HU-GFP and DAPI co-localize on the Escherichia coli nucleoid.

The heterodimeric HU protein, one of the most abundant DNA binding proteins, plays a pleiotropic role in bacteria. Among others, HU was shown to contribute to the maintenance of DNA superhelical density in Escherichia coli. By its properties HU shares some traits with histones and HMG proteins. More recently, its specific binding to DNA recombination and repair intermediates suggests that HU should be considered as a DNA damage sensor. For all these reasons, it will be of interest to follow the localization of HU within the living bacterial cells. To this end, we constructed HU-GFP fusion proteins and compared by microscopy the GFP green fluorescence with images of the nucleoid after DAPI staining. We show that DAPI and HU-GFP colocalize on the E. coli nucleoid. HU, therefore, can be considered as a natural tracer of DNA in the living bacterial cell.