A plasmid-encoded H-NS protein selectively binds its own plasmid.

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Conjugative plasmids are mobile genetic elements crucial for bacterial adaptation and trait dissemination, including antibiotic resistance.
• Nucleoid-associated proteins like H-NS play vital roles in bacterial chromosome organization and gene regulation.
• Some conjugative plasmids encode H-NS homologues, potentially aiding plasmid maintenance and mitigating carriage costs.

Purpose of the Study:

• To characterize HppX _CROD2, an H-NS homologue from the IncX4 plasmid pCROD2, and investigate its DNA-binding properties.
• To determine if HppX _CROD2 exhibits specificity for its cognate plasmid compared to the host chromosome or other plasmids.
• To elucidate the mechanism and implications of plasmid-specific DNA binding by HppX _CROD2.

Main Methods:

• Expression and purification of HppX _CROD2.
• Electrophoretic mobility shift assays (EMSAs) to assess DNA binding affinity.
• DNA-binding studies using plasmid DNA, chromosomal DNA, and synthetic DNA fragments.
• Heterologous expression of HppX _CROD2 in *Escherichia coli*.

Main Results:

• HppX _CROD2 demonstrated a strong preference for binding the IncX4 plasmid pCROD2 over chromosomal DNA or other plasmids.
• Chromosomally encoded H-NS showed no sequence preference between plasmid and chromosomal DNA.
• HppX _CROD2 binding to its cognate DNA sequence was significantly reduced when cloned into a laboratory plasmid, indicating context-dependent binding.
• This phenomenon was termed "cognate plasmid specificity".

Conclusions:

• HppX _CROD2 exhibits remarkable cognate plasmid specificity, binding preferentially to DNA within its native plasmid context.
• This specificity is likely mediated by recognition of plasmid-specific DNA topology influenced by plasmid-encoded topoisomerases.
• Cognate plasmid specificity provides a mechanism for plasmids to regulate their own genes independently of host DNA and minimize disruption of host gene expression.