The Twin Arginine Transport System Appears To Be Essential for Viability in Sinorhizobium meliloti

ABSTRACT

The twin arginine transport (Tat) system is responsible for transporting prefolded proteins to the periplasmic space. The Tat pathway has been implicated in many bacterial cellular functions, including motility, biofilm formation, and pathogenesis and symbiosis. Since the annotation of Sinorhizobium meliloti Rm1021 genome suggests that there may be up to 94 putative Tat substrates, we hypothesized that characterizing the twin arginine transport system in this organism might yield unique data that could help in the understanding of twin arginine transport. To initiate this work we attempted a targeted mutagenesis of the tat locus. Despite repeated attempts using a number of different types of media, the attempts at mutation construction were unsuccessful unless the experiment was carried out in a strain that was merodiploid for tatABC. In addition, it was shown that a plasmid carrying tatABC was stable in the absence of antibiotic selection in a tat deletion background. Finally, fluorescence microscopy and live/dead assays of these cultures show a high proportion of dead and irregularly shaped cells, suggesting that the loss of tatABC is inversely correlated with viability. Taken together, the results of this work provide evidence that the twin arginine transport system of S. meliloti appears to be essential for viability under all the conditions that we had tested.