NATURAL HORIZONTAL TRANSFER OF A NAPHTHALENE DIOXYGENASE GENE BETWEENBACTERIA NATIVE TO A COAL TAR-CONTAMINATED FIELD SITE

Horizontal transfer of genes responsible for pollutant biodegradation may play a key role in the evolution of bacterial populations and the adaptation of microbial communities to environmental contaminants. How ever, field evidence for horizontal gene transfer between microorganis ms has traditionally been very difficult to obtain. In this study, the sequences of the 168 rRNA and naphthalene dioxygenase iron-sulfur pro tein (nahAc) genes of nine naphthalene-degrading bacteria isolated fro m a coal tar waste-contaminated site, as well as a naphthalene-degradi ng bacterium from a contaminated site in Washington state and two arch etypal naphthalene-degrading strains, were compared. Seven strains fro m the study site had a single nahAc allele, whereas the 168 rRNA gene sequences of the strains differed by as much as 7.9%. No nahAc alleles from the site were identical to those of the archetypal strains, alth ough the predominant allele was closely related to that of Pseudomonas putida NCIB 9816-4, isolated in the British Isles. However, one site- derived nahAc allele was identical to that of the Washington state str ain. Lack of phylogenetic congruence of the nahAc and 16S rRNA genes i ndicates that relatively recent in situ horizontal transfer of the nah Ac gene has occurred, possibly as a direct or indirect consequence of pollutant contamination. Alkaline lysis plasmid preparations and pulse d-field gel electrophoresis have revealed the presence of plasmids ran ging in size from 70 to 88 kb in all site isolates. Southern hybridiza tions with a 407-bp nahAc probe have suggested that the nahAc gene is plasmid borne in all the site isolates but one, a strain isolated from subsurface sediment 400 m upstream from the source of the other site isolates. In this strain and in the naphthalene-degrading strain from Washington state, nahAc appears to be chromosomally located. In additi on, one site isolate may carry nahAc on both chromosome and plasmid. W ithin the group of bacteria with identical nahAc sequences, the Southe rn hybridizations showed that the gene was distributed between plasmid s of different sizes and a chromosome. This suggests that plasmid modi fication after transfer may have been effected by transposons. Horizon tal transfer of catabolic genes may play a significant role in the acc limation of microbial communities to pollutants.