Generally, TBDRs have a membrane-spanning barrel domain consisting of 22 antiparallel β-strands and an N-terminus region containing a short, conserved motif called the TonB box, which is essential for the interaction between TBDRs and the C-terminal region of TonB 2, 18. In addition, the upregulation of particular TBDR-like genes has been observed in Sphingomonas wittichii RW1 and a Pseudomonas strain during their growth in the presence of dioxin and vanillin, respectively, evoking the possibility that TBDRs participate in the outer membrane transport and catabolism of aromatic compounds 16, 17. Recently, novel functions of the active transporters, TonB-dependent receptors, have been documented, such as in the uptake of rare earth metals, membrane homeostasis, and secretion of proteins 13, 14, 15.

It was envisaged that TBDRs would have more diverse functions than is currently known based on the fact that many Gram-negative bacteria have a large number of TBDR-like genes in their genomes, whose functions are unknown 9, 10.īacterial outer membrane transporters of aromatic compounds reported thus far have included passive transporters such as OpdK and OmpW, which are a vanillate-specific channel of Pseudomonas aeruginosa PAO1 and a naphthalene porin of Pseudomonas fluorescens, respectively 11, 12. Although there are examples where the TonB system transports certain siderophores containing aromatic groups 7, 8, its involvement in the uptake and catabolism of aromatic compounds was previously unknown. A number of TonB systems involved in the uptake of siderophores, vitamin B12 and saccharides have been identified 4, 5, 6. TBDRs mediate substrate-specific transport across the outer membrane, utilizing energy derived from the proton motive force transmitted from the TonB−ExbB−ExbD complex (TonB complex) located in the inner membrane (TonB system) 2, 3. In the outer membrane, transport of nutrients is achieved by passive transporters, such as porins and substrate-specific channels, and active transporters, TonB-dependent receptors (TBDRs) 1. The cell envelope of Gram-negative bacteria comprises outer and inner membranes therefore, nutrients must be passed through these two membranes to utilize them in the bacterial cells. Our results will provide an important basis for establishing engineered strains optimized for use in lignin valorisation. Furthermore, we found that overexpression of the corresponding TBDR gene enhanced the uptake of this biphenyl compound, contributing to the improved rate of platform chemical production. Here, we show that the outer membrane transport of a biphenyl compound derived from lignin is mediated by the TonB system in a Gram-negative bacterium capable of degrading lignin-derived aromatic compounds, Sphingobium sp. Although a number of TonB systems involved in the uptake of siderophores, vitamin B12 and saccharides have been identified, their involvement in the uptake and catabolism of aromatic compounds was previously unknown. TonB-dependent receptors (TBDRs) mediate substrate-specific transport across the outer membrane, utilizing energy derived from the proton motive force transmitted from the TonB−ExbB−ExbD complex located in the inner membrane (TonB system).

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