Single molecule tracking reveals the role of transitory dynamics of nucleoid-associated protein HU in organizing the bacterial chromosome

HU is the most conserved nucleoid-associated protein in eubacteria and has been implicated as a key player in global chromosome organization. The mechanism of HU-mediated nucleoid organization, however, remains poorly understood. Using single molecule tracking coupled with genetic manipulations, we characterized the dynamics of HU in live Escherichia coli cells. We found that native HU dimers bind and unbind chromosomal DNAs weakly and transitorily across the entire nucleoid volume but remain nucleoid-localized, reminiscent of random diffusion in a liquid phase-separated, membrane-less "macro-compartment" distinct from the remaining cytosol. Mutating three key surface lysine residues of HU nearly entirely abolished the weak and transitory interactions of HU with DNA and led to severe cell growth and DNA segregation defects, suggesting the importance of HUs interactions with chromosomal DNA mediated by the positively charged surface. A conserved proline residue important for recognizing bent and cruciform DNAs such as that in recombination intermediates, similarly abolished HUs rapid and transitory DNA interaction dynamics but had little impact on its apparent binding stability with nonspecific chromosomal DNAs. Interestingly, the proline residue appeared to be important for HU{beta} dimer formation as mutating this residue makes HU{beta} behave similarly to HU2 dimers. Finally, we find that while prior evidence has found HU capable of depositing nucleoid-associated noncoding RNAs onto cruciform DNA structures, deletion of these specific naRNAs or inhibition of global transcription had a relatively minor effect on HU dynamics irrespective altered nucleoid compaction. Our results suggest a model of chromosome organization mediated by weak, transient interactions of HU, a substantial deviation from nucleoid-like proteins such as histones. Such collective sum of the numerous weak, transitory binding events of HU with nonspecific chromosome DNAs could generates a "force" to maintain a dynamic, fluid nucleoid with enough flexibility to rapidly facilitate global topological processes such as replication or nucleoid segregation.