Regulation of pel genes, major virulence factors in the plant pathogen bacterium Dickeya dadantii, is mediated by cooperative binding of the nucleoid-associated protein H-NS

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Dickeya dadantii is a pathogen infecting a wide range of plant species. Soft rot, the visible symptom, is mainly due to production of pectate lyases (Pels) that can destroy plant cell walls. Previously, we found that nucleoid-associated protein (NAP) H-NS is a key regulator of pel gene expression. The primary binding sites of this NAP have been determined here by footprinting experiments on the pelD gene, encoding an essential virulence factor. Quantitative analysis of DNAse I footprints and surface plasmon resonance imagery experiments further revealed that high-affinity binding sites initiate cooperative binding to establish the nucleoprotein structure required for gene expression silencing. Mutations in the primary binding sites resulted in reduction or loss of repression by H-NS. Overall, these data suggest that H-NS represses pelD, and by inference, other pel genes, by a cooperative binding mechanism, through oligomerization of H-NS molecules.