Catabolic linear plasmids have been found in various soil bacteria, particularly in rhodococci. The linear plasmids of actinobacteria with biodegrada-tive capacities are typical invertrons (Sakaguchi 1990), containing terminal inverted repeats and terminal proteins covalently bound to each 5' end. Some Gram-negative bacteria, e.g., the haloalkane degrader Xanthobacter au-totrophicus GJ10, were also reported to harbor linear catabolic plasmids, but it is not yet known whether proteins are attached to their ends, and the sequences of their telomeres also have not been investigated yet. Some other linear replicons of Gram-negative bacteria, such as N15 of Escherichia coli, are prophages which are not integrated into the bacterial chromosome but exist as linear plasmid molecules with covalently closed hairpin ends (Hert-wig 2007, in this volume); however, note that these DNA elements do not code for catabolic traits.

There is much more information on circular plasmids involved in the biodegradation of carbon compounds (especially xenobiotics) than on linear catabolic plasmids (for a review, see, e.g., Nojiri et al. 2004), but some of the catabolic plasmids mentioned in the literature have not been fully characterized, and it is not always clear whether they are circular or linear. Reports on catabolic plasmids have often focused on the characterization of degradative genes or gene clusters and the phenotypic properties conferred by these genes; however, recent analyses of the complete nucleotide sequences of linear catabolic plasmids have significantly enhanced our understanding of these replicons. Ongoing and future genomic approaches will further improve insight into the structure and evolution and—together with biochemical studies—into the function of linear plasmids. This review gives an overview of catabolic linear plasmids, with an emphasis on the organization and physiological role of catabolic gene clusters.

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