Association of newly discovered IS elements with the dichloromethane utilization genes of methylotrophic bacteria.

Dichloromethane (DCM) dehalogenases enable facultative methylotrophic bacteria to utilize DCM as sole carbon and energy source. DCM-degrading aerobic methylotrophic bacteria expressing a type A DCM dehalogenase were previously shown to share a conserved 4.2 kb BamHI DNA fragment containing the dehalogenase structural gene, dcmA, and dcmR, the gene encoding a putative regulatory protein. Sequence analysis of a 10 kb DNA fragment including this region led to the identification of three types of insertion sequences identified as IS1354, IS1355 and IS1357, and also two ORFs, orf353 and orf192, of unknown function. Two identical copies of element IS1354 flank the conserved 4.2 kb fragment as a direct repeat. The occurrence of these newly identified IS elements was shown to be limited to DCM-utilizing methylotrophs containing a type A DCM dehalogenase. The organization of the corresponding dcm regions in 12 DCM-utilizing strains was examined by hybridization analysis using IS-specific probes. Six different groups could be defined on the basis of the occurrence, position and copy number of IS sequences. All groups shared a conserved 5.6 kb core region with dcmA, dcmR, orf353 and orf192 as well as IS1357. One group of strains including Pseudomonas sp. DM1 contained two copies of this conserved core region. The high degree of sequence conservation observed within the genomic region responsible for DCM utilization and the occurrence of clusters of insertion sequences in the vicinity of the dcm genes suggest that a transposon is involved in the horizontal transfer of the DCM-utilization character among methylotrophic bacteria.