ACTIVATION OF ESCHERICHIA-COLI RIBOSOMAL-RNA TRANSCRIPTION BY FIS DURING A GROWTH-CYCLE

rRNA transcription in Escherichia coli is activated by the FIS protein , which binds upstream of rrnp(1) promoters and interacts directly wit h RNA polymerase. Analysis of the contribution of FIS to nn transcript ion under changing physiological conditions is complicated by several factors: the wide variation in cellular FIS concentrations with growth conditions, the contributions of several other regulatory systems to rRNA synthesis, and the pleiotropy of fis mutations. In this report, w e show by in vivo footprinting and Western blot analysis that occupanc y of the rrnBp(1) FIS sites correlates with cellular levels of FIS. We find, using two methods of measurement (pulse induction of a FIS-acti vated hybrid promoter and primer extension from an unstable transcript made from rrnBp(1)), that the extent of transcription activation by F IS parallels the degree of FIS site occupancy and therefore cellular F IS levels. FIS activates transcription throughout exponential growth a t low culture density, but rrnp(1) transcription increases independent ly of FIS immediately following upshift, before FIS accumulates. These results support the model that FIS is one of a set of overlapping sig nals that together contribute to transcription from rrnp(1) promoters during steady-state growth.