PCR AMPLIFICATION OF THE FAS-1 GENE FOR THE DETECTION OF VIRULENT-STRAINS OF RHODOCOCCUS FASCIANS

Thirty-six isolates of the fasciation-inducing bacterium Rhodococcus f ascians were evaluated for the presence and location of the fas-1 gene , which codes for an isopentenyl transferase, the committed step in cy tokinin biosynthesis. The presence of fas-1 was determined by PCR usin g a set of primers to the most conserved regions of the gene and by So uthern hybridization to genomic digests using the PCR fragment as a pr obe. Both methods revealed the presence of the gene in 18 virulent iso lates and the absence of the gene in 18 avirulent isolates. Thus, ther e is a strong relationship between the presence of the gene and virule nce of the organism. The location of fas-1 was determined by probing b lots of linear and circular DNA. For most of the virulent isolates, th e gene was localized to a 200+/-10 kb linear plasmid. Three virulent i solates lacked a plasmid of this size, but contained fas-1 either on a linear plasmid of 130 kb or on a large circular plasmid.