Basic and Acidic Regions Flanking the HMG Domain of Maize HMGa Modulate the Interactions with DNA and the Self-Association of the Protein

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Abstract
The maize HMGa protein is a typical member of the family of plant chromosomal HMG1-like proteins. The HMG domain of HMGa is flanked by a basic N-terminal domain characteristic for plant HMG1-like proteins, and is linked to the acidic C-terminal domain by a short basic region. Various derivatives of the HMGa protein were expressed in Escherichia coli and purified. The individual HMG domain can functionally complement the defect of the HU-like chromatin-associated Hbsu protein in Bacillus subtilis. The basic N-terminal domain which contacts DNA enhances the affinity of the protein for linear DNA, whereas it has little effect on the structure-specific binding to DNA minicircles. The acidic C-terminal domain reduces the affinity of HMGa for linear DNA, but does not affect to the same extent the recognition of DNA structure which is an intrinsic property of the HMG domain. The efficiency of the HMGa constructs to facilitate circularization of short DNA fragments in the presence of DNA ligase is like the binding to linear DNA altered by the basic and acidic domains flanking the HMG domain, while the supercoiling activity of HMGa is only slightly influenced by the same regions. Both the basic N-terminal and the acidic C-terminal domains contribute directly to the self-association of HMGa in the presence of DNA. Collectively, these findings suggest that the intrinsic properties of the HMG domain can be modulated within the HMGa protein by the basic and acidic domains.