Interaction of DNA-binding protein HU from Bacillus stearothermophilus (HUBst) with coliphage T2 DNA was investigated by means of a single-duplex DNA chain visualization method using fluorescence microscopy. Fluorescence microscopic images of coliphage T2 DNA molecules were observed as a function of HUBst concentration. The average fluorescence image size of T2 DNA decreased with increase in HUBst concentration to a size comparable to that of a DNA globule induced by polyethylene glycol (PEG) and multivalent cation (MVC). The change to globule-like DNA proceeded gradually and monotonously, in contrast to the coil-globule transition of DNA induced by PEG and MVC. The histogram of the fluorescence image length was essentially a single-modal one throughout the process of conformational change. These results indicate that the process of shrinking of DNA from a random coil to a globule-like one is not of a transitional nature. The interaction of HUBst with DNA and the mechanism of shrinkage are concluded to be different from those of PEG-induced and MVC-induced coil-globule transition of DNA. (C) 2002 Elsevier Science.