THE PHYLOGENETIC DISTRIBUTION OF A TRANSPOSABLE DIOXYGENASE FROM THE NIAGARA RIVER WATERSHED

Horizontal gene transfer in the Bacteria has been demonstrated to occu r under natural conditions. The ecological impact of gene transfer eve nts depends on the new genetic material being expressed in recipient o rganisms, and on natural selection processes operating on these recipi ents. The phylogenetic distribution of cbaAB genes for chlorobenzoate 3,4-(4,5)-dioxygenase, which are carried within Tn5271 on the IncP bet a plasmid pBRC60, was investigated using isolates from freshwater micr ocosms and from the Niagara River watershed. The latter included isola tes from surface water, groundwater and bioremediation reactor samples . The cbaAB genes have become integrated, through interspecific transf er, primarily into species of the beta Proteobacteria (44/48 isolates) . Only four isolates, identified as Pseudomonas fluorescens (3/48) and Xanthomonas maltophilia (1/48), belonged to the Gamma Proteobacteria, despite the observation that pBRC60 was capable of mobilizing these g enes into a wide range of beta and Gamma Proteobacteria in the laborat ory. The natural host range correlated with the distribution of the me ta-ring-fission pathway for metabolism of protocatechuates formed when the cbaAB genes were expressed (45/48 isolates). We proposed the hypo thesis that natural selection has favoured recipients that successfull y integrate the activity of the transferred dioxygenase with the conse rved meta ring-fission pathway. The hypothesis was tested by transferr ing a plasmid construct containing the cbaAB genes into type strains r epresentative of the beta and Gamma Proteobacteria. The concept of app lying mobile catabolic genes to probe the phylogenetic distribution of compatible degradative pathways is discussed.