Dissemination of metaldehyde catabolic pathways is driven by mobile genetic elements in Proteobacteria

Bioremediation of metaldehyde from drinking water using metaldehyde-degrading strains has recently emerged as a promising alternative. A combined Next Generation and Third Generation sequencing approach was used to obtain full genomes for the model metaldehyde degraders Acinetobacter calcoaceticus E1 and Sphingobium CMET-H. In the former, IS91 and IS6-family insertion sequences (ISs) were found surrounding the metaldehyde-degrading gene cluster located in plasmid pAME76. This cluster was also located in plasmids in most metaldehyde-degrading ß- and ?-Proteobacteria that we analysed. For Sphingobium CMET-H sequence analysis suggested a phytanoyl-CoA family oxygenase as a candidate for metaldehyde degrading gene. Heterologous gene expression in E. coli alongside degradation tests verified its functional significance and the degrading gene was henceforth called mahS. It was found that mahS is hosted within the conjugative plasmid pSM1 and its genetic context indicates that a crossover between the metaldehyde and acetoin degradation pathways is plausible. Here, specific replicons and ISs responsible for maintaining and dispersing metaldehyde-degrading genes in a, ß and ?-Proteobacteria were identified and described. In addition, the first gene implicated in metaldehyde utilisation in an a-Proteobacteria was uncovered for which insights into a possible degradation pathway are provided.