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A small protein that mediates the activation of a two-component system by another two-component system
L F Kox1, M M Wösten, E A Groisman
1Department of Molecular Microbiology, Howard Hughes Medical Institute, Washington University School of Medicine, St Louis, MO 63110-1093, USA.
Abstract:
The PmrA-PmrB two-component system of Salmonella enterica controls resistance to the peptide antibiotic polymyxin B and to several antimicrobial proteins from human neutrophils. Transcription of PmrA-activated genes is induced by high iron, but can also be promoted by growth in low magnesium in a process that requires another two-component system, PhoP-PhoQ. Here, we define the genetic basis for the interaction between the PhoP-PhoQ and PmrA-PmrB systems. We have identified pmrD as a PhoP-activated gene that mediates the transcriptional activation of PmrA-regulated genes during growth in low magnesium. When transcription of pmrD is driven from a heterologous promoter, expression of PmrA-activated genes occurs even at repressing magnesium concentrations and becomes independent of the phoP and phoQ genes. The PmrD effect is specific for PmrA-regulated genes and requires functional PmrA and PmrB proteins. A pmrD mutant is sensitive to polymyxin if grown in low magnesium, but resistant if grown in high iron. The PmrD protein controls the activity of the PmrA-PmrB system at a post-transcriptional level.
Insights
Salmonella enterica
Area of Science:
- Microbiology
- Bacterial Pathogenesis
- Molecular Biology
Background:
- The PmrA-PmrB two-component system in *Salmonella enterica* regulates resistance to polymyxin B and neutrophil antimicrobial proteins.
- This system's gene transcription is induced by high iron and also by low magnesium, involving the PhoP-PhoQ system.
- Understanding the interplay between these systems is crucial for bacterial resistance mechanisms.
Purpose of the Study:
- To elucidate the genetic basis for the interaction between the PhoP-PhoQ and PmrA-PmrB two-component systems in *Salmonella enterica*.
- To identify the specific gene responsible for mediating PmrA-PmrB system activation under low magnesium conditions.
Main Methods:
- Genetic analysis to identify genes involved in the interaction between PhoP-PhoQ and PmrA-PmrB systems.
- Gene expression studies using heterologous promoters to assess the role of identified genes.
- Mutant analysis (*pmrD* mutant) to evaluate polymyxin sensitivity under different growth conditions (low magnesium, high iron).
Main Results:
- The gene *pmrD* was identified as a PhoP-activated gene essential for PmrA-regulated gene activation during growth in low magnesium.
- Ectopic expression of *pmrD* bypassed the requirement for PhoP-PhoQ and activated PmrA-regulated genes even at repressing magnesium levels.
- The PmrD protein acts post-transcriptionally to control the PmrA-PmrB system's activity, specifically for PmrA-regulated genes.
Conclusions:
- The PmrD protein is a key mediator linking the PhoP-PhoQ and PmrA-PmrB systems in *Salmonella enterica*.
- PmrD enables polymyxin resistance under low magnesium conditions by modulating the PmrA-PmrB system post-transcriptionally.
- This finding reveals a novel regulatory mechanism for antimicrobial resistance in *Salmonella*.