The plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria possesses a type III secretion (T3S) system which is encoded in the 23-kb hypersensitive response and pathogenicity (hrp) gene cluster. The T3S system is essential for pathogenicity in susceptible hosts and the induction of the hypersensitive response in resistant plants. In this study, we revisited the operon structure of the right part of the hrp gene cluster. Based on complementation experiments of transposon insertions and reverse-transcription polymerase chain reaction analyses, the hrpD operon contains hrcQ, hrcR, hrcS, and hpaA, whereas hrcD, hrpD6, and hrpE belong to the hrpE operon. We determined the transcriptional start site of the hrpE operon and showed that there is a promoter upstream of hrcD containing a plant-inducible promoter box. Conserved secondary mRNA structures in the intergenic region between hrpD6 and hrpE suggest a posttranscriptional regulated expression of hrpE. Based on comparisons of different hrp gene clusters and the analysis of evolutionary rates, we propose that the hrpE transcriptional unit was integrated into the hrp gene cluster at a later time.
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