FIS MODULATES GROWTH PHASE-DEPENDENT TOPOLOGICAL TRANSITIONS OF DNA IN ESCHERICHIA-COLI

The Escherichia coli DNA-binding protein FIS serves as a DNA architect ural factor in two unrelated enzymatic reactions, the site-specific in version of DNA and transcriptional activation of stable RNA promoters. In both these processes, FIS facilitates the assembly and dynamic tra nsitions of two structurally distinct nucleoprotein complexes. We have proposed previously that, in these systems, FIS stabilizes writhed DN A microloops by binding at multiple helically phased sites in DNA. How ever, FIS also binds and bends DNA at many non-specific sites and, at its maximum levels in the early exponential phase, FIS could potential ly occupy a considerable part of the E. coil chromosome. Here, we show that fis affects growth phase-specific alterations in the supercoilin g level of DNA. Expression of fis accelerates the accumulation of mode rately supercoiled plasmids in stationary phase, which are stabilized by FIS after nutritional shift-up. In accordance with such a function, FIS modulates the relaxing and supercoiling activities of topoisomera ses in vitro in a way that keeps RNA in a moderately supercoiled state . Our results suggest that the primary role of FIS is to modulate chro mosomal dynamics during bacterial growth.