Vol. 56, Issue 5, 982-988, November 1999

Lead Inhibition of DNA-Binding Mechanism of Cys₂His₂ Zinc Finger Proteins

Jay S. Hanas, Justin S. Rodgers, John A. Bantle,¹ and Yong-Gang Cheng

Department of Biochemistry and Molecular Biology, University of Oklahoma College of Medicine, Oklahoma City, Oklahoma

The association of lead with chromatin in cells suggests that deleterious metal effects may in part be mediated through alterations in gene function. To elucidate if and how lead may alter DNA binding of cysteine-rich zinc finger proteins, lead ions were analyzed for their ability to alter the DNA binding mechanism of the Cys₂His₂ zinc finger protein transcription factor IIIA (TFIIIA). As assayed by DNase I protection, the interaction of TFIIIA with the 50-bp internal control region of the 5S ribosomal gene was partially inhibited by 5 µM lead ions and completely inhibited by 10 to 20 µM lead ions. Preincubation of free TFIIIA with lead resulted in DNA-binding inhibition, whereas preincubation of a TFIIIA/5S RNA complex with lead did not result in DNA-binding inhibition. Because 5S RNA binds TFIIIA zinc fingers, this result is consistent with an inhibition mechanism via lead binding to zinc fingers. The complete loss of DNase I protection on the 5S gene indicates the mechanism of inhibition minimally involves the N-terminal fingers of TFIIIA. Inhibition was not readily reversible and occurred in the presence of an excess of -mercaptoethanol. Inhibition kinetics were fast, progressing to completion in ~5 min. Millimolar concentrations of sulfhydryl-specific arsenic ions were not inhibitory for TFIIIA binding. Micromolar concentrations of lead inhibited DNA binding by Sp1, another Cys₂His₂ finger protein, but not by the nonfinger protein AP2. Inhibition of Cys₂His₂ zinc finger transcription factors by lead ions at concentrations near those known to have deleterious physiological effects points to new molecular mechanisms for lead toxicity in promoting disease.