Differential effects of heavy metals on the binding of Xenopus upstream binding factor(xUBF) to DNA

Xenopus upstream binding factor (xUBF) is a transcription factor for RNA polymerase I which contains multiple DNA-binding motifs. Among these DNA-binding motifs,HMG box I is essential for promoting RNA polymerase I-dependent rRNA gene transcription. Gel shift assay indicated that the binding of recombinant HMG box I to a 136-bp linear DNA probe was significantly inhibited by Cd2+ at 1 μM.The formation of larger protein-DNA complexes was particularly sensitive to Cd2+.The intereaction between HMG box I and DNA was completely inhibited by 10 μM of Cd2+,yet this intereaction was not inhibited by the same concentration of Ca2+. Hg2+ at 0.1μM began to cause abnormal band shifting, and protein-DNA bands disappeared to the wells of a polyacrylamide gel in the presence of 10 μM of Hg2+, reflecting that a drastic change in the conformation of HMG box I-DNA had occurred. The binding of HMG box I to DNA was slightly disturbed by As3+ at 1 μM and was significantly affected at 10 μM.Our results suggest that inhibition of the normal binding of UBF to its target DNA may be one of the mechanisms of heavy metal-induced inhbbtion of RNA synthesis.