Functional roles of multiple Ton complex genes in a Sphingobium degrader of lignin-derived aromatic compounds

Masaya Fujita,Naofumi Kamimura,Eiji Masai,Mizuki Kondo,Koki Shibata,Shodai Yano,Shojiro Hishiyama

doi:10.1038/s41598-021-01756-8

Abstract

TonB-dependent transporters (TBDTs) mediate outer membrane transport of nutrients using the energy derived from proton motive force transmitted from the TonB–ExbB–ExbD complex localized in the inner membrane. Recently, we discovered ddvT encoding a TBDT responsible for the uptake of a 5,5-type lignin-derived dimer in Sphingobium sp. strain SYK-6. Furthermore, overexpression of ddvT in an SYK-6-derivative strain enhanced its uptake capacity, improving the rate of platform chemical production. Thus, understanding the uptake system of lignin-derived aromatics is fundamental for microbial conversion-based lignin valorization. Here we examined whether multiple tonB-, exbB-, and exbD-like genes in SYK-6 contribute to the outer membrane transport of lignin-derived aromatics. The disruption of tonB2–6 and exbB3 did not reduce the capacity of SYK-6 to convert or grow on lignin-derived aromatics. In contrast, the introduction of the tonB1–exbB1–exbD1–exbD2 operon genes into SYK-6, which could not be disrupted, promoted the conversion of β-O-4-, β-5-, β-1-, β-β-, and 5,5-type dimers and monomers, such as ferulate, vanillate, syringate, and protocatechuate. These results suggest that TonB-dependent uptake involving the tonB1 operon genes is responsible for the outer membrane transport of the above aromatics. Additionally, exbB2/tolQ and exbD3/tolR were suggested to constitute the Tol-Pal system that maintains the outer membrane integrity.

Highlights

TonB-dependent transporters (TBDTs) mediate outer membrane transport of nutrients using the energy derived from proton motive force transmitted from the TonB–ExbB–ExbD complex localized in the inner membrane
The Ton complex composed of TonB1–ExbB1–ExbD1–ExbD2 was suggested to be involved in the uptake of DDVA mediated by DdvT
No reduction in the conversion capacity of ∆ompW compared to the wild type was observed, indicating that ompW is unnecessary to uptake lignin-derived aromatic compounds tested in this study

Summary

Introduction

TonB-dependent transporters (TBDTs) mediate outer membrane transport of nutrients using the energy derived from proton motive force transmitted from the TonB–ExbB–ExbD complex localized in the inner membrane. The introduction of the tonB1–exbB1–exbD1–exbD2 operon genes into SYK-6, which could not be disrupted, promoted the conversion of β-O-4-, β-5-, β-1-, β-β-, and 5,5type dimers and monomers, such as ferulate, vanillate, syringate, and protocatechuate These results suggest that TonB-dependent uptake involving the tonB1 operon genes is responsible for the outer membrane transport of the above aromatics. We have demonstrated that a TBDT (DdvT) mediates the uptake of 5,5′-dehydrodivanillate (DDVA), a lignin-derived 5,5-type dimer, across the outer membrane in SYK-6. The genomes of strains of Sphingomonadaceae, which include many strains capable of degrading recalcitrant aromatic compounds, contain many TBDT-like genes, suggesting that the Ton system is involved in the outer membrane transport of these aromatic c ompounds[25]. A recent study strongly suggests that TBDTs are involved in the uptake of aromatic compounds, such as benzo[a] pyrene in Novosphingobium pentaromativorans US6-1, which degrades polyaromatic h ydrocarbons[26]

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Conclusion