We have investigated the in vivo DNA supercoiling sensitivity of the Escherichia coli tRNA(1tyr) gene (tyrT) promoter in its normal chromosomal location. Here, the native tyrT promoter is found to be exquisitely sensitive to mutations and to drugs which alter the level of DNA supercoiling. We show that the response of the tyrT promoter to supercoiling is qualitatively similar to that of a known supercoiling-sensitive tRNA gene promoter, hisR. Specifically, treatments which increase in vivo DNA supercoiling levels enhance transcription of these tRNA genes. Particularly striking is the strong enhancement of expression from both promoters by a transposon insertion mutation in the topA gene encoding DNA toposisomerase I. This phenotypic effect can be complemented by providing active topoisomerase I in trans from a recombinant plasmid. Interestingly, it can also be complemented by overexpression of the genes encoding the subunits of DNA topoisomerase IV. We believe that this is the first demonstration that DNA topoisomerase IV can influence gene expression and it suggests that DNA topoisomerase I is partially redundant, at least in E. coli.
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