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 promoter (UP) element] both in vivo and in vitro. This UP element is contacted by sigma(28) holoenzyme bound at the flagellin promoter and binds the isolated alpha 2 subassembly of RNA polymerase. The UP element increases the affinity of RNA polymerase for the flagellin promoter and stimulates transcription when initiation is limited by the rate of RNA polymerase binding. Comparison with other promoters in the flagellar regulon reveals a bipartite architecture: the -35 and -10 elements confer specificity for sigma(28), while promoter strength is determined largely by upstream DNA sequences.