Alternative sigma factor RpoN and its modulation protein YhbH are indispensable for Erwinia amylovora virulence.

In Erwinia amylovora, ECF (extracytoplasmic functions) alternative sigma factor HrpL regulates the transcription of hrp ( hypersensitive response and pathogenicity)-type III secretion system ( T3SS) genes by binding to a consensus sequence known as the hrp box in hrp gene promoters. In turn, the expression of hrpL has been proposed to be positively controlled by alternative sigma factor 54 (σ54) ( RpoN) and HrpS, a member of the σ54 enhancer-binding proteins ( EBPs). However, the function of RpoN has not been characterized genetically in E. amylovora. In this study, we investigated the role of RpoN, a nitrogen limitation sigma factor, and its modulation protein YhbH, a novel ribosome-associated protein, in E. amylovora virulence. Our results showed that mutations in hrp S, hrp L, rpo N and yhbH, but not yfiA and rmf3, resulted in a nonpathogenic phenotype on immature pear fruits and apple shoots. Consistently, the expression of T3SS genes, including hrp L, dsp E, hrp N and hrp A, was barely detected in hrp S, hrp L, rpo N and yhbH mutants. These mutants were also not capable of eliciting a hypersensitive response ( HR) on tobacco; however, the overexpression of hrpL using an inducible promoter rescued the HR-eliciting abilities of these mutants. These results suggest that a sigma factor cascade exists in the regulatory networks of E. amylovora and regulates important virulence factors. On the basis of this study and previously reported data, a model is proposed for the regulation of T3SS in E. amylovora. [ABSTRACT FROM AUTHOR]