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A conserved sequence motif associated with transcription of avr genes was identified in the promoter regions of six Pseudomonas syringae pv. syringae Pss61 hrp operons. A 34-bp fragment carrying this motif was cloned from the HrpZ promoter region and was shown to confer HrpL-dependent promoter activity. Expression of pathogenicity and host range determinants in P. syringae strains is thus directed by the apparent alternate sigma factor HrpL.
Xiao et al. (Sun,) studied this question.