The DNA supercoiling-sensitive expression of the Salmonella typhimurium his operon requires the his attenuator and is modulated by anaerobiosis and by osmolarity.

Bacterial cells possess a subset of genes whose expression correlates with changes in DNA supercoiling brought about by anaerobic growth and by growth at high osmolarity. It has been shown previously that expression of the histidine biosynthetic operon of Salmonella typhimurium is derepressed by relaxation of supercoiled DNA. Here, we confirm that a his::MudJ operon fusion in S. typhimurium can be induced by treatment with the DNA gyrase inhibitor novobiocin in a dose-dependent manner, and show that the level of derepression is higher in stationary phase than in mid-exponential phase cultures. Furthermore, expression of his is repressed by anaerobiosis and by osmolarity, two environmental parameters which increase the negative supercoiling of bacterial DNA. Novobiocin induction of his is also repressed by growing the cells either at high osmolarity or anaerobically. Both environmental repression and novobiocin induction of his require the his attenuator. In addition, derepression of his expression by novobiocin and its repression by anaerobiosis or osmolarity are independent of the stringent response gene, relA.