Figure 1.

Reporter constructs for the isolation of class 2 and class 3 promoter mutants. (a) Class 2 transcription was assayed separately from class 3 transcription by putting FlhD₄C₂ and σ²⁸ expression under the control of inducible promoters. (b) When autoclaved chlortetracycline was added, flhDC was expressed from its tetracycline-inducible promoter and resulted in activation of class 2 transcription at 87% of the wildtype level. (c) When arabinose was added, fliA (σ²⁸) was expressed from its arabinose-inducible promoter and activated class 3 transcription at 162% of the wildtype level. Transcription was assayed using a lac-fusion inserted within the first or second genes of the flagellar operon.