The phylogenetic distribution of a transposable dioxygenase from the Niagara River watershed


  • Professor Campbell Wyndham is Director of the Institute of Biology, Carleton University. The primary goal of the research carried out in his laboratory is to understand how bacterial populations respond to environmental contaminants. This paper resulted from the integration of several studies by students working on different aspects of a long-term Hyde Park chemical landfill project. Cindy Nakatsu is now Assistant Professor at Purdue University. Roberta Fulthorpe is now Assistant Professor at Scarborough College, University of Toronto. Barbara Holland contributed to this work as an undergraduate student and later as a technical assistant. Michelle Peel is currently completing her Ph.D. on the distribution of different chlorobenzoate and biphenyl catabolic genotypes at Hyde Park.

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Horizontal gene transfer in the Bacteria has been demonstrated to occur under natural conditions. The ecological impact of gene transfer events depends on the new genetic material being expressed in recipient organisms, and on natural selection processes operating on these recipients. The phylogenetic distribution of cbaAB genes for chlorobenzoate 3,4-(4,5)-dioxygenase, which are carried within Tn5271 on the IncPβ plas-mid pBRC60, was investigated using isolates from freshwater microcosms and from the Niagara River watershed. The latter included isolates from surface water, groundwater and bioremediation reactor samples. The cbaAB genes have become integrated, through interspecific transfer, primarily into species of the β Proteobacteria (44/48 isolates). Only four isolates, identified as Pseudomonas fluorescens (3/48) and Xanthomonas maltophilia (1/48), belonged to the Γ Proteobacteria, despite the observation that pBRC60 was capable of mobilizing these genes into a wide range of β and Γ Proteobacteria in the laboratory. The natural host range correlated with the distribution of the meta-ring-fission pathway for metabolism of protocatechuates formed when the cbaAB genes were expressed (45/48 isolates). We proposed the hypothesis that natural selection has favoured recipients that successfully integrate the activity of the transferred dioxygenase with the conserved meta ring-fission pathway. The hypothesis was tested by transferring a plasmid construct containing the cbaAB genes into type strains representative of the β and γ Proteobacteria. The concept of applying mobile catabolic genes to probe the phylogenetic distribution of compatible degradative pathways is discussed.