The att locus of Rhodococcus fascians strain D188 is essential for full virulence on tobacco through the production of an autoregulatory compound

The ability of Rhodococcus fascians strain D188 to provoke leafy gall forma tion on a variety of plant species is correlated with the linear plasmid pF iD188, on which different pathogenicity loci were identified. The att locus affects the severity of symptom development on tobacco, whereas the fas lo cus is essential for virulence. To gain insight into the function of the at t locus, sequence and expression analyses were performed. The att locus con tains nine open reading frames homologous to arginine and beta -lactam bios ynthetic genes. aft gene expression is transcriptionally induced by leafy g all extracts, but not by extracts of uninfected plants, and depends on the attR gene that encodes a LysR-type transcriptional regulator. The att locus proves to be essential for the formation of inducing factors (IFs) that ar e present in gall extracts. Because the induction of the las locus also req uires the presence of IFs in gall extracts, the att locus is proposed to pl ay an important role in regulating the expression of the virulence loci of R. fascians.