Mercury Inactivates Transcription and the Generalized Transcription Factor TFB in the Archaeon Sulfolobus solfataricus

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Abstract
Mercury has a long history as an antimicrobial agent effective against eukaryotic and prokaryotic organisms. Despite its prolonged use, the basis for mercury toxicity in prokaryotes is not well understood. Archaea, like bacteria, are prokaryotes but they use a simplified version of the eukaryotic transcription apparatus. This study examined the mechanism of mercury toxicity to the archaeal prokaryote Sulfolobus solfataricus. In vivo challenge with mercuric chloride instantaneously blocked cell division, eliciting a cytostatic response at submicromolar concentrations and a cytocidal response at micromolar concentrations. The cytostatic response was accompanied by a 70% reduction in bulk RNA synthesis and elevated rates of degradation of several transcripts, including tfb-1, tfb-2, and lacS. Whole-cell extracts prepared from mercuric chloride-treated cells or from cell extracts treated in vitro failed to support in vitro transcription of 16S rRNAp and lacSp promoters. Extract-mixing experiments with treated and untreated extracts excluded the occurrence of negative-acting factors in the mercury-treated cell extracts. Addition of transcription factor B (TFB), a general transcription factor homolog of eukaryotic TFIIB, to mercury-treated cell extracts restored >50% of in vitro transcription activity. Consistent with this finding, mercuric ion treatment of TFB in vitro inactivated its ability to restore the in vitro transcription activity of TFB-immunodepleted cell extracts. These findings indicate that the toxicity of mercuric ion in S. solfataricus is in part the consequence of transcription inhibition due to TFB-1 inactivation.