DNA footprinting shows which sigma factor binds to a promoter in response to osmotic shock. The promoter for the osmY gene is recognized by σ38, a sigma factor that regulates gene expression in response to osmotic shock. (Other σ38-targeted genes behave similarly.) (a) Footprinting analysis shows that the promoter is bound by σ38. (b) When cells are subjected to osmotic shock, transcription of osmY increases. The graphs show the fraction of total transcripts that derive from either σ38 or σ70 promoters as a function of increasing osmotic shock.