Abstract
In mesophilic prokaryotes, the DNA-binding protein HU participates in nucleoid organization as well as in regulation of DNA-dependent processes. Little is known about nucleoid organization in thermophilic eubacteria. We show here that HU from the hyperthermophilic eubacterium Thermotoga maritima HU bends DNA and constrains negative DNA supercoils in the presence of topoisomerase I. However, while binding to a single site occludes ∼35 bp, association of T. maritima HU with DNA of sufficient length to accommodate multiple protomers results in an apparent shorter occluded site size. Such complexes consist of ordered arrays of protomers, as revealed by the periodicity of DNase I cleavage. Association of TmHU with plasmid DNA yields a complex that is remarkably resistant to DNase I-mediated degradation. TmHU is the only member of this protein family capable of occluding a 35 bp nonspecific site in duplex DNA; we propose that this property allows TmHU to form exceedingly stable associations in which DNA flanking the kinks is sandwiched between adjacent proteins. We suggest that T. maritima HU serves an architectural function when associating with a single 35 bp site, but generates a very stable and compact aggregate at higher protein concentrations that organizes and protects the genomic DNA.
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