showed that HU also plays an important role in the regulation of many genes in response to environmental changes and adaptation to stress, including changes in osmolarity, acid stress, SOS induction, and anaerobiosis [13, 22] HU is a heterodimer formed by an α-subunit and a β-subunit, which are encoded by the hupA and hupB genes, respectively [23, 24, 25, 26, 27] and are differentially stimulated during cold shock [28] HU is a transcriptional dual regulator. HU-1 (HupB) and HU-2 (HupA) are closely related but differ by 28 residues, and they have a high content of hydrophobic residues represented mostly by alanine. There is no sequence homology between the proteins HU-1 and HU-2 compared to any of the five histones of different eukaryotes studied [25] HU forms high-affinity complexes with DNA containing sharp bends, kinks, branched and bulged structures, or single-strand breaks and loops [3, 29, 30, 31, 32] Three dimeric forms of HU exist in Escherichia coli: two homodimers, EcHUα₂ and EcHUβ₂, and a heterodimer, HUαβ [33] All these forms are in thermal equilibrium between two dimeric conformations (N₂↔I₂) that vary in their secondary structure content [34] Based on NMR spectroscopy, it was revealed that the hupA and hupB genes code for the EcHUα and EcHUβ polypeptide chains, respectively (70% identity, 90% homology) [35, 36]. Based on high-temperature molecular dynamics simulation and NMR experiments, information has been obtained about the structural and dynamic features at the atomic level for the N₂↔I₂ thermal transition of the EcHUβ₂ homodimer [34] A 3D model has been proposed for the major I₂ conformation of EcHUβ₂ [34] It has been observed that the EcHUβ₂ conformation appears to be structurally more stable than that of EcHUα₂[36]. The crystal structure of the DNA-HUαβ complex has been resolved [37] In the presence of poly(P), HupA is efficiently degraded by Lon [38] The presence of HU depends on the phase of growth [8] and it is distributed uniformly in the nucleoid [10] Currently, no inducer for this regulator has been reported in the literature. A hupA mutation affected the production of cell surface structures in strains lacking a cysH gene [39] When the N terminal is blocked with an artificial extension, the process of tetramerization of HupA and HupB is blocked, suggesting that this terminal is important in the process of heterodimerization, because the tetramerization appears to be essential to form heterodimers [36]. Based on proteomic studies, it was determined that the activities of nucleoid-associated proteins (NAPs) such as H-NS, HU, IHF, and FIS are also modulated by posttranslational modifications (PTMs) that signal differences in growth conditions; because none of the NAPs is known to bind any signaling ligands, these growth condition signals may be compensated for by these PTMs [40] DNA regulated by the NAPs (HU and Fis) directly or indirectly contributes to the symmetrical wave pattern of the base-pair substitution (BPS) rate [41] HU belongs to a family of DNA architectural proteins and acts mostly as a regulatory or accessory factor, stabilizing a correct nucleoprotein complex [42, 43, 44] For example, HU introduces negative supercoiling in covalently closed circular DNA in the presence of topoisomerase I [45, 46] Negative supercoiling induces significant looping under any appreciable tension [47] On the other hand, HU binds preferentially to negatively supercoiled DNA than to relaxed DNA, and the tetrameric form is the predominant conformation of the protein on supercoiled DNA [48]. HU remodels nucleoids during cell growth and environmental adaptation by promoting the formation of a condensed core surrounded by less-condensed isolated domains. It serves as a general microbial mechanism for transcriptional regulation to synchronize genetic responses during the cell cycle and to adapt to changing environments. [49] The negative supercoiling of DNA in Fis, H-NS, and HU transcription factors modulates their 3D shape and determines their molecular dynamics [50] The circular supercoiled DNA dynamically folds in particular higher-order structures (hyperplectonemes) [50] HU binds DNA without sequence specificity and with low affinity, but at low concentrations HU induces bends and at high concentrations HU induces the formation of rigid filaments [9, 27, 51, 52, 53]. The binding of this protein is destabilized when the tension of the double helix increases [54] but the presence of Mg²⁺ enhances the binding of HU to DNA with high tension and the protein enhances the stability of the double-stranded DNA under overstretching [55]. In addition, Mg²⁺ increases the mobility of HU on the DNA [55]. Relationships between HU/IHF sequence, DNA-binding properties, and other protein features were identified based on sequence comparisons of thousands of known histone-like proteins from diverse bacterial phyla, in addition to a comparative modeling that demonstrated that HU protein 3D folding is even more conservative than the HU sequence [56] The effects of HU protein enhancements on DNA flexibility and the cyclization rate have been determined, based on a Monte Carlo approach, by Czapla et al. [57] Based on studies of single-molecule tracking, it was determined that on average at any time, 23% of HU molecules are in long-lived DNA-bound complexes at target sites, 77% are transiently, nonspecifically DNA-bound, and only 0.4% are in free diffusion between DNA strands; a diffusion coefficient of 0.33 μm²s^-1 was calculated [58]. The HU sliding is largely regulated by DNA bending dynamics [59] Based on the modified Pearson correlation coefficient (MPCC), a significant positive correlation between the RNAP and HU experimental distributions is robust [60] The Hu-α subunit plays significant and positive roles in promoting mixing and relaxation of the chromosome [61] Exogenous HU is able to bind to lipopolysaccharides of the cellular outer membrane and to extracellular DNA through the same motifs of the protein [62]. This binding facilitates a clumping of bacterial cells, a role that appears to be useful in biofilm formation [62]. The expression of hupA decreased in an rstA mutant and influenced the replication. RstA might affect initiation of replication through regulating the expression of the α-subunit of HU [63] Review: [64]