Sequence Analysis of α-helix-turn-α-helix Motif Region of HrpX in Plant Pathogenic Xanthomonas

The carboxyl terminal half of the HrpX regulatory protein in 33 strains among 14 pathovars of the Xanthomonos species carries two potential α-helix-turn-α-helix (HTH) motifs characteristic of AraC/XylS regulatory proteins. The putative amino acid sequence in the first HTH was completely identical, and an amino acid substitution was observed in a few strains in the second HTH. These findings strongly suggest that the first HTH recognizes an identical target sequence in the cognate promoter, and the second HTH makes specific contact with a nearly identical target sequence responsible for the transcriptional machinery involved in hrp genes.