Previous Article | Next Article ![]()
Journal of Bacteriology, December 2005, p. 8181-8184, Vol. 187, No. 23
0021-9193/05/$08.00+0 doi:10.1128/JB.187.23.8181-8184.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
,
Eun-Hee Lee,1,3,
Anthony B. Law,1,3
Yih-Ling Tzeng,2 and
William M. Shafer1,2,3*
Departments of Microbiology and Immunology,1 Medicine, Emory University School of Medicine, Atlanta, Georgia 30322,2 Laboratories of Bacterial Pathogenesis, Medical Research Service, VA Medical Center, Decatur, Georgia 300333
Received 16 June 2005/ Accepted 15 September 2005
|
|
|---|
|
|
|---|
The orange and yellow pigments that produce the "golden" colonies of S. aureus are the products of a C30 triterpenoid biosynthetic pathway (22, 23). The yellow carotenoid pigment is produced through 4,4'-diaponeurosporene, which is then converted to an orange end product, staphyloxanthin. The protein products of the proposed crtMN operon (29) are responsible for the first stage of carotenoid pigment production. CrtM, a dehydrosqualene synthase, combines two molecules of farnesyl pyrophosphate to form 4,4'-diapophytoene. The 4,4'-diapophytoene then undergoes three rounds of dehydrogenation, directed by CrtN, a 4,4'-diapophytoene desaturase, to produce the intermediate yellow pigment 4,4'-diaponeurosporene (26). The dehydrogenation and conversion of this pigment to staphyloxanthin are thought to be carried out by one or more of the protein products of the four-gene open reading frame orf1-orf4 (GenBank accession number X97985) (28).
Production of pigment is influenced by the rsbUVWsigB system (9, 18, 24, 25). Specifically, the alternative sigma factor SigB (31) positively regulates expression of the crtMN operon (2). In experiments using a genetic derivative of S. aureus strain 8325-4 carrying Tn551, we determined that the susceptibility of S. aureus to an antimicrobial peptide of human lysosomal cathepsin G (CG117-136) is associated with expression of cspA (16), which has been annotated (www.tigr.org) as a cold shock gene in S. aureus. We subsequently found that transfer of a cspA null mutation to the highly pigmented strain COL confirmed a relationship between cspA and staphylococcal susceptibility to CG117-136. Surprisingly, however, cspA-negative mutants of strain COL lost the capacity to produce pigment. We now report that CspA regulates pigment production in S.aureus through a SigB-dependent mechanism.
Bacterial strains, plasmids used, growth conditions, and analysis of pigment. The strains of Escherichia coli and S. aureus and the plasmids used in this investigation are listed in Table 1. To determine if pigment production is temperature dependent, two 250-ml flasks containing 50 ml tryptic soy broth (TSB) plus the appropriate antibiotic were inoculated (1:100 dilution) from an overnight culture of a specific strain and incubated at 30°C or 15°C. Cultures were sampled every 24 to 30 h until pigmentation was visible. A 20-ml sample of each culture was centrifuged at 6,000 x g for 10 min. To determine the concentration of carotenoid pigment present in each strain, the methanol extraction procedure of Morikawa et al. (24) was employed using S. aureus strains grown at 30°C for 30 h in 50 ml of TSB.
|
View this table: [in a new window] |
TABLE 1. Bacterial strains and plasmids
|
(17, 27) that carried the cspA::Km cassette from SKC31, and a transductant (strain SKC32) was identified (Table 1). Transcription of S. aureus genes was monitored by quantitative real-time reverse transcription-PCR (qRT-PCR), using RNA extracted from 15-h TSB cultures grown at 30°C. RNA was extracted from approximately 5.0 x 109 CFU, resuspended in Tris-EDTA (pH 7.6), that were lysed using the FastProtein blue matrix (Qbiogene) in a FastPrep (FP120) Bio 101 Savant instrument. RNA was separated from the cell lysate using the RNeasy minikit (QIAGEN, Inc.).
The 16S rRNA RT-PCR products were used to normalize the concentration of transcripts from selected genes (data not presented). These genes were the alkaline shock protein, asp23, which is regulated by the secondary sigma factor SigB (8), crtN, and sigB. Specific primers used in qRT-PCR are listed in Table 2. qRT-PCR was performed on an iCycler iQ real-time PCR detection system (Bio-Rad); 0.5 µg of total RNA was used as the template for cDNA synthesis that employed a random hexamer and SuperScript II reverse transcriptase (Invitrogen). iQ SYBR Green Supermix (Bio-Rad) was used in a total reaction volume of 25 µl with 200 nM of 5' and 3' primers and twofold-diluted RT reaction mixtures as templates for each PCR.
|
View this table: [in a new window] |
TABLE 2. Oligonucleotide primers
|
Loss of CspA results in decreased pigment production by S. aureus. Unlike the parental strain COL, a broth culture of strain SKC31 (isogenic to COL but with cspA::Km) failed to produce the characteristic orange pigment after 30 h of incubation at 30°C (Fig. 1); similar observations were made for broth cultures of strains COL and SKC31 incubated at 15°C or 37°C (data not presented). Pigment (orange) production could, however, be restored in strain SKC31 by placing cspA in trans (strain SKC31/pBCSA) but the strain remained nonpigmented when only the vector (pBT2) was introduced (Fig. 1). Through measurements of the methanol-extractable carotenoid pigment (24) in these strains, we found (Table 3) that strain SKC31 produced substantially less pigment than parental strain COL and that pigment production was restored by expressing cspA in trans (strain SKC31/pBCSA). The low level of the slightly yellow pigment produced by strain SKC31 was likely the intermediate pigment 4,4'-diaponeurosporene (29).
![]() View larger version (38K): [in a new window] |
FIG. 1. Visual comparison of pigment production by the cold shock gene of S. aureus, cspA. Each pellet, at the bottom of a 50-ml conical polystyrene centrifuge tube, represents 20 ml of a 50-ml culture that was harvested after growth at 30°C for 30 h. The strains used were the wild-type S. aureus COL, the cspA::Km insertion-deletion mutant SKC31, SKC31(pBCSA), an SKC31 mutant complemented in trans by a 528-bp insert of the cspA gene in the pBT2 vector, and SKC31(pBT2), an SKC31 mutant control carrying only the pBT2 vector.
|
|
View this table: [in a new window] |
TABLE 3. Comparison of methanol-extracted carotenoid pigments from strains of Staphylococcus aureus
|
![]() View larger version (32K): [in a new window] |
FIG. 2. qRT-PCR comparison of transcription of selected S. aureus genes. Shown are the relative levels of transcripts of the asp23, crtN, and sigB genes from S. aureus strains COL, SKC31, SKC31(pBT2), and SKC31(pBCSA). Expression of each gene was normalized to the expression of the reference gene for 16S rRNA from each strain. Relative values of expression in the test strains were calculated and compared to that of parental strain COL. All values are averages (±standard deviation) from triplicate analyses that used three independent preparations of RNA.
|
Based on the above results and similar findings (data not presented) made with pigmented strains Newman and SH1000 (12), we propose that CspA modulates pigment production in S. aureus. This regulatory scheme likely involves control of expression of the crtMN operon and possibly other genes (e.g., orf1-orf4) (28). It is not yet clear whether CspA controls crtMN expression directly or indirectly because loss of CspA production was shown to modulate expression of sigB and a SigB-regulated gene (asp23) by two- and fourfold, respectively. In our hands, loss of the sigB operon in strain COL (strain IK183) has a more dramatic impact on levels of pigment than loss of cspA alone; a strain (SKC32) with mutations in both the rsbUVWsigB operon and cspA produces a level of carotenoid similar to that of the single mutant lacking SigB (Table 3). Importantly, neither SigB nor pigment production is phenotypically linked to antimicrobial peptide susceptibility (16).
Although the mechanism by which CspA regulates pigment production in a SigB-dependent manner is not yet clear, our data indicate that it does not involve stabilization of transcripts. Rather, it appears that CspA acts as an enhancer of transcription. In support of this hypothesis, the CspB protein of Bacillus subtilis (21) has been reported (10) to bind to ATTGG box elements and the complementary pentanucleotide sequence CCAAT in promoter regions of certain genes. Such binding in vivo of CspB to partial single-stranded DNA (32) could function to stabilize an open complex of RNA polymerase. The pentanucleotide sequence has been termed the cold shock domain (Y-box) that is recognized by a family of eukaryotic transcription factors (30). In E. coli, CspA binds to the promoter of gyrA, facilitating its transcription during cold shock (15), and it can also bind to the hns promoter region (3). It has also been proposed to bind RNA (14). Our analysis of the genes involved in pigment production in strain COL (www.tigr.org) revealed a potential CCAAT DNA-binding site for CspA upstream of the promoter for crtMN transcription, a site within crtM, six sites within or adjacent to orf3 and orf4, and one site within sigB (data not presented). The importance of these sites in CspA-mediated regulation of pigment production will require additional investigation.
This work was supported by NIH grant AI43316 (W.M.S.). W.M.S. is the recipient of a Senior Research Career Scientist Award from the VA Medical Service.
Present address: Midwestern University, Glendale, AZ 85308. ![]()
These authors contributed equally to this work. ![]()
|
|
|---|
B-dependent transcription of sigB and asp23 in two different Staphylococcus aureus strains. Mol. Gen. Genet. 261:558-566.[CrossRef][Medline]
B activity depends on RsbU in Staphylococcus aureus. J. Bacteriol. 183:1843-1852.
-lysin gene (hld) in the regulation of virulence genes by the accessory gene regulator (agr) in Staphylococcus aureus 8325-4. EMBO J. 9:1391-1399.[Medline]
B in Staphylococcus aureus reveals its function as a global regulator of virulence genes. J. Bacteriol. 180:4814-4820.
B, urges Staphylococcus aureus to thicken the cell wall and to resist ß-lactams. Biochem. Biophys. Res. Commun. 288:385-389.[CrossRef][Medline]
B activity in Staphylococcus aureus is controlled by RsbU and an additional factor(s) during bacterial growth. Infect. Immun. 69:7858-7865.
This article has been cited by other articles:
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»