can cause DNA condensation and promotes the formation of higher-order nucleoprotein complexes depending on the concentrations of potassium chloride and magnesium chloride. The higher-order DNA organization by IHF in vivo is as yet unclear. H-NS A distinguishable feature of histone-like or heat-stable nucleoid structuring protein (H-NS) from other NAPs is the ability to switch from the homodimeric form at relatively low concentrations (<1 x 10−5 M) to an oligomeric state at higher levels. Because of oligomerization properties, H-NS spreads laterally along AT-rich DNA in a nucleation reaction, where high-affinity sites function as nucleation centers. The spreading of H-NS on DNA results in two opposite outcomes depending on the magnesium concentration in the reaction. At low magnesium concentration (< 2 mM), H-NS forms rigid nucleoprotein filaments whereas it forms inter- and intra-molecular bridges at higher magnesium concentrations (> 5 mM). The formation of rigid filaments results in straightening of DNA with no condensation whereas the bridging causes substantial DNA folding. Analysis of H-NS binding in the genome by ChIP-Seq assays provided indirect evidence for the spreading of H-NS on DNA in vivo. H-NS binds selectively to 458 regions in the genome. Although H-NS has been demonstrated to prefer curved DNA formed by repeated A-tracks in DNA sequences the basis of the selective binding is the presence of a conserved sequence motif found in AT-rich regions. More importantly, the frequent occurrence of the sequence motif within an H-NS binding region that can re-enforce the cooperative protein-protein interactions, and the unusually long length of the binding region are consistent with the spreading of the protein. Whether the filament formation or DNA bridging is prevalent in vivo depends on the physiological concentration of magnesium inside the cell. If the magnesium concentration is uniformly low (< 5 mM), H-NS would form rigid nucleoprotein filaments in vivo. Alternatively, if there is an uneven distribution of magnesium in the cell, it could promote both DNA bridging and stiffening but in different regions of the nucleoid. Furthermore, H-NS is best known as a global gene silencer that preferentially inhibits transcription of horizontally transferred genes and it is the rigid filament that leads to gene silencing. Taken together, it appears that the formation of rigid filaments is the most likely outcome of H-NS-DNA interactions in vivo that leads to gene silencing but does not induce DNA condensation. Consistently, the absence of H-NS does not change the nucleoid volume. However,