pFiD188, the Linear Virulence Plasmid of Rhodococcus fascians D188

Rhodococcus fascians is currently the only phytopathogen of which the virulence genes occur on a linear plasmid. To get insight into the origin of this replicon and into the virulence strategy of this broad-spectrum phytopathogen, the sequence of the linear plasmid of strain D188, pFiD188, was determined. Analysis of the 198,917 bp revealed four syntenic regions with linear plasmids of R. erythropolis, R. jostii, and R. opacus, suggesting a common origin of these replicons. Mutational analysis of pFi_086 and pFi_102, similar to cutinases and type IV peptidases, respectively, showed that conserved region R2 was involved in plasmid dispersal and pointed toward a novel function for actinobacterial cutinases in conjugation. Additionally, pFiD188 had three regions that were unique for R. fascians. Functional analysis of the stk and nrp loci of regions U2 and U3, respectively, indicated that their role in symptom development was limited compared with that of the previously identified fas, att, and hyp virulence loci situated in region U1. Thus, pFiD188 is a typical rhodococcal linear plasmid with a composite structure that encodes core functions involved in plasmid maintenance and accessory functions, some possibly acquired through horizontal gene transfer, implicated in virulence and the interaction with the host.