Role of the twin-arginine translocase (tat) system in iron uptake in Listeria monocytogenes.

The twin-arginine translocase (Tat) complex is a unique system that translocates folded proteins across the cytoplasmic membrane. In this study, the Tat transporter system in Listeria monocytogenes was characterized to determine the role of Tat in the iron uptake pathway. A putative tatAC operon, containing conserved Fur-binding sequences in the promoter region, has been predicted to encode Tat-translocase components. Another operon, fepCAB, with a putative Fur-binding sequence in the promoter, close to TatAC, was identified in the complementary strands of L. monocytogenes. Electrophoretic mobility shift assay showed that the listerial Fur-repressor binds to the promoter of the tatAC operon, suggesting that tat is under Fur regulation. Using a heterologous system in a reporter assay, FepB was translocated across the membrane. Mutations in tatC and fepB were constructed to determine the roles of Tat and FepB, respectively. In a whole-cell ferric reductase assay, the fepB and tatC mutants were found to have reduced levels of ferric reductase activities compared with those of the isogenic parent strain. Although ferric reductase activity has been demonstrated in Listeria, a conventional ferric reductase encoding sequence does not appear to be present in its genome. Hence, we propose that fepB encodes a ferric reductase enzyme, which is translocated by the Tat-translocase system onto the bacterial cell surface, and plays an important role in the reductive iron uptake process in L. monocytogenes.