Author Correction: Iron acquisition system of Sphingobium sp. strain SYK-6, a degrader of lignin-derived aromatic compounds

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Iron acquisition system of Sphingobium sp. strain SYK-6, a degrader of lignin-derived aromatic compounds

Iron, an essential element for all organisms, acts as a cofactor of enzymes in bacterial degradation of recalcitrant aromatic compounds. The bacterial family, Sphingomonadaceae comprises various degraders of recalcitrant aromatic compounds; however, little is known about their iron acquisition system. Here, we investigated the iron acquisition system in a model bacterium capable of degrading lignin-derived aromatics, Sphingobium sp. strain SYK-6. Analyses of SYK-6 mutants revealed that FiuA (SLG_34550), a TonB-dependent receptor (TBDR), was the major outer membrane iron transporter. Three other TBDRs encoded by SLG_04340, SLG_04380, and SLG_10860 also participated in iron uptake, and tonB2 (SLG_34540), one of the six tonB comprising the Ton complex which enables TBDR-mediated transport was critical for iron uptake. The ferrous iron transporter FeoB (SLG_36840) played an important role in iron uptake across the inner membrane. The promoter activities of most of the iron uptake genes were induced under iron-limited conditions, and their regulation is controlled by SLG_29410 encoding the ferric uptake regulator, Fur. Although feoB, among all the iron uptake genes identified is highly conserved in Sphingomonad strains, the outer membrane transporters seem to be diversified. Elucidation of the iron acquisition system promises better understanding of the bacterial degradation mechanisms of aromatic compounds.

Transferrin Binding Protein B and Transferrin Binding Protein A2 Expand the Transferrin Recognition Range of Histophilus somni

ABSTRACT The bacterial bipartite transferrin receptor is an iron acquisition system that several important human and animal pathogens require for survival. It consists of the TonB-dependent transporter transferrin binding protein A (TbpA) and the surface lipoprotein transferrin binding protein B (TbpB). Curiously, the Tbps are only found in host-specific pathogens and are themselves host specific, meaning that they will bind to the transferrin of their host species but not to the transferrins of other animal species. While this phenomenon has long been established, neither the steps in the evolutionary process that led to this exquisite adaptation for the host nor the steps that could alter it are known. We sought to gain insight into these processes by studying Tbp specificity in Histophilus somni, an economically important pathogen of cattle. A past study showed that whole cells of H. somni specifically bind bovine transferrin but not transferrin from sheep and goats, two bovids whose transferrins share 93% amino acid sequence identity with bovine transferrin. To our surprise, we found that H. somni can use sheep and goat transferrins as iron sources for growth and that HsTbpB, but not HsTbpA, has detectable affinity for sheep and goat transferrins. Furthermore, a third transferrin binding protein found in H. somni, HsTbpA2, also showed affinity for sheep and goat transferrins. Our results suggest that H. somni TbpB and TbpA2 may contribute to broadening the host transferrin recognition range of H. somni. IMPORTANCE Host-restricted pathogens infect a single host species or a narrow range of host species. Histophilus somni, a pathogen that incurs severe economic losses for the cattle industry, infects cattle, sheep, and goats but not other mammals. The transferrin binding proteins, TbpA and TbpB, are thought to be a key iron acquisition system in H. somni; however, despite their importance, H. somni TbpA and TbpB were previously shown to be cattle transferrin specific. In our study, we find that H. somni TbpB and another little-studied Tbp, TbpA2, bind sheep and goat transferrins, as well as bovine transferrin. Our results suggest that TbpB and TbpA2 may allow for host range expansion and provide a mechanism for how host specificity in Tbp-encoding pathogens can be altered.

Iron acquisition system of Sphingobium sp. strain SYK-6, a degrader of lignin-derived aromatic compounds

Iron, an essential element for all organisms, acts as a cofactor of enzymes in bacterial degradation of recalcitrant aromatic compounds. The bacterial family, Sphingomonadaceae comprises various degraders of recalcitrant aromatic compounds; however, little is known about their iron acquisition system. Here, we investigated the iron acquisition system in a model bacterium capable of degrading lignin-derived aromatics, Sphingobium sp. strain SYK-6. Analyses of SYK-6 mutants revealed that FiuA (SLG_34550), a TonB-dependent receptor (TBDR), was the major outer membrane iron transporter. Three other TBDRs encoded by SLG_04340, SLG_04380, and SLG_10860 also participated in iron uptake, and tonB2 (SLG_34550), one of the six tonB comprising the Ton complex which enables TBDR-mediated transport was critical for iron uptake. The ferrous iron transporter FeoB (SLG_36840) played an important role in iron uptake across the inner membrane. The promoter activities of most of the iron uptake genes were induced under iron-limited conditions, and their regulation is controlled by SLG_29410 encoding the ferric uptake regulator, Fur. Although feoB, among all the iron uptake genes identified is highly conserved in Sphingomonad strains, the outer membrane transporters seem to be diversified. Elucidation of the iron acquisition system promises better understanding of the bacterial degradation mechanisms of aromatic compounds.

Transferrin binding protein B and Transferrin binding protein A 2 expand the transferrin recognition range of Histophilus somni

The bacterial bipartite transferrin receptor is an iron acquisition system that is required for survival by several key human and animal pathogens. It consists of the TonB-dependent transporter Transferrin binding protein A (TbpA) and the surface lipoprotein Transferrin binding protein B (TbpB). Curiously, the Tbps are only found in host specific pathogens, and are themselves host specific, meaning that they will bind to the transferrin of their host species, but not to those of other animal species. While this phenomenon has long been established, neither the steps in the evolutionary process that led to this exquisite adaptation for the host, nor the steps that could alter it, are known. We sought to gain insight into these processes by studying Tbp specificity in Histophilus somni, a major pathogen of cattle. A past study showed that whole cells of H. somni specifically bind bovine transferrin, but not transferrin from sheep and goats, two bovids whose transferrins share 93% amino acid sequence identity with bovine transferrin. To our surprise, we found that H. somni can use sheep and goat transferrins as iron sources for growth, and that HsTbpB, but not HsTbpA, has detectable affinity for sheep and goat transferrins. Furthermore, a third transferrin binding protein, HsTbpA2, also showed affinity for sheep and goat transferrins. Our results show that H. somni TbpB and TbpA2 act to broaden the host transferrin recognition range of H. somni.ImportanceHost restricted pathogens infect a single host species or a narrow range of host species. Histophilus somni, a pathogen that incurs severe economic losses for the cattle industry, infects cattle, sheep, and goats, but not other mammals. The transferrin binding proteins, TbpA and TbpB, are thought to be a key iron acquisition system in H. somni, however, surprisingly, they were also shown to be cattle transferrin-specific. In our study we find that H. somni TbpB, and another little-studied Tbp, TbpA2, bind sheep and goat transferrins as well as bovine transferrin. Our results suggest that TbpA2 may have allowed for host range expansion, and provide a mechanism for how host specificity in Tbp containing pathogens can be altered.

Siderophore-Mediated Iron Acquisition Enhances Resistance to Oxidative and Aromatic Compound Stress inCupriavidus necatorJMP134

ABSTRACTMany bacteria secrete siderophores to enhance iron uptake under iron-restricted conditions. In this study, we found thatCupriavidus necatorJMP134, a well-known aromatic pollutant-degrading bacterium, produces an unknown carboxylate-type siderophore named cupriabactin to overcome iron limitation. Using genome mining, targeted mutagenesis, and biochemical analysis, we discovered an operon containing six open reading frames (cubA–F) in theC. necatorJMP134 genome that encodes proteins required for the biosynthesis and uptake of cupriabactin. As the dominant siderophore ofC. necatorJMP134, cupriabactin promotes the growth ofC. necatorJMP134 under iron-limited conditions via enhanced ferric iron uptake. Furthermore, we demonstrated that the iron concentration-dependent expression of thecuboperon is mediated by the ferric uptake regulator (Fur). Physiological analyses revealed that the cupriabactin-mediated iron acquisition system influences swimming motility, biofilm formation, and resistance to oxidative and aromatic compound stress inC. necatorJMP134. In conclusion, we identified a carboxylate-type siderophore named cupriabactin, which plays important roles in iron scavenging, bacterial motility, biofilm formation, and stress resistance.IMPORTANCESince siderophores have been widely exploited for agricultural, environmental, and medical applications, the identification and characterization of new siderophores from different habitats and organisms will have great beneficial applications. Here, we identified a novel siderophore-producing gene cluster inC. necatorJMP134. This gene cluster produces a previously unknown carboxylate siderophore, cupriabactin. Physiological analyses revealed that the cupriabactin-mediated iron acquisition system influences swimming motility, biofilm formation, and oxidative stress resistance. Most notably, this system also plays important roles in increasing the resistance ofC. necatorJMP134 to stress caused by aromatic compounds, which provide a promising strategy to engineer more efficient approaches to degrade aromatic pollutants.