TonB-dependent transport systems (consisting of TonB-dependent transporters, TonB, ExbB, and ExbD) facilitate uptake of nutrients across the bacterial outer membrane. Tol-Pal systems contain components related to those of TonB systems. In this study, we investigated TonB-dependent transport systems in Helicobacter pylori. H. pylori genomes contain three tonB paralogs (designated tonB1, tonB2, and tonB3/tolA), three exbB paralogs, three exbD paralogs, six genes encoding putative TonB-dependent transporters, tolB, and pal. By using immunopurification and mass spectrometry methods, we show that TonB2, ExbB2, and ExbD2 co-purify with each other, consistent with the corresponding genes being in the same predicted operon. The tonB1 gene is not linked to exbB or exbD genes, but we show that ExbB3 and ExbD3 co-purify with TonB1. The tonB3/tolA gene is in a predicted operon containing exbB1/tolQ, exbD1/tolR, tolB, and pal. We show that TonB3/TolA co-purifies with ExbB1/TolQ and ExbD1/TolR, suggesting that TonB3/TolA is a component of the Tol-Pal system. We show that H. pylori tonB1 is essential for H. pylori growth, tonB2 mutants require supplemental iron for growth, and tonB3/tolA is non-essential for growth in vitro. ICP-MS experiments showed that a tonB2 mutant contains reduced levels of nickel compared to a wild-type strain, and correspondingly, the tonB2 mutant exhibits reduced urease activity. These results indicate that the TonB2 system has an important role in H. pylori acquisition of nickel and iron. We propose that the TonB1 system has a key role in the acquisition of multiple nutrients required for H. pylori viability.
Keywords: TonB; TonB-dependent transporter; essential genes; iron; metals; nickel; protein complex.