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- Title
Exotoxin A production in Pseudomonas aeruginosa requires the iron-regulated pvdS gene encoding an alternative sigma factor.
- Authors
Ochsner, Urs A.; Johnson, Zaiga; Lamont, Iain L.; Cunliffe, Heather E.; Vasil, Michael L.
- Abstract
Exotoxin A (ETA) is secreted by <em>Pseudomonas aeruginosa</em> under iron-limiting growth conditions. The ETA structural gene, <em>toxA</em>, is regulated at the transcriptional level by the gene products of the <em>regAB</em> operon. The expression of both <em>toxA</em> and <em>regAB</em> is repressed under iron-replete conditions, suggesting a role for the f̱erric u̱ptake ṟegulator (Fur) in regulation of ETA synthesis; however, the Fur protein does not interact directly with the <em>toxA</em> or the <em>regAB</em> promoters. Evidence is presented that the iron control of ETA synthesis is mediated by a Fur-regulated alternative sigma factor, PvdS, which had initially been identified as a positive activator for the production of the siderophore pyoverdin. In a Δ<em>pvdS</em> deletion mutant, ETA was produced at low levels of less than 5% compared to wild type, but still in response to iron starvation, and introduction of a functional <em>pvdS</em> gene on a plasmid fully restored wild-type levels and normal iron regulation of ETA synthesis. Therefore, a functional <em>pvdS</em> locus is essential for ETA production. Neither <em>toxA</em> nor <em>regAB</em> mRNA was detectable in a Δ<em>pvdS</em> mutant. Over-expression of <em>pvdS</em> from the <em>tac</em> promoter on a plasmid resulted in a high-level and iron-independent production of ETA in wild-type PAO1, in the Δ<em>pvdS</em> strain, but not in a Δ<em>regA</em> strain as a host. These findings suggest that PvdS is required for the activation of the <em>regAB</em> promoters. The transcription of <em>regAB</em> and <em>toxA</em> after induction of the P<em>tac-pvdS</em> gene was monitored in cells grown in high-iron medium. While both <em>regAB</em> and <em>toxA</em> were highly expressed during all growth phases under microaerobic conditions, <em>toxA</em> transcripts were detected only during the exponential but not the early stationary phase of growth under aerobic conditions. These results suggest that a second regulatory mechanism besides the Fur-PvdS system is involved in iron regulation of ETA production.
- Subjects
BACTERIAL toxins; PSEUDOMONAS aeruginosa; SIDEROPHORES; GENETIC regulation; PLASMIDS; MESSENGER RNA; REGULATION of cell growth
- Publication
Molecular Microbiology, 1996, Vol 21, Issue 5, p1019
- ISSN
0950-382X
- Publication type
Article
- DOI
10.1046/j.1365-2958.1996.481425.x