| Summary: | Bacteria make a huge contribution to the purification of the environment from toxic stable pollutants of anthropogenic and natural origin due to the diversity of their enzyme systems. For example, the ability to decompose 3-chlorobenzoate (3CBA) by the four representative genera of Actinobacteria, such as <i>Rhodococcus</i>, <i>Gordonia</i>, <i>Microbacterium</i>, and <i>Arthrobacter</i>, was studied. In most cases, the formation of 4-chlorocatechol as the only key intermediate during the decomposition of 3CBA was observed. However, <i>Rhodococcus opacus</i> strain 1CP was an exception, whose cells decomposed 3CBA via both 3-chloro- and 4-chlorocatechol. The enzyme 3-Chlorobenzoate 1,2-dioxygenase (3CBDO) induced during the growth of these bacteria in the presence of 3CBA differed significantly in substrate specificity from the benzoate dioxygenases induced upon growth in the presence of benzoate. The <i>R. opacus</i> 6a strain was found to contain genes encoding chlorocatechol 1,2-dioxygenase, chloromuconate cycloisomerase, and dienelactone hydrolase, whose nucleotide sequence was 100% consistent with the sequences of the corresponding genes encoding the enzymes of the modified 4-chlorocatechol <i>ortho</i>-cleavage pathway of the strain <i>R. opacus</i> 1CP. However, the gene encoding chloromuconolactone dehalogenase (<i>clcF</i>) was not found in the representatives of the actinomycete genera, including <i>Gordonia</i> and <i>Arthrobacter</i>. A linear mega-plasmid carrying 3-chlorocatechol degradation genes remained stable after maintaining the <i>R. opacus</i> 1CP strain on an agar-rich medium for 25 years. In general, a similar plasmid was absent in actinobacteria of other genera, as well as in closely related species of <i>R. opacus</i> 6a.
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