Locus of Enterocyte Effacement-encoded Regulator ( Ler) of Pathogenic Escherichia coli Competes Off Histone-like Nucleoid-structuring Protein ( H-NS) through Noncooperative DNA Binding

Background: Ler alleviates H-NS-mediated gene silencing. Results: LerDNA binding properties and Ler effects on H-NSDNA binding are investigated using magnetic tweezers and atomic force microscopy. Conclusion: Ler binds noncooperatively to stiffen and fold DNA, and it can replace prebound H-NS in the conditions tested. Significance: The findings provide new insights into the molecular mechanism of anti-silencing by Ler. The locus of enterocyte effacement-encoded regulator (Ler) of enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) functions to activate transcription of virulence genes silenced by the histone-like nucleoid-structuring protein (H-NS). Despite its important role in the bacterial gene regulation, the binding mode of Ler to DNA and its mechanism in alleviating genes repressed by H-NS are largely unknown. In this study, we use magnetic tweezers to demonstrate that Ler binds extended DNA through a largely noncooperative process, which results in DNA stiffening and DNA folding depending on protein concentration. We also show that Ler can replace prebound H-NS on DNA over a range of potassium and magnesium concentrations. Our findings reveal the DNA binding properties of Ler and shed light to further understand the anti-silencing activity of Ler.