Evolution of a chlorobenzene degradative pathway among bacteria in a contaminated groundwater mediated by a genomic island in Ralstonia

Abstract : Horizontal gene transfer is widely documented as one of the main mechanisms responsible for the evolution of new characteristics in microorganisms. Evidence for horizontal gene transfer comes mainly from three types of experiments: (i) laboratory 'mating' experiments with isolated pure cultures of microorganisms or deliberately released plasmid donating bacteria into more complex communities; (ii) studies of various types of mobile DNA and demonstration of the mechanistic details of transfer machinery; and (iii) indirectly inferred evidence from sequence comparisons among different isolated microorganisms. For example, Herrick eta/. ( 1997) studied the naphthalene degradation characteristics of bacterial isolates in the same area. By phylogenetic comparisons, they concluded that the allele for the nahAc gene in seven isolates was incongruent to that for 16S rRNA. suggesting recent horizontal transfer of the nah genes. Similarly, by comparing 16S rONA sequences and tfdA genes from bacteria degrading 2,4-dichlor9Phenoxyacet!c acid (2,4- D), it was concluded that (orne) tfdA genes must have been transferred by horizontal movement (McGowan eta/., 1998). Movement of genes for atrazine degradation was inferred from sequence comparisons of bacteria isolated at various geographic regions (de Souza eta/., 1998). Also, more or less accidentally, Peters eta/. (1997) discovered the occurrence of a plasmid for phenol degradation that was identical to a plasmid from a strain introduced deliberately into a river basin to speed up phenol degradation after industrial spills.