A natural single nucleotide mutation in the small regulatory RNA ArcZ of Dickeya solani switches off the antimicrobial activities against yeast and bacteria.

The necrotrophic plant pathogenic bacterium Dickeya solani emerged in the potato agrosystem in Europe. All isolated strains of D. solani contain several large polyketide synthase/non-ribosomal peptide synthetase (PKS/NRPS) gene clusters. Analogy with genes described in other bacteria suggests that the clusters ooc and zms are involved in the production of secondary metabolites of the oocydin and zeamine families, respectively. A third cluster named sol was recently shown to produce an antifungal molecule. In this study, we constructed mutants impaired in each of the three secondary metabolite clusters sol, ooc, and zms to compare first the phenotype of the D. solani wild-type strain D s0432-1 with its associated mutants. We demonstrated the antimicrobial functions of these three PKS/NRPS clusters against bacteria, yeasts or fungi. The cluster sol, conserved in several other Dickeya species, produces a secondary metabolite inhibiting yeasts. Phenotyping and comparative genomics of different D. solani wild-type isolates revealed that the small regulatory RNA ArcZ plays a major role in the control of the clusters sol and zms. A single-point mutation, conserved in some Dickeya wild-type strains, including the D. solani type strain IPO 2222, impairs the ArcZ function by affecting its processing into an active form.