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Transcriptional regulation of the iron-responsive sigma factor gene pbrA
R Sexton1, P R Gill, D N Dowling
1Microbiology Department, University College, Cork, Ireland.
Abstract:
In response to the intracellular iron concentration Pseudomonas fluorescens M114 coordinately regulates the production of pseudobactin M114, its cognate receptor PbuA, and a casein protease. Transcriptional initiation of this coordinate iron-stress response requires the sigma factor PbrA. PbrA is a member of the ECF (Extracytoplasmic function) subgroup of the sigma 70 family of eubacterial RNA polymerase sigma factors. Regulatory studies of the pbrA gene utilising promoter-lacZ transcriptional fusions demonstrate that expression of pbrA dictates the cellular response to iron. pbrA is transcribed in all phases of iron-limited growth but maximally at late-logarithmic to stationary phase. pbrA expression is independent of autoregulatory control but is strictly repressed in iron-rich conditions in a Fur-dependent fashion. Constitutive expression of pbrA from an inducible tac promoter permits the induction of PbrA-dependent transcription and pseudobactin M114 biosynthesis in high-iron conditions. A PbrA consensus sequences was derived from significant DNA sequence homologies observed within the "-25 bp" and "-16 bp" regions conserved among all PbrA-dependent promoters. The predicted PbrA target promoter consensus is homologous for the promoter recognition sites for other environmentally responsive ECF sigma factors.
Insights
Pseudomonas fluorescens M114 regulates iron-stress responses via the sigma factor PbrA. PbrA controls pseudobactin M114 production and is repressed by iron, but its constitutive expression bypasses this repression.
Area of Science:
- Microbiology
- Molecular Biology
- Bacterial Genetics
Background:
- Pseudomonas fluorescens M114 coordinates iron-stress responses, including pseudobactin M114 and receptor PbuA production.
- This response is regulated by the sigma factor PbrA, a member of the ECF subgroup of sigma 70 factors.
Purpose of the Study:
- To investigate the role of the sigma factor PbrA in regulating the iron-stress response in Pseudomonas fluorescens M114.
- To elucidate the transcriptional regulation of the pbrA gene and its impact on cellular iron homeostasis.
Main Methods:
- Promoter-lacZ transcriptional fusions were used to study pbrA gene expression.
- Analysis of pbrA expression under various iron concentrations and growth phases.
- Identification of PbrA consensus sequences through DNA sequence homology.
Main Results:
- pbrA expression dictates the cellular iron-stress response and is maximal during late-logarithmic to stationary phases of iron-limited growth.
- pbrA expression is independent of autoregulation but repressed by iron in a Fur-dependent manner.
- Constitutive expression of pbrA enabled pseudobactin M114 biosynthesis even under high-iron conditions.
Conclusions:
- PbrA is a key regulator of the iron-stress response in Pseudomonas fluorescens M114.
- The PbrA consensus promoter sequence shows homology to other ECF sigma factor binding sites, suggesting conserved regulatory mechanisms.