DNA binding ability of histone-like protein HPhA is negatively affected by interaction with Pb2+

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(2+), Ca(2+), Zn(2+), Pb(2+) for their effect on the recombinant HPhA (rHPhA)-DNA binding. We found that only Pb(2+) was able to reduce the formation of rHPhA-DNA complex using gel mobility shift assays. Equilibrium dialysis showed that Pb(2+) bound to rHPhA by a 2:1 ratio. The interaction of Pb(2+) 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(2+) 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(2+) 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(2+). Synchronous fluorescence spectrometry analysis revealed that Pb(2+) was able to affect the polarity of the amino acids that near the Trp and Tyr residues. These results together showed that Pb(2+) interact with the recombinant rHPhA and this interaction negatively affect the ability of rHPhA to form complex with DNA molecules.