Catabolic Plasmids

Formost of this century agriculture and industry haveused a wide range of synthetic chemicals which appear to have no counterparts in nature. Many of these compounds, by their very nature and complexity, are resistant to degradation when released into soil, water and air. Unfortunately, the repeated use and release of such synthetics has become an everyday occurrence, resulting in widespread environmental pollution. Where degradation does occur it is carried out almost exclusively by microorganisms, predominantly bacteria. The complexity of these molecules is such that to degrade them to simple and harmless end products, many enzymatic steps are required. For many soil bacteria these degradative (catabolic) activities are encoded by large groups of genes clustered on the main chromosome or on catabolic plasmids. Such plasmids are usually large (80 to 4 200 kb) with one or more clusters of multicistronic transcriptional units, possessing up to 10–15 degradative genes. In many instances catabolic plasmids are selftransmissible by cell-to-cell contact (conjugation) and are transferred freely throughout diverse soil bacterial populations (broad host range), leading to novel combinations of degradative genes capable of degrading the most complex, the most recalcitrant and the most persistent of synthetic molecules. In some instances these catabolic gene clusters occur within transposable elements and therefore they move unfettered between plasmids and the main chromosome. Rearrangements of existing genes, the activities of transposable elements and the broad host range of catabolic plasmids provide powerful mechanisms for the evolution of bacteria capable of rapidly degrading and recycling mutagenic, carcinogenic and/or teratogenic chemicals such as dioxins, trichloroethylene (TCE), polychlorinated biphenyls (PCBs), polybrominated biphenyls (PBBs), dichlorodiphenyltrichloroethane (DDT), g1,2,3,4,5,6-hexachlorocyclohexane and phenoxyherbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The evolution and spread of bacterial genes involved in the degradation of xenobiotic molecules is a powerful and positive development in the fight against environmental pollution. Bacterial degraders