THE INFLUENCE OF DNA TOPOLOGY ON THE ENVIRONMENTAL-REGULATION OF A PH-REGULATED LOCUS IN SALMONELLA-TYPHIMURIUM

Salmonella typhimurium is exposed to major shifts in H+ concentration both in its natural and pathogenic environments. The organism undergoe s extensive changes in gene expression in response to these pH fluctua tions. A current question of regulatory biology is how a change in ext ernal pH selectively modulates transcription. We have analysed the exp ression of one such pH-regulated locus, aniG, and found it is controll ed by several additional environmental conditions including osmolarity and oxygen. For factors such as osmolarity and anaerobiosis, an envir onmentally triggered change in DNA supercoiling has been suggested as a means for controlling gene expression. Thus, environmentally induced changes in DNA topology were explored as a possible common means for establishing the multiple controls on aniG. The involvement of DNA sup ercoiling in the genetic response of S. typhimurium to external pH has not previously been defined. This report establishes that alkaline en vironments lower the linking number of reporter plasmids when compared to acidic environments. A consistent pattern was then established whe reby conditions or mutations leading to either increased or decreased negative supercoiling were associated with altered expression of aniG. A similar relationship was observed for another environmentally regul ated locus, proU. The DNA topology effects on aniG expression were dep endent on the presence of EarA, the negative regulator of aniG. These data can be explained by a model in which repressor-operator interacti ons are very sensitive to changes in operator conformation. These envi ronmentally induced topological influences on operator DNA structure c ontribute to the magnitude of pH control exerted upon aniG.