Escherichia coli tat mutant strains are able to transport maltose in the absence of an active malE gene

The twin-arginine transport (Tat) system is a prokaryotic protein transport system. Escherichia coli mutants in this pathway show a defect in cell separation during cell division, resulting in destabilization and permeability of the outer membrane. Maltose uptake is catalysed by a membrane-bound transporter of the ATP binding cassette (ABC) superfamily, where MalE is the essential periplasmic binding protein component. Here, we report that tat mutants are unexpectedly able to transport maltose in the absence of malE. This observation is specific to the MalE component since co-inactivation of malF, which encodes one of the channel components of the transporter, completely abolishes maltose transport even when the Tat system is inactivated. Genetic repair of the outer membrane leaky phenotype of the tat mutant strain re-established the absolute requirement for MalE in maltose uptake. In addition, we demonstrate that phenotypic repair of the outer membrane defect of the tat strain can also be achieved chemically by the inclusion of high concentrations of calcium or magnesium in the growth medium.