Concluding Remarks

The comparison of linear catabolic plasmids sequenced so far with related replicons suggests that many linear plasmids are mosaic-like structures that may have resulted from a series of insertions, deletions, transpositions, and recombination events. The observation that putative IS elements or IS-related genetic structures are often associated with catabolic gene clusters supports this hypothesis. Very similar catabolic gene clusters, and thus metabolic pathways, are often shared by phylogenetically different bacteria, suggesting that mobile genetic elements (transferable plasmids, transposons, "genomic islands") have been distributed by horizontal DNA transfer. Horizontal gene transfer likely is a major factor promoting the dispersal of catabolic capabilities and the evolution of bacterial degradation pathways.

Acknowledgements The financial support of our work on linear Arthrobacter plasmid pAL1 by the Deutsche Forschungsgemeinschaft (FE 383/11-1) is gratefully acknowledged.

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