PCR amplification of the fas‐1 gene for the detection of virulent strains of Rhodococcus fascians

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Thirty‐six isolates of the fasciation‐inducing bacterium Rhodococcus fascians were evaluated for the presence and location of the fas‐1 gene, which codes for an isopentenyl transferase, the committed step in cytokinin biosynthesis. The presence of fas‐1 was determined by PCR using a set of primers to the most conserved regions of the gene and by Southern hybridization to genomic digests using the PCR fragment as a probe. Both methods revealed the presence of the gene in 18 virulent isolates and the absence of the gene in 18 avirulent isolates. Thus, there is a strong relationship between the presence of the gene and virulence of the organism. The location of fas‐1 was determined by probing blots of linear and circular DNA. For most of the virulent isolates, the gene was localized to a 200 ± 10 kb linear plasmid. Three virulent isolates lacked a plasmid of this size, but contained fas‐1 either on a linear plasmid of 130 kb or on a large circular plasmid.