The site-specific integration of genetic elements may modulate thermostable protease production, a virulence factor in Dichelobacter nodosus, the causative agent of ovine footrot

The Gram-negative anaerobe Dichelobacter nodosus is the causative agent of footrot in sheep. The authors have previously characterized two genetic elements, the intA (vap) and intB elements, which integrate into the genome of D. nodosus. In the virulent strain A198 there are two copies of the intA element. One copy is integrated into the 3' end of the tRNA-ser(GCU) gene, close to the aspartokinase (askA) gene, and the second copy is integrated into the 3' end of the tRNA-ser(GGA) gene, next to the polynucleotide phosphorylase (pnpA) gene. In this study, a new genetic element was identified in the benign strain C305, the intC element, integrated into the 3' end of the tRNA-ser(GCU) gene, next to askA. The intC element was found in most D. nodosus strains, both benign and virulent, which were examined, and was integrated into tRNA-ser(GCU) in most strains. Between the askA and tRNA-ser(GCU) genes, a gene (designated glpA), was identified whose predicted protein product has very high amino acid identity with RsmA from the plant pathogen Erwinia carotovora. RsmA acts as a global repressor of pathogenicity in E. carotovora, by repressing the production of extracellular enzymes. In virulent strains of D. nodosus the intA element was found to be integrated next to pnpA, and either the intA or intC element was integrated next to glpA. By contrast, all but one of the benign strains had intB at one or both of these two positions, and the one exception had neither intA, intB nor intC at one position. The loss of the intC element from the virulent strain 1311 resulted in loss of thermostable protease activity, a virulence factor in D. nodosus. A model for virulence is proposed whereby integration of the intA and intC genetic elements modulates virulence by altering the expression of glpA, pnpA, tRNA-ser(GCU) and tRNA-ser(GGA).