TorR/TorS Two-Component system resists extreme acid environment by regulating the key response factor RpoS in Escherichia coli

Response to acid stress is critical for Escherichia coli to successfully complete its life-cycle. Acid resistance is an indispensable mechanism that allows neutralophilic bacteria, such as E. coli, to survive in the gastrointestinal tract. Escherichia coli acid tolerance has been extensively studied over the past decades, and most studies have focused on mechanisms of gene regulation. Bacterial two-component signal transduction systems sense and respond to external environmental changes through regulating genes expression. However, there has been little research on the mechanism of the TorR/TorS system in acid resistance, and how TorR/TorS regulate the expression of acid-resistant genes is still unclear. We found that TorR/TorS deletion in E. coli cells led to a growth defect in extreme acid conditions, and this defect might depend on the nutritional conditions and growth phase. TorS/TorR sensed an extremely acidic environment, and this TorR phosphorylation process might not be entirely dependent on TorS. RNA-seq and RT-qPCR results suggested that TorR regulated expressions of gadB, gadC, hdeA, gadE, mdtE, mdtF, gadX, and slp acid-resistant genes. Compared with wild-type cells, the stress response factor RpoS levels and its expressions were significantly decreased in & UDelta; torR cells stimulated by extreme acid. And under these circumstances, the expression of iraM was significantly reduced to 0.6-fold in & UDelta; torR cells. Electrophoretic mobility shift assay showed that TorR-His6 could interact with the rpoS promoter sequence in vitro. beta-galactosidase activity assay results approved that TorR might bind the rpoS promoter region in vivo. After the mutation of the TorR-box in the rpoS promoter region, these interactions were no longer observed. Taken together, we propose that TorS and potential Hanks model Ser/Thr kinase received an external acid stress signal and then phosphorylated TorR, which guided the expressions of a variety of acid resistance genes. Moreover, TorR coped with extreme acid environments through RpoS, levels of which might be maintained by IraM. Finally, TorR may confer E. coli with the ability to resist gastric acid, allowing the bacterium to reach the surface of the terminal ileum and large intestine mucosal epithelial cells through the gastric acid barrier, and establish colonization and pathogenicity.