of pppGpp was done by adopting the calibration values from ppGpp, due to the analogous structure. The results of the intracellular metabolite measurements represent the means of 4 biological replicates and were normalized to the cellular dry weight.
The cells were grown in a modified low-phosphate CDM (0.4 mM phosphate, buffered at a pH of 7.2 with 40 mM MOPS [morpholinepropanesulfonic acid]) to an OD600 of 0.5, at which point [32P]H3PO4 (50 μCi/ml) was added to the cultures. After 3 h of growth, the cells were shifted into CDM containing 0.5 μg/ml mupirocin or into CDM with and without Leu/Val for an additional 30 min. For the shift experiments, the bacteria were filtered using a 0.22-μm sterile filter with vacuum, washed twice with sterile 1% PBS, and resuspended in the appropriate medium. For the extraction of nucleotides, 200 μl ice-cold 2 M formic acid was added per ml of culture, followed by incubation for 30 min on ice. The bacteria were lysed with 0.5 ml of zirconia-silica beads (0.1-mm diameter) in a high-speed homogenizer, and the extracts were centrifuged at 12,000 × g for 5 min at 4°C. An equal volume of phenol-chloroform-isoamyl alcohol (50:49:1 [vol/vol/vol]) saturated with deionized water was added to the resulting supernatant. The mixture was agitated and centrifuged at 12,000 × g for 5 min at 4°C. Ten microliters of the resulting supernatant were spotted onto a polyethyleneimine-cellulose thin-layer sheet (Macherey&Nagel). For the one-dimensional thin-layer chromatography (TLC) analysis, 1.2 M KH2PO4 (pH 3.5) was used as the chromatographic solvent.
Female BALB/c mice (16 to 18 g) were purchased from Charles River (Sulzfeld, Germany), housed in polypropylene cages, and given food and water ad libitum. S. aureus isolates were cultured for 18 h in B medium, washed three times with sterile PBS, and suspended in sterile PBS to 3 × 107 CFU/100 μl. As a control, selected dilutions were plated on B agar. Mice were inoculated with 100 μl of S. aureus via the tail vein. Control mice were treated with sterile PBS. Ten mice were used for each strain. Five days after challenge, kidneys were aseptically harvested and used for CFU enumeration and histological analysis, respectively. For the CFU count, organs of 7 mice were homogenized in 3 ml of PBS using Dispomix (Bio-Budget Technologies GmbH, Krefeld, Germany). Serial dilutions of the organ homogenates were cultured on mannitol salt-phenol red agar plates for at least 48 h at 37°C. CFU were calculated as CFU/organ. The statistical significance of the bacterial load was determined using a Mann Whitney test.
For thin sections of kidney tissues, the kidneys of intravenously (i.v.) infected mice were removed, embedded in O.C.T Tissue Tek (Vogel, Gießen, Germany), and shock frozen in liquid nitrogen. Five-micrometer cryosections were thin sliced, mounted on Superfrost slides (Langenbrinck, Emmendingen, Germany), and stored at −80°C until use. For staphylococcal protein A staining, slides were dried and subsequently fixed with prechilled acetone for 5 min. Endogenous peroxidase was blocked with 30% H2O2/methanol for 5 min. Slides were incubated with rabbit IgG (Dianova, Hamburg, Germany) (11 mg/ml) diluted 1:100 in 10% fetal calf serum (FCS)-PBS, followed by incubation with a peroxidase-coupled goat anti-rabbit IgG (Fab)2 fragment (Dianova) diluted 1:100 in 10% FCS. As substrate, 0.1% diaminobenzidine-tetrahydrochloride (AppliChem, Darmstadt, Germany) dissolved in PBS was used. Tissue was counterstained with Mayer's hematoxylin (Merck, Darmstadt, Germany). Slides were embedded in Roti-Histokitt (Carl-Roth, Karlsruhe, Germany) and dried overnight. Images were captured with an Olympus BX51 microscope equipped with a digital DP71 camera, and images were processed using CellB software (Olympus, Leinfelden, Germany).
In each of the sequenced S. aureus genomes, three genes with significant homology to the (p)ppGpp synthase domain of RelA from E. coli can be identified. SAOUHSC_02811 and SAOUHSC_00942 code for small proteins with a putative (p)ppGpp synthase domain only; due to sequence homology with proteins characterized in Streptococcus mutans (32), these are designated relP and relQ, respectively. SAOUHSC_01742 shows a high degree of homology to (p)ppGpp synthases of other firmicutes, as well as to both the N-terminal and C-terminal domains of RelA and SpoT from E. coli. In accordance with the nomenclature used in recent reviews (46, 56), we will refer to this enzyme as RSH. Previous attempts to disrupt the rsh gene in S. aureus failed, indicating that RSH is essential for growth and/or survival in this organism (16). Therefore, we first constructed conditional mutants using the pMUTIN4 vector system. pMUTIN4 is a suicide vector that allows a gene in the chromosome to be connected to the IPTG-inducible Pspac promoter (51). A 5′ rsh fragment was cloned into pMUTIN4 and transformed into S. aureus. The plasmid integrated into the chromosomal rsh gene, creating a Pspac-rsh fusion in which rsh transcription can be controlled by IPTG. As expected, the conditional mutant strains were not able to grow without the addition of IPTG to the medium, supporting the essentiality of the rsh gene product for the growth of S. aureus (Fig. (Fig.1).1). An IPTG dose-dependent increase of the growth rate could be observed (Fig. (Fig.1A1A).
Based on results obtained by analyzing (p)ppGpp synthases in proteobacteria, we hypothesized that in the RSH enzyme, the hydrolase domain but not the synthase domain is important for viability. E. coli exhibits the bifunctional enzyme SpoT with a (p)ppGpp synthase and a hydrolase domain and the monofunctional enzyme RelA with synthase activity only. It was shown that it is impossible to obtain a spoT mutant in a relA-positive background (58). This indicates that the hydrolase activity of SpoT is essential to degrade the otherwise RelA-dependent toxic accumulation of (p)ppGpp in the cell. To obtain an rshsyn mutant, we deleted conserved residues at amino acid (aa) positions 308 to 310 (SAOUHSC_01742) of the synthase domain of RSH. For RSHSeq of Streptococcus dysgalactiae subsp. equisimilis, it could be shown that mutation of any of these residues leads to defective synthase but intact hydrolase activity of the enzyme (23).
Deletions of the amino acids YQS in the native chromosomal rsh gene of S. aureus could be readily achieved using markerless chromosomal mutagenesis as described previously (3). In contrast to the conditional rsh mutants, the rshsyn mutants showed no growth defect in complex medium (Fig. (Fig.1B)1B) or in CDM (data not shown). Thus, the observation that deletion mutants of rsh are not viable in S. aureus is due to the missing hydrolase activity of RSH. The most plausible explanation is that RSH is required to detoxify molecules synthesized by alternative (p)ppGpp synthases like RelP or RelQ.
We aimed to analyze whether and under which conditions RSH contributes to (p)ppGpp synthesis in S. aureus. RSH contains a typical C-terminal domain which may sense the accumulation of uncharged tRNA after amino acid deprivation, leading to a shift toward synthase-ON/hydrolase-OFF conformation of the bifunctional enzyme (23). The stringent response was induced by mupirocin in bacteria grown to mid-exponential growth phase in complex medium. Alternatively, bacteria grown in CDM were transferred into CDM lacking leucine and valine. Both amino acids were necessary for optimal growth of S. aureus, and downshift of the strain into medium lacking these amino acids resulted in immediate growth inhibition. After induction of the stringent response, nucleotides were detected in cell extracts by TLC (Fig. 2A and B) and by LC-MS (Fig. (Fig.2C).2C). With both methods, the alarmones pppGpp and ppGpp were detectable in the wild type under stringent conditions, with higher concentrations of pppGpp than of ppGpp. The addition of mupirocin elicited a higher (p)ppGpp synthesis rate than did Leu/Val deprivation. In contrast, neither of the inducing conditions resulted in an increase of (p)ppGpp in the rshsyn mutant. This confirmed the hypothesis that the synthase domain is indeed nonfunctional in the rshsyn mutant. A slight spot corresponding to ppGpp was detectable in the rshsyn mutant by TLC analysis. In LC-MS, a weak signal of ppGpp was also detectable in the rsh mutant under stringent conditions, which might be derived from the activity of RelP and/or RelQ.
As expected, a significant decrease in GTP levels was observed after the induction of both stringent conditions in the wild type only and was thus clearly related to the pppGpp synthase activity of RSH.
The nucleotide analysis showed that RSH is the major enzyme responsible for the stringent response under amino acid-limited conditions induced by either the addition of mupirocin or Leu/Val deprivation. The response is characterized by synthesis of (p)ppGpp and a decrease of the intracellular GTP pool.
Since RSH is clearly responsible for (p)ppGpp synthesis under conditions of amino acid deprivation, it was of interest whether the growth of the rshsyn mutant was affected under such conditions. First, we tested resistance toward mupirocin. Mupirocin acts as a potent inhibitor of Ile-tRNA synthetase and, as such, mimics isoleucine starvation. rshsyn mutants of strains Newman and HG001, respectively, were both shown to be more sensitive toward mupirocin than the wild types, with up to 4-fold decreases of the MICs and MBCs (Table (Table3).3). There was no difference between the rshsyn mutants and the wild types with regard to sensitivity toward other classes of antibiotics (vancomycin and ciprofloxacin) not involved in amino acid availability (data not shown).
We next monitored growth in CDM lacking single amino acids. Without Leu/Val, S. aureus strains were strongly inhibited in growth. To test whether RSH is important for survival under these conditions, CFU were determined at the time points indicated in Fig. Fig.3.3. In medium lacking Leu/Val, the rshsyn mutant showed a significant reduction of CFU/ml compared to the growth of the wild type after 24 h; whereas a slight increase in CFU was observed for the wild type, only 4% of inoculated mutant bacteria were reculturable. Of note, there was no significant drop in the OD600 after 24 h for the rshsyn mutant. This indicates that RSH-dependent (p)ppGpp synthesis is required for bacterial survival under conditions of nutrient limitation.
Mupirocin treatment and Leu/Val deprivation resulted in the stringent response as indicated by (p)ppGpp synthesis via RSH. This is most probably accompanied by a specific modulation of the transcription of at least some genes. We selected a subset of genes which are hallmarks of the stringent control in other bacteria (15, 26) and were shown in microarray analysis to be repressed in S. aureus after the addition of mupirocin (2). We chose to induce the stringent response via Leu/Val deprivation since this may be a more physiological condition than mupirocin treatment. Northern blot analysis revealed that transcription of infB (coding for translation initiation factor IF-2), guaC (coding for GMP reductase), rpsL (coding for ribosomal protein S12), and tsf (coding for translation elongation factor Ts) was markedly decreased after 30 min of Leu/Val deprivation in the wild type but not in the rshsyn mutant (Fig. (Fig.4).4). The integration of a complete rsh gene into the chromosome of the rshsyn mutant restored the wild-type phenotype (Fig. (Fig.4,4, last lane). Thus, repression of the selected genes after Leu/Val downshift is clearly dependent on RSH synthase activity.
An interaction between CodY and the stringent response was shown in several other firmicutes (4, 24, 33). In S. aureus, several of the genes which were found to be repressed by CodY (45) also appeared to be upregulated after mupirocin treatment (2). For further analysis, we selected 4 operons of the CodY regulon which are preceded by a predicted CodY binding motif: the prototypic ilvDBC-leuABC-ilvA operon and ilvE coding for enzymes essential for branched-chain amino acid biosynthesis, brnQ1 coding for a branched-chain amino acid permease, and SAOUHSC_02932 coding for a putative amino acid permease. All 4 operons were clearly upregulated after Leu/Val deprivation in the wild type and the rsh-complemented strain but not in the rshsyn mutant strain (Fig. (Fig.55).
The ongoing repression of the selected genes in the rshsyn mutant is clearly CodY dependent since, in a codY mutant, as well as in a codY rshsyn double mutant, these genes are derepressed under all conditions (shown for SAOUHSC_02932) (Fig. (Fig.6).6). In contrast, RSH-repressed genes appeared to be independent of the CodY background, as exemplified by the results for infB.
In other pathogens, it was shown that the stringent control is tightly linked with other regulatory pathways involved in virulence gene expression (18). Thus, we extended our analysis to include the influence of the stringent control on the expression of major virulence regulators, namely, the regulatory RNA III encoded in the quorum sensing system agr, the two-component system saePQRS, and the transcription factor sarA. All three virulence regulators appeared to be upregulated in the wild type and in rshsyn mutants after mupirocin treatment for 30 min (Fig. (Fig.7).7). The elevated levels of transcription of sarA after mupirocin treatment were even more pronounced in the rshsyn mutants. These results indicate that the stimulating effects of mupirocin on virulence regulatory loci are largely independent of (p)ppGpp synthesis via RSH.
We next analyzed whether induction of the stringent response via Leu/Val deprivation has an effect similar to that of mupirocin addition (Fig. (Fig.8A).8A). In fact, enhanced sarA and sae transcription was observed in the wild type, as well as in the rsh mutant, after shifting the bacteria into medium lacking Leu/Val for 30 min.
We hypothesized that the rsh-independent effects on sae and sarA transcript levels under stringent conditions may be due to transcript stabilization rather than to promoter activation. Therefore, we determined transcript stabilities by treatment with rifampin, an antibiotic which inhibits the de novo synthesis of RNA. Strains at the exponential growth phase were shifted into medium with and without Leu/Val or mupirocin and grown for a further 30 min. Then, rifampin (500 μg/ml) was added to the cultures for the times indicated in Fig. Fig.8B8B.
We could confirm that, for instance, the sarA-encoding transcripts are stabilized after Leu/Val downshift (Fig. (Fig.8B).8B). A similar stabilization of transcripts was observed after mupirocin treatment (data not shown), which is consistent with the results of Anderson et al. (2).
The role of the RSH-mediated stringent response and CodY regulation for virulence was investigated in a hematogenic kidney abscess model with S. aureus HG001, the rshsyn mutant, a codY deletion mutant, and an rshsyn codY double mutant (Fig. (Fig.9).9). Mice were challenged with 3 × 107 CFU via the tail vein. The body weight of the mice was followed over time, and bacterial loads in kidneys were determined 5 days after infection. Our analysis revealed a significantly lower virulence of the rshsyn mutant than of the wild type (P < 0.01). In contrast, there was no significant difference between the virulence of the codY mutant or the codY rshsyn double mutant compared to that of the wild type. Similarly, body weight decreased significantly (P < 0.01) in mice challenged with the wild type, in contrast to those challenged with the rshsyn mutant (data not shown). Histological analysis of mice challenged with the wild type or the codY rshsyn double mutant showed localized bacteria and infiltrated polymorphonuclear leukocytes in the renal cortex and also in the medulla/papilla of the kidney (Fig. 10A and B). In both cases, the tissue shows typical signs of hematogenous (pyelo) nephritis. In mice challenged with the rshsyn mutant, the bacterial load was below the detection limit and no polymorphonuclear neutrophilic leukocyte infiltration was observed (Fig. 10C).
In the work presented here, we could confirm previous observations (16) that the bifunctional enzyme RSH is essential for the growth of S. aureus. This is a peculiarity of S. aureus since, in other firmicutes, rsh mutants are viable and have been employed to decipher the basic mechanisms of the stringent control in those organisms (1, 4, 26, 31, 38, 49, 54). The essentiality of RSH in S. aureus is due to its hydrolase activity, since mutants in which only the synthase domain of RSH was inactivated remain viable. This led to the hypothesis that other (p)ppGpp synthases in addition to RSH are active in S. aureus and that RSH is required to hydrolyze these molecules to prevent a toxic accumulation. (p)ppGpp is probably synthesized by one or both of two additional enzymes (RelP and RelQ) with putative (p)ppGpp synthase activity encoded in the genome of S. aureus. They are highly homologous to enzymes already shown to contribute to (p)ppGpp synthesis in other organisms (32, 41) However, RSH is clearly the main enzyme required for (p)ppGpp synthesis after amino acid deprivation in S. aureus. This could be shown by the addition of mupirocin or by shifting bacterial cultures into medium lacking Leu/Val. Both conditions resulted in detectable synthesis of pppGpp and ppGpp within 30 min of induction in the wild type but not in the rshsyn mutant. Under these conditions, the small proteins RelP and RelQ do not contribute significantly to the stringent response. However, it is conceivable that they are responsible for the baseline level of ppGpp observed in the rshsyn mutant (Fig. (Fig.2)2) and that they are important under different stress conditions.
In contrast to rsh deletion strains, the RSH synthase mutants of S. aureus strain HG001 or Newman, respectively, were not impaired in growth under nutrient-rich conditions. However, the RSH synthase mutants certainly showed a higher sensitivity toward mupirocin than the wild type, similar to relA mutants of E. coli (5) or Streptococcus pneumoniae (26). One could speculate that the upregulation of genes involved in isoleucine biosynthesis observed in the wild type but not in the rshsyn mutant results in an increased intracellular isoleucine level. Isoleucine can antagonize mupirocin, thereby increasing the MIC of mupirocin in S. aureus (55). Additionally, the rshsyn mutant was not able to survive conditions of amino acid deprivation, indicated by a decrease of CFU. The lethality of the rshsyn mutant during amino acid limitation may be due to the inability of the strain to stop replication. It was proposed by Wang et al. (53) that rapid replication arrest mediated by (p)ppGpp helps to maintain genomic stability by preventing deleterious consequences associated with continuous replication in starved cells.
We employed the rshsyn mutants to gain insights into the stringent response to amino acid starvation in S. aureus. We show that mupirocin and Leu/Val deprivation have a similar impact on the mRNA levels of most of the genes analyzed. Only part of the response is mediated via RSH-driven (p)ppGpp synthesis since, for instance, higher levels of mRNA encoding major virulence regulators (RNA III, SaeRS, and SarA) were seen in the wild type, as well as in the rshsyn mutants, after mupirocin treatment (Fig. (Fig.7).7). These effects are probably due to stabilization of the transcripts under conditions in which translation is impaired. It was proposed that ribosome stalling after binding of uncharged tRNA to the A site affects the lifetime of bacterial mRNA. Ribosomes associated with mRNA act as protective barriers and influence the susceptibility of mRNA to RNase attack (11). These ribosomes mask internal endonuclease cleavage sites and, additionally, blockade the 5′ end, which can impede access to internal endonuclease cleavage sites. Transcript stabilization after mupirocin treatment seems to affect different mRNA species to different degrees (2) and may have a regulatory function. Whether higher mRNA levels of the regulatory loci are paralleled by increased protein expression and higher target gene expression remains to be determined.
Besides the RSH synthase-independent effects on transcript stabilization, amino acid deprivation also results in RSH synthase-driven (p)ppGpp synthesis, which is accompanied by repression of genes involved in ribosomal turnover and translation and activation of genes involved in amino acid metabolism and transport. This coordinated gene expression probably allows the pathogen to adapt to starvation environments, such as those encountered during certain stages of infection. In other pathogens, (p)ppGpp-dependent activation of virulence genes is mediated by regulatory proteins, such as Sigma factors or transcription factors (4, 17, 24, 33, 44, 46). Here, we focused on the analysis of genes belonging to the previously characterized CodY regulon of S. aureus (45). Stringent conditions evoked by Leu/Val deprivation clearly resulted in the derepression of CodY-regulated genes in S. aureus. For these genes, mutation of codY has an opposing and dominant effect compared to that of rsh mutation. In line with the assumption that induction of the stringent control results in derepression of CodY target genes, a codY rshsyn double mutant expressed CodY target genes under noninduced conditions.
Analysis of CodY in several firmicutes has shown that binding of branched-chain amino acids and/or GTP to CodY results in the repression of target gene promoters. In S. aureus, a decreased GTP pool was observed under stringent conditions (Fig. (Fig.2C).2C). For B. subtilis (21, 34) and Listeria monocytogenes (4), a direct link between the GTP drop and derepression of CodY target genes was demonstrated. Whether stringent conditions also influence the intracellular isoleucine pool remains to be determined. Previously, we showed that isoleucine is necessary and sufficient for the repression of CodY target genes in S. aureus (45). Of note, in S. mutans, there was also a clear link between the stringent response and CodY which was shown to be independent of variations in the GTP level.
A regulatory link between the stringent control and CodY during infection was recently demonstrated for L. monocytogenes (4), with results very similar to those for S. aureus. In both organisms, an rsh mutant was clearly attenuated in an animal model of infection, whereas additional codY mutation restored virulence in both organisms. Thus, the attenuation of the rsh mutant could be due at least in part to ongoing repression of CodY-regulated genes under condition which would result in derepression in the wild type. The expression of these genes is obviously relevant for virulence. In the model applied here, the bacteria are usually efficiently cleared within the bloodstream. Some bacteria can reach the kidney, where they multiply and persist. One may assume that the bacteria there encounter an amino acid limitation which evokes the stringent control. Under such conditions, (p)ppGpp may be essential not only for growth but also for survival. The survival of the rshsyn mutant under in vivo conditions may be diminished, as was shown in vitro, under conditions of amino acid deprivation.
We thank Vittoria Bisanzio for excellent technical assistance.
The work was supported by grants to C.W. and K.O. from the Deutsche Forschungsgemeinschaft TR34. M. Liebeke was the recipient of a fellowship from the Alfried Krupp von Bohlen und Halbach-Stiftung Foundation Functional Genomics Approach in Infection Biology program.
Editor: A. J. Bäumler
Published ahead of print on 8 March 2010.