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 ele ments, 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 e lement. One copy is integrated into the 3' end of the tRNA-ser(GCU) gene, c lose to the aspartokinase (askA) gene, and the second copy is integrated in to the 3' end of the tRNA-ser(GGA) gene, next to the polynucleotide phospho rylase (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. no dosus strains, both benign and virulent, which were examined, and was integ rated into tRNA-ser(GCU) in most strains. Between the askA and tRNA-ser(GCU ) genes, a gene (designated glpA), was identified whose predicted protein p roduct 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 virul ent 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. B y contrast, all but one of the benign strains had intB at one or both of th ese 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 r esulted 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 o f glpA, pnpA, tRNA-ser(GCU) and tRNA-ser(GGA).