Haloalkane-utilizing Rhodococcus strains isolated from geographically distinct locations possess a highly conserved gene cluster encoding haloalkane catabolism

The sequences of the 16S rRNA and haloalkane dehalogenase (dhaA) genes of f ive gram-positive haloalkane-utilizing bacteria isolated from contaminated sites in Europe, Japan, and the United States and of the archetypal haloalk ane-degrading bacterium Rhodococcus sp. strain NCIMB13064 were compared. Th e 16S rRNA gene sequences showed less than 1% sequence divergence, and all haloalkane degraders clearly belonged to the genus Rhodococcus. All strains shared a completely conserved dhaA gene, suggesting that the dhaA genes we re recently derived from a common ancestor. The genetic organization of the dhaA gene region in each of the haloalkane degraders was examined by hybri dization analysis and DNA sequencing. Three different groups could be defin ed on the basis of the extent of the conserved dhaA segment. The minimal st ructure present in all strains consisted of a conserved region of 12.5 kb, which included the haloalkane-degradative gene cluster that was previously found in strain NCIMB13064. Plasmids of different sizes were found in all s trains. Southern hybridization analysis with a dhaA gene probe suggested th at all haloalkane degraders carry the dhaA gene region both on the chromoso me and on a plasmid (70 to 100 kb). This suggests that an ancestral plasmid was transferred between these Rhodococcus strains and subsequently has und ergone insertions or deletions. In addition, transposition events and/or pl asmid integration may be responsible for positioning the dhaA gene region o n the chromosome. The data suggest that the haloalkane dehalogenase gene re gions of these gram-positive haloalkane-utilizing bacteria are composed of a single catabolic gene cluster that was recently distributed world-wide.