¹³C-NMR relaxation study on mobility of the DNA-binding arm of HU

The mobility of the DNA-binding arm of HU protein was studied by 13C-NMR spectroscopy. The correlation times τc of Phe47Cα in the body and Gly60Cα in the arm of HU were determined for HU and HU-DNA complex. The value of τc of Phe47Cα is 2-4 times larger than that of Gly60Cα irrespective of the presence or absence of DNA. These results show that Gly60Cα undergoes more rapid motion than Phe47Cα. The increase in correlation time on addition of DNA is greater for Gly60Cα than for Phe47Cα. This suggests that the addition of DNA influences more significantly the motion of Gly60Cα than that of Phe47Cα. These results are in accord with the X-ray result, in which the top part of the arm is not visible. Gly60Cα in the arm is thus more mobile than Phe47Cα in the body, and the mobility of Gly60Cα is reduced by the DNA binding.