Comparative genomics unravels mechanisms of genetic adaptation for the catabolism of the phenylurea herbicide linuron in Variovorax

Biodegradation of the phenylurea herbicide linuron appears a specialization within a specific clade of the Variovorax genus. The linuron catabolic ability is likely acquired by horizontal gene transfer but the mechanisms involved are not known. The full genome sequences of six linuron degrading Variovorax strains isolated from geographically distant locations were analyzed to acquire insight in the mechanisms of genetic adaptation towards linuron metabolism in Variovorax. Whole genome sequence analysis confirmed the phylogenetic position of the linuron degraders in a separate clade within Variovorax and indicated their unlikely origin from a common ancestral linuron degrader. The linuron degraders differentiated from non-degraders by the presence of multiple plasmids of 20 to 839 kb, including plasmids of unknown plasmid groups. The linuron catabolic gene clusters showed (i) high conservation and synteny and (ii) strain-dependent distribution among the different plasmids. All were bordered by IS1071 elements forming composite transposon structures appointing IS1071 as key for catabolic gene recruitment. Most of the strain carried at least one broad host range plasmid that might have been a second instrument for catabolic gene acquisition. We conclude that clade 1Variovorax strains, despite their different geographical origin, made use of a limited genetic repertoire to acquire linuron biodegradation.