PROMOTER ARCHITECTURE IN THE FLAGELLAR REGULON OF BACILLUS-SUBTILIS -HIGH-LEVEL EXPRESSION OF FLAGELLIN BY THE SIGMA(D) RNA-POLYMERASE REQUIRES AN UPSTREAM PROMOTER ELEMENT

Flagellin is one of the most abundant proteins in motile bacteria, yet its expression requires a low abundance sigma factor (sigma(28)). We show that transcription from the Bacillus subtilis flagellin promoter is stimulated 20-fold by an upstream A+T-rich region [upstream promote r (UP) element] both in vivo and in vitro. This UP element is contacte d by sigma(28) holoenzyme bound at the flagellin promoter and binds th e isolated alpha 2 subassembly of RNA polymerase. The UP element incre ases the affinity of RNA polymerase for the flagellin promoter and sti mulates transcription when initiation is limited by the rate of RNA po lymerase binding. Comparison with other promoters in the flagellar reg ulon reveals a bipartite architecture: the -35 and -10 elements confer specificity for sigma(28), while promoter strength is determined larg ely by upstream DNA sequences.