We have determined the entire nucleotide sequence of a 4.4 kbp fragment of pMOP, a plasmid involved in 4-methylphthalate catabolism in (formerly ) Pc701. Two complete ORFs were identified and termed and encodes a 4-methylphthalate permease which is a member of a superfamily of symport proteins found in both prokaryotes and eukaryotes. Functionality was assigned to MopB by detailed analysis of the predicted amino acid sequence, resulting in the identification of 12 hydrophobic membrane-spanning domains and motifs associated with this class of protein. An assay was developed to demonstrate MopB function in substrate uptake. Of 4-methylphthalate, 4-hydroxyisophthalate, benzoate, -toluate and phthalate, only uptake of 4-methylphthalate and phthalate was demonstrated, suggesting that two carboxyl groups in the position are essential for substrate recognition. The predicted protein MopA showed significant levels of homology to reductase proteins implicated in aromatic and aliphatic catabolism, and contained motifs recognized as binding the ADP and flavin moieties of FAD/NAD. Northern hybridization experiments determined that and are cotranscribed, but expression was only seen in cells grown on 4-methylphthalate and not in cells grown on closely related structural analogues, including phthalate. and may be situated at the 3′-terminus of a cistron about 10 kbp in size. The isolation and characterization of a 4-methylphthalate permease gene may lead to the identification of other permeases involved in bacterial biodegradation processes and possibly the construction of strains with enhanced degradative abilities.


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