x (1/cell volume in liter) x 103. Cell volume in liter ( 2 x 10−15) was determined by assuming volume of the E. coli cell to be 2 μm3. 1 Binding affinity refers to equilibrium dissociation constant (Kd) in molar units (M). ND = not determined HU Histone-like protein from E. coli strain U93 (HU) is an evolutionarily conserved protein in bacteria. HU exists in E. coli as homo- and heterodimers of two subunits HUα and HUβ sharing 69% amino acid identity. Although it is referred to as a histone-like protein, close functional relatives of HU in eukaryotes are high-mobility group (HMG) proteins, and not histones. HU is a non-sequence specific DNA binding protein. It binds with low-affinity to any linear DNA. However, it preferentially binds with high-affinity to a structurally distorted DNA. Examples of distorted DNA substrates include cruciform DNA, bulged DNA, dsDNA containing a single-stranded break such as nicks, gaps, or forks. Furthermore, HU specifically binds and stabilizes a protein-mediated DNA loop. In the structurally specific DNA binding mode, HU recognizes a common structural motif defined by bends or kinks created by distortion, whereas it binds to a linear DNA by locking the phosphate backbone. While the high-affinity structurally-specific binding is required for specialized functions of HU such as site-specific recombination, DNA repair, DNA replication initiation, and gene regulation, it appears that the low-affinity general binding is involved in DNA condensation. In chromatin-immunoprecipitation coupled with DNA sequencing (ChIP-Seq), HU does not reveal any specific binding events. Instead, it displays a uniform binding across the genome presumably reflecting its mostly weak, non-sequence specific binding, thus masking the high-affinity binding in vivo. In strains lacking HU, the nucleoid is "decondensed", consistent with a role of HU in DNA compaction. The following in vitro studies suggest possible mechanisms of how HU might condense and organize DNA in vivo. Not only HU stably binds to distorted DNA with bends, it induces flexible bends even in a linear DNA at less than 100 nM concentration. In contrast, HU shows the opposite architectural effect on DNA at higher physiologically-relevant concentrations. It forms rigid nucleoprotein filaments causing the straitening of DNA and not the bending. The filaments can further form a DNA network (DNA bunching) expandable both laterally and medially because of the HU-HU multimerization triggered by the non-sequence-specific DNA binding. How are these behaviors of HU relevant inside the cell? The formation of filaments