DNA supercoiling and the leu-500 promoter mutation of Salmonella typhimurium.

DNA supercoiling is an important, but relatively poorly understood factor which influences promoter function. leu-500 is a point mutation in the promoter of the leucine operon of Salmonella typhimurium which confers leucine auxotrophy. It can be phenotypically suppressed by mutations in the topA gene, which encodes topoisomerase I, implicating DNA supercoiling in the regulation of this promoter. We have demonstrated that phenotypic suppression of this mutant promoter is transcriptional, and that topA mutations restore function to the mutant promoter. Transcription from the leu-500 promoter was examined in a series of strains harbouring topA and tos (presumptive gyr) mutations, each of which exhibits a different level of in vivo plasmid supercoiling. Promoter function did not correlate with the level of supercoiling but rather with the presence or absence of a functional topA gene. Furthermore, when cloned onto a multicopy plasmid, the leu-500 promoter failed to function, even in a topA background. Thus, local rather than global changes in DNA topology are implicated in the activation of this promoter.