Dehalogenases in Soil Bacteria

A number of bacterial strains were isolated from soil, by direct plating and enrichment culture, which were able to grow on chlorinated alkanoic acids as the sole carbon and energy source. Sixteen of these isolates were divided into five groups on the basis of differences in their dehalogenase activities towards monochloroacetic acid (MCA), dichloroacetic acid (DCA), 2-monochloropropionic acid (2MCPA) and 2,2-dichloropropionic acid (22DCPA). Disc gel electrophoresis of crude extracts identified four distinct dehalogenases exhibiting different electrophoretic mobilities. The groups were characterized by the possession of one or more of these enzymes although one of the groups was split into two on the basis of the possession of two different dehalogenases. In some cases, dehalogenases with the same electrophoretic mobilities, from different isolates, appeared to be identical enzymes, while in others, enzymes from different isolates, with the same electrophoretic mobilities, had different substrate activity profiles. Representatives of each group were grown in continuous-flow culture with either MCA or 2MCPA as the growth-limiting substrate. In all but one instance the dehalogenase profiles were different from those of the same organism grown in closed culture and were seen to be dependent on both growth rate and the growth-limiting substrate. The same strains were seen to possess different plasmids with molecular weights ranging from 109 to 190 Md. Five plasmids were characterized on the basis of their restriction endonuclease digest patterns. Curing experiments indicated that these plasmids encoded for the dehalogenase activities and resistance towards tellurium or mercury, selenium and tellurium. The significance of these results in terms of the evolution of dehalogenase activity is discussed and a model presented which relates the required period of enrichment and the common dehalogenases expressed by each bacterial strain to the presence of the five plasmids.