TBDT or TonB dependant transporter (see [[Ton system]]) are outermembrane transporters that are often involved in uptake of nutrients but also utilised by [[colicins]], [[bacteriocin]] and phage's that rely on the [[Ton system]] to power their conformational change. the energy transfer is mediated by the N-terminal 5-7 residue TonB-box located on the globular plug domain. This TonB box can be located on different regions of the plug domain sometimes extending into the periplasm and other times tucked inside the beta-barrel which makes them difficult to see in many structural biology techniques. Upon ligand binding to the TonB dependant transporter, the TonB box becomes exposed and the interaction with TonB leads to conformational changes in the TBD transporter that are coupled to ligand transport across the OM. Some TBDTs contain a periplasmic signalling domain that is involved in Cell surface signalling and sigma/anti-sigma factors. Ligand binding to the transporter causes a signal to be transduced to the anti-sigma factor in the periplasm that interacts directly with the transporter, resulting in the release of the sigma factor from the cytoplasmic face of the inner membrane where it can go onto to influence gene expression. in many cases this signal results in proteolysis (such as by RseP) of the anti-sigma factor which is the driving force behind sigma factor release ![[20250727_233112 1.png|250]] ## **Iron chelation** iron is an extremely important part of bacterial cell survival and flourishing so it is a large target for disruption and development of antimicrobials. however not only do they enable their own virulence we can also use them to get bacteria to take up antimicrobials, which in the case of gram negative bacteria can be the hardest part. (see [[Maintenance of asymmetry, OM Biogenesis]]) **Xeno/siderophore uptake** Iron uptake in bacteria consists of two components the siderophores which chelate and bind iron with high affinity and the outermembrane pores that transport them. siderophores are small the iron chelating molecules produced by microorganisms themselves and xenosiderophores are siderophores that are produced by other species microbes that the examined species is able to take up. **FuhA** (AKA TonA) is a TonB-dependant transporter that is utilised by T1 phages however this adsorption (attachment) can only occur if the cell is energised / tonB is present as it is energy requiring. interestingly T5 phages can also use FuhA but the adsorption itself causes a conformational change in T5 that allows the insertion of genomic material (this means that loss of a proton gradient across the IM would prevent T1 phage adsorption but not T5). That is a bit of a side track. Ferrichrome (siderophore) is transported by FuhA **FepA** is another iron uptaking TonB dependant transporter thats that binds its siderophore **enterobactin** however it can also bind to colicin B and D. Colicins are toxic proteins that kills *E. coli* . when investigating *E. coli* that are resistant to colicins they found they generally have issues with the [[Ton system]] or FepA, interestingly as a side effect of this resistance they can not effectively take up iron which causes an overproduction of enterochelin because the gene is derepressed via the inhibition of fur (Braun et al., 2023) **[[FoxA]]** is another Iron uptaking TonB-dependant transporter found in P. aeruginosa. upon ligand binding a conformational change leads to the exposure of the TonB Box which allows for TonB binding and energisation of the transporter. a Thiopeptide has been shown to bind to FoxA and interact in a way that facilitates its uptake ## Cobalamin//B12 Bacteria need B12, also known as cobalamin, as it it is an important cofactor for a variety of essential enzymes and pathways such as Methionine synthase in *Escherichia coli* or ribonucleotide reductase in *Pseudomonas aeruginosa*. Synthesis of cobalamin is not something conserved across all bacteria and even in the ones that do synthesise it, it is not energetically favourable; synthesis of B12 requires around 30 enzymatic steps. To mitigate this cost (or lack of synthesis pathway) bacteria have evolved ways to take up B12 from their environment. The major way that gram-negative bacteria like *E. coli* do this is via **BtuB** an outermembrane TonB-dependant transporter. #### BtuB ![[1NQE.png|250]] PDB: 1NQE *Escherichia coli* extracellular loop: top BtuB forms the outer membrane TonB dependant transporter that takes up B12. It follows the typical TBDT structure of having a β-barrel, an inner plug domain and large extracellular loops. B12 uptake and the activities of BtuB is regulated by a adenosylcobalamin sensing [[Riboswitch]] whose conformational change results in repression of BtuB translation by preventing mRNA binding to the ribosome. Further to this is repression by small RNAs OmrA which binds to a different region of the mRNA preventing translation(Bastet et al., 2024). **References** - [Bastet, L., Korepanov, A.P., Jagodnik, J., Grondin, J.P., Lamontagne, A.-M., Guillier, M. and Lafontaine, D.A. (2024). Riboswitch and small RNAs modulate btuB translation initiation in Escherichia coli and trigger distinct mRNA regulatory mechanisms. _Nucleic acids research_, [online] 52(10), pp.5852–5865. doi:https://doi.org/10.1093/nar/gkae347.](https://academic.oup.com/nar/article/52/10/5852/7671312?login=false) - [Bastiaansen, K.C., Ibañez, A., Ramos, J.L., Bitter, W. and Llamas, M.A. (2014). The Prc and RseP proteases control bacterial cell-surface signalling activity. _Environmental Microbiology_, 16(8), pp.2433–2443. doi:https://doi.org/10.1111/1462-2920.12371.](https://enviromicro-journals.onlinelibrary.wiley.com/doi/full/10.1111/1462-2920.12371?saml_referrer) - [Braun, V., Ratliff, A.C., Celia, H. and Buchanan, S.K. (2023). Energization of Outer Membrane Transport by the ExbB ExbD Molecular Motor. _Journal of Bacteriology_, 205(6). doi:https://doi.org/10.1128/jb.00035-23.](https://pmc.ncbi.nlm.nih.gov/articles/PMC10294619/) - [Noinaj, N., Guillier, M., Barnard, T.J. and Buchanan, S.K. (2010). TonB-Dependent Transporters: Regulation, Structure, and Function. _Annual Review of Microbiology_, [online] 64(1), pp.43–60. doi:https://doi.org/10.1146/annurev.micro.112408.134247.](https://www.annualreviews.org/content/journals/10.1146/annurev.micro.112408.134247) - [Raux, E., Schubert, H.L. and Warren*, M.J. (2000). Biosynthesis of cobalamin (vitamin B12): a bacterial conundrum. _Cellular and Molecular Life Sciences_, 57(13), pp.1880–1893. doi:https://doi.org/10.1007/pl00000670.](https://link.springer.com/article/10.1007/PL00000670)