Leafy gall formation is controlled by fasR, an AraC-type regulatory gene in Rhodococcus fascians

Rhodococcus fascians can interact with many plant species and induce the fo rmation of either leafy galls or fasciations. To provoke symptoms, R. fasci ans strain D188 requires pathogenicity genes that are located on a linear p lasmid, pFiD188. The fas genes are essential for virulence and constitute a n operon that encodes, among other functions, a cytokinin synthase gene, Ex pression of the fas genes is induced by extracts of infected plant tissue o nly. We have isolated an AraC-type regulatory gene,fasR, located on pFiD188 , which is indispensable for pathogenesis and for fas gene expression The c ombined results of our experiments show that in vitro expression of the fas genes in a defined medium is strictly regulated and that several environme ntal factors (PH, carbon and nitrogen sources, phosphate and oxygen content , and cell density) and regulatory proteins are involved, We further show t hat expression of the fas genes is controlled at both the transcriptional a nd the translational levers. The complex expression pattern probably reflec ts the necessity of integrating a multitude of signals and underlines the i mportance of the fas operon in the pathogenicity of R. fascians.