A single molecule analysis of H-NS uncouples DNA binding affinity from DNA specificity

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Heat-stable nucleoid structuring protein (H-NS) is vital for gene silencing in prokaryotes and pathogenesis.
• The precise molecular mechanism of H-NS-mediated gene silencing remains incompletely understood.
• H-NS is known to regulate a significant portion of the bacterial genome, approximately 5%.

Purpose of the Study:

• To elucidate the molecular mechanism of H-NS DNA binding and gene silencing.
• To quantify the binding affinity of H-NS to specific DNA sites using a single-molecule counting approach.
• To investigate the role of charged residues in the H-NS linker domain in DNA binding.

Main Methods:

• Single-molecule counting to measure dissociation constants (KD) of H-NS to DNA.
• Hairpin unzipping assay to differentiate non-specific binding from nucleation site binding.
• Analysis of H-NS binding to both A/T-rich and G/C-rich DNA sequences.

Main Results:

• Charged residues in the H-NS linker domain significantly contribute to DNA binding affinity.
• H-NS binding affinity to A/T-rich and G/C-rich DNA sites was found to be nearly identical, contrary to previous reports.
• A dissociation constant (KD) of approximately 60 nM was measured for initial non-specific DNA binding via electrostatic interactions.

Conclusions:

• H-NS initially binds non-specifically to DNA through electrostatic interactions mediated by its linker domain.
• This initial binding facilitates the search for specific nucleation sites, which are crucial for subsequent polymerization and gene silencing.
• The findings provide a refined model for H-NS function in bacterial gene regulation and pathogenesis.