An efflux transporter PbrA and a phosphatase PbrB cooperate in a lead-resistance mechanism in bacteria Original paper

What was studied?

This study focused on the lead resistance mechanisms in Cupriavidus metallidurans CH34, particularly exploring the role of two key components: the PbrA efflux transporter and the PbrB phosphatase. The research examined how these proteins work in tandem to mitigate the toxic effects of lead (Pb²⁺) by effluxing the metal and sequestering it as a less toxic phosphate salt. The study investigated the metal specificity of the PbrA transporter and the enzymatic activity of PbrB, using genetically modified strains of C. metallidurans to analyze their responses to lead, zinc, and cadmium exposure. Through functional assays and complementation tests, the study aimed to elucidate the molecular mechanism of lead detoxification involving PbrA and PbrB and to identify how these proteins collaborate in conferring resistance.

Who was studied?

The study focused on Cupriavidus metallidurans CH34, a bacterium known for its ability to resist a range of heavy metals, including lead. Specifically, the pbrABCD operon was examined, which encodes proteins involved in lead resistance, including PbrA (a P1B-type ATPase) and PbrB (a C55-PP phosphatase). These genes were expressed in C. metallidurans DN440, a strain genetically modified to lack certain metal resistance genes, allowing the researchers to assess how the pbr operon conferred resistance to lead, zinc, and cadmium. Additionally, the study used Escherichia coli strains for complementary experiments involving PbrBC and phosphatase activity. These bacterial strains were carefully selected to examine the efficiency of the resistance mechanisms when exposed to various metal ions, with specific attention paid to the interaction between PbrA and PbrB.

Most important findings

The research found that PbrA, a P1B-type ATPase, was involved in the efflux of not only lead (Pb²⁺) but also zinc (Zn²⁺) and cadmium (Cd²⁺), suggesting a broader role for PbrA in heavy metal transport. However, the co-expression of PbrA and PbrB resulted in a specific increase in lead resistance, demonstrating that while PbrA exported multiple metals, lead sequestration via PbrB was crucial for enhanced resistance. PbrB was identified as a C55-PP phosphatase, and its ability to sequester lead as a phosphate salt was central to the mechanism. When expressed in conjunction with PbrA, PbrB helped precipitate lead, reducing its bioavailability and preventing further toxicity. Interestingly, expression of PbrA alone did not confer significant resistance to lead, while PbrB alone also failed to provide effective detoxification, underscoring the necessity of both proteins for efficient lead resistance. The study also highlighted that the PbrBC fusion protein could function independently of PbrC, further supporting the idea that PbrB alone is sufficient for lead detoxification.

Key implications

The findings have significant implications for understanding metal resistance mechanisms in bacteria, particularly in environments contaminated with heavy metals like lead. The cooperation between PbrA and PbrB provides a model for how bacteria might detoxify and handle toxic metals, which could inform strategies for bioremediation. The discovery that PbrB functions as a C55-PP phosphatase and is involved in lead sequestration offers new insights into bacterial adaptation to metal stress, especially regarding lead. These findings could have applications in designing microbial systems to reduce environmental lead contamination or to study the microbial resistance mechanisms that could be leveraged in clinical settings, where lead toxicity is a concern. Moreover, the broader applicability of PbrA and PbrB to other heavy metals suggests that these proteins could be targets for engineering bacteria with enhanced resistance to a range of contaminants. The study also emphasizes the importance of cooperative systems in bacterial resistance, where efflux pumps and enzymatic sequestration mechanisms work together to minimize metal toxicity.