Characterization and transcriptional analysis of an ECF sigma factor from Xanthomonas campestris pv. campestris

The genomic DNA segment encoding the rpoE gene and its flanking region was cloned from Xanthomonas campestris pv. campestris strain 11 (Xc11). The transcriptional start site of rpoE was located at nucleotide G, which is 33 nucleotides preceding the putative translation initiation codon of rpoE, and a extracytoplasmic function sigma factors (sigma(E))-dependent promoter was identified with -35 (5'-GAACTT-3') and -10 (5'-TCTCA-3') consensus sequences. The protein encoded by rpoE gene acted as a sigma (sigma) factor and was sufficient to direct core RNA polymerase to the rpoE promoter and to stimulate initiation of transcription in vitro. The specific binding of the reconstituted Esigma(E) holoenzyme with the Xc11 rpoE promoter was demonstrated by gel retardation assay and DNAse I footprint analysis. This study clearly demonstrated that the rpoE-rseA-mucD genomic organization of X. campestris is similar to that found in Xylella fastidiosa; however, expression of rpoE in X. campestris is autoregulated by its own sigma(E)-dependent promoter.