Here, we have investigated the possible effect of UV-B light on the folding/unfolding properties and stability of Arabidopsis thaliana MYB4 (AtMYB4) transcription factor in vitro by using biophysical approaches. Urea-induced equilibrium unfolding analyses have shown relatively higher stability of the wild-type recombinant AtMYB4 protein than the N-terminal deletion forms after UV-B exposure. However, as compared to wild-type form, AtMYB4Δ2 protein, lacking both the two N-terminal MYB domains, showed appreciable alteration in the secondary structure following UV-B exposure. UV-B irradiated AtMYB4Δ2 also displayed higher propensity of aggregation in light scattering experiments, indicating importance of the N-terminal modules in regulating the stability of AtMYB4 under UV-B stress. DNA binding assays have indicated specific binding activity of AtMYB4 to a putative MYB4 binding motif located about 212 bp upstream relative to transcription start site of AtMYB4 gene promoter, while relatively weak DNA binding activity was detected for another putative MYB4 motif located at -908 bp in AtMYB4 promoter. Gel shift and fluorescence anisotropy studies have shown increased binding affinity of UV-B exposed AtMYB4 to the promoter proximal MYB4 motif. ChIP assay has revealed binding of AtMYB4 to the promoter proximal (-212 position) MYB4 motif (ACCAAAC) in vivo. Docking experiments further revealed mechanistic detail of AtMYB4 interaction with the putative binding motifs. Overall, our results have indicated that the N-terminal 62–116 amino acid residues constituting the second MYB domain plays an important role in maintaining the stability of the C-terminal region and the overall stability of the protein, while a promoter proximal MYB-motif in AtMYB4 promoter may involve in the regulation of its own expression under UV-B light.
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