Mechanism of TonB-dependent transport system in Halomonas alkalicola CICC 11012s in response to alkaline stress.

Halomonas alkalicola CICC 11012s can grow at pH 12.5, the highest pH at which the organisms in the genus Halomonas can grow. Genomic analysis reveals that H. alkalicola adapts to alkaline stress using a variety of adaptive strategies; however, the detailed mechanism for its growth at high-alkaline conditions has not been elucidated. Therefore, in this study, the adaptations of H. alkalicola in response to extreme alkaline stress were investigated. A sharp decrease of alkaliphilic tolerance was observed in mutants E. coli ΔEctonB and H. alkalicola ΔHatonB. Expressions of the gene clusters encoding TonB-dependent transport system and iron complex transport system in H. alkalicola grown under extreme alkaline conditions were markedly up-regulated. We then compared the intracellular ionic iron content and iron-chelating ability of mutant strain with those of wild-type strain to understand the influence of TonB-dependent transport system on the alkaline responses. The results indicated that the presence of TonB-dependent transport system increased the alkaline tolerance of H. alkalicola grown at high-alkaline conditions, but had no effects when the strain was grown at neutral pH and low-alkaline conditions. Meanwhile, the presence of this system increased the transport and accumulation of ionic irons to maintain intracellular metabolic homeostasis, which in turn could increase the tolerance of the strain to extreme alkaline conditions. Based on the results, we established a model representing the interactions between TonB-dependent transport system, alkaline tolerance, and intracellular ionic iron that could help deepen the understanding of the alkaline response mechanism of alkaliphilic bacteria.