ESCHERICHIA-COLI TYRT GENE-TRANSCRIPTION IS SENSITIVE TO DNA SUPERCOILING IN ITS NATIVE CHROMOSOMAL CONTEXT - EFFECT OF DNA TOPOISOMERASE-IV OVEREXPRESSION ON TYRT PROMOTER FUNCTION

We have investigated the in vivo DNA supercoiling sensitivity of the E scherichia coil tRNA(1)(tyr) gene (tyrT) promoter in its normal chromo somal location. Here, the native tyrT promoter is found to be exquisit ely sensitive to mutations and to drugs which alter the level of DNA s upercoiling. We show that the response of the tyrT promoter to superco iling is qualitatively similar to that of a known supercoiling-sensiti ve tRNA gene promoter, hisR. Specifically, treatments which increase i n vivo DNA supercoiling levels enhance transcription of these tRNA gen es. Particularly striking is the strong enhancement of expression from both promoters by a transposon insertion mutation in the topA gene en coding DNA topoisomerase I. This phenotypic effect can be complemented by providing active topoisomerase I in trans from a recombinant plasm id. Interestingly, it can also be complemented by overexpression of th e genes encoding the subunits of DNA topoisomerase IV. We believe that this is the first demonstration that DNA topoisomerase IV can influen ce gene expression and it suggests that DNA topoisomerase I is partial ly redundant, at least in E. coli.