DNA binding ability of histone-like protein HPhA is negatively affected by interaction with Pb

The histone-like protein (HPhA) is highly homologous to the eukaryotic histones and has the ability to bind to the DNA molecules. In this study, we tested divalent metal ions Mg, Ca, Zn, Pb for their effect on the recombinant HPhA (rHPhA)-DNA binding. Wefound that only Pb was able to reduce the formation of rHPhA-DNA complex using gel mobility shift assays. Equilibrium dialysis showed that Pb bound to rHPhA by a 2:1 ratio. The interaction of Pb and rHPhA was further studied by spectroscopic method including fluorescence, ultraviolet visible (UV-Vis) absorption, and circular dichroism (CD) spectroscopies. Fluorescent spectroscopy results suggested that Pb and rHPhA formed a complex that caused internal quenching of the fluorescence of the protein at the ground state, and the quenching is predominately static and mixed with dynamic quenching. UV-Vis absorption spectrum results showed Pb caused a slightly blue shift of the maximum absorption wavelength of rHPhA which indicated the reduction of the protein's hydrophobicity. The CD spectrum further indicated that a reduction of the α-helix content of rHPhA occurred upon binding to Pb. Synchronous fluorescence spectrometry analysis revealed that Pb was able to affect the polarity of the amino acids that near the Trp and Tyr residues. These results together showed that Pb interact with the recombinant rHPhA and this interaction negatively affect the ability of rHPhA to form complex with DNA molecules.