Abstract
The widely conserved twin-arginine translocases (Tat) allow the transport of fully folded cofactor-containing proteins across biological membranes. In doing so, these translocases serve different biological functions ranging from energy conversion to cell division. In the Gram-positive soil bacterium Bacillus subtilis, the Tat machinery is essential for effective growth in media lacking iron or NaCl. It was previously shown that this phenomenon relates to the Tat-dependent export of the heme-containing peroxidase EfeB, which converts Fe2+ to Fe3+ at the expense of hydrogen peroxide. However, the reasons why the majority of tat mutant bacteria perish upon dilution in NaCl-deprived medium and how, after several hours, a sub-population adapts to this condition was unknown. Here we show that, upon growth in the absence of NaCl, the bacteria face two major problems, namely severe oxidative stress at the membrane and starvation leading to death. The tat mutant cells can overcome these challenges if they are fed with arginine, which implies that severe arginine depletion is a major cause of death and resumed arginine synthesis permits their survival. Altogether, our findings show that the Tat system of B. subtilis is needed to preclude severe oxidative stress and starvation upon sudden drops in the environmental Na+ concentration as caused by flooding or rain.
Highlights
Bacterial twin-arginine translocation (Tat) pathways can translocate fully folded cargo proteins across the cytoplasmic membrane [1,2,89]
The re sults obtained by confocal fluorescence microscopy showed that the tat mutant bacteria remained viable during the early time points after dilution into Lysogeny Broth (LB) without NaCl, and that substantial numbers of dead bacteria were only detectable when the optical density readings at 600 nm (OD600) of the cultures with tat mutant bacteria started to decline at the entry into the lysis phase
Our present findings show that tat mutant B. subtilis cells that are diluted into a NaCl-free growth medium have to face two major problems
Summary
Bacterial twin-arginine translocation (Tat) pathways can translocate fully folded cargo proteins across the cytoplasmic membrane [1,2,89]. According to current models for protein translocation via the Tat pathway in bacteria and the thylakoidal membrane of chloroplasts, the TatA and TatC subunits of Bacillus collectively form a docking complex for cargo proteins [1]. The translocation process is initiated when a protein with the correct RRsignal peptide interacts with this docking complex. The docking complex proofreads the signal peptide and in serts the substrate into the membrane, which requires the recruitment of additional TatA subunits. These may either form a transmembrane pore or weaken the membrane, thereby facilitating translocation of the cargo protein [6,7]. Once the translocation process has been initiated, the signal peptide is proteolytically removed from the cargo protein by signal peptidase [11,12,13], allowing release of the translocated protein at the trans side of the membrane [14]
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