ABSTRACT
Mercury has harmful effects in both rodents and humans. In rodent tissue culture cells exposed to HgCl(2), the metal ions were observed to concentrate in cell nuclei and to associate with chromatin. Thus, transcription factors and other proteins associated with chromatin are possible targets of mercuric ion toxicity. In this study, mercuric ions were found to inhibit the DNA binding activity of the Cys(2)His(2) zinc finger proteins transcription factor IIIA (TFIIIA) and Sp1. These factors are prototypes of the largest eukaryotic protein superfamily. Neither the presence of excess zinc ions nor beta-mercaptoethanol prevented inhibition by mercuric ions. Mercuric ions also inhibited DNA binding by the non-zinc finger protein AP2. Zinc finger-DNA binding was inhibited when both TFIIIA/5S RNA complex and TFIIIA alone were preincubated with concentrations as low as 15 microM mercuric ion. Inhibition occurred in less than 1 min and was not readily reversible. Mercuric ions also inhibited the digestion of DNA by the restriction enzymes BamHI or EcoRI. Inhibition of transcription factors as well as potentially other DNA binding proteins by micromolar concentrations of mercuric ion suggests additional biochemical mechanisms for mercury toxicity in promoting disease via alterations in gene transcription patterns.
Subject(s)
DNA-Binding Proteins/metabolism , DNA/drug effects , Mercury/pharmacology , Transcription Factors/metabolism , Animals , DNA/metabolism , DNA Restriction Enzymes/metabolism , DNA-Binding Proteins/chemistry , Drug Interactions , Female , Ovary/drug effects , Ovary/physiology , RNA, Ribosomal, 5S/genetics , RNA, Ribosomal, 5S/metabolism , Sp1 Transcription Factor/metabolism , Transcription Factor AP-2 , Transcription Factor TFIIIA , Transcription Factors/chemistry , Xenopus Proteins , Xenopus laevis , Zinc/pharmacology , Zinc Fingers/drug effectsABSTRACT
The inhibition of ribosomal RNA (rRNA) maturation by 5-fluorouridine (FUrd) in Novikoff hepatoma cells appears to depend upon the incorporation of the analog into the 45 S rRNA precursor. Precursor synthesized in the presence of FUrd is not processed into mature rRNA, but precursor synthesized in the absence of the analog is processed normally after the addition of the drug. The effect of FUrd on rRNA maturation is concentration dependent. At a concentration of 1 X 10(-4)M, the analog completely inhibits the formation of mature 18 S and 28 S rRNA; while at a concentration of 1 X 10(-7)M, the analog has no significant effect on rRNA maturation. These results suggest that some minimum degree of analog substitution is necessary to inhibit the maturation process. In addition to its inhibition of maturation, FUrd also inhibits the transcription of 45 S rRNA precursor. However, this effect of the drug is less complete and more time dependent that the effect on maturation. The inhibition of rRNA maturation by FUrd persists after removal of the analog from the culture medium. Cells that had been exposed to FUrd for 2 hr were unable to process 45 S rRNA precursor 20 hr after removal of the drug from the medium.
