Cloning and sequencing of the Haemophilus influenzae exbB and exbD genes

Cell permeability, β-lactamase activity, and transport contribute to high level of resistance to ampicillin in Lysobacter enzymogenes

Discovery of multidrug resistance (MDR) in environmental microorganisms provides unique resources for uncovering antibiotic resistomes, which could be vital to predict future emergence of MDR pathogens. Our previous studies indicated that Lysobacter sp. conferred intrinsic resistance to multiple antibiotics at high levels, especially ampicillin, the first broad-spectrum β-lactam antibiotics against both Gram-positive and Gram-negative bacteria. However, the underlying molecular mechanisms for resistance to ampicillin in Lysobacter enzymogenes strain C3 (LeC3) remain unknown. In this study, screening a Tn5 transposon mutant library of LeC3 recovered 12 mutants with decreased ampicillin resistance, and three mutants (i.e., tatC, lebla, and lpp) were selected for further characterization. Our results revealed that genes encoding β-lactamase (lebla) and twin-arginine translocation (tatC) system for β-lactamase transport played a pivotal role in conferring ampicillin resistance in L. enzymogenes. It was also demonstrated that the lpp gene was not only involved in resistance against β-lactams but also conferred resistance to multiple antibiotics in L. enzymogenes. Permeability assay results indicated that decreased MDR in the lpp mutant was in part due to its higher cellular permeability. Furthermore, our results showed that the difference of LeC3 and L. antibioticus strain LaATCC29479 in ampicillin susceptibility was partly due to their differences in cellular permeability, but not due to β-lactamase activities.

Structure and function of seed storage proteins in faba bean (Vicia faba L.)

The protein subunit is the most important basic unit of protein, and its study can unravel the structure and function of seed storage proteins in faba bean. In this study, we identified six specific protein subunits in Faba bean (cv. Qinghai 13) combining liquid chromatography (LC), liquid chromatography-electronic spray ionization mass (LC-ESI-MS/MS) and bio-information technology. The results suggested a diversity of seed storage proteins in faba bean, and a total of 16 proteins (four GroEL molecular chaperones and 12 plant-specific proteins) were identified from 97-, 96-, 64-, 47-, 42-, and 38-kD-specific protein subunits in faba bean based on the peptide sequence. We also analyzed the composition and abundance of the amino acids, the physicochemical characteristics, secondary structure, three-dimensional structure, transmembrane domain, and possible subcellular localization of these identified proteins in faba bean seed, and finally predicted function and structure. The three-dimensional structures were generated based on homologous modeling, and the protein function was analyzed based on the annotation from the non-redundant protein database (NR database, NCBI) and function analysis of optimal modeling. The objective of this study was to identify the seed storage proteins in faba bean and confirm the structure and function of these proteins. Our results can be useful for the study of protein nutrition and achieve breeding goals for optimal protein quality in faba bean.

Bordetella pertussis TonB, a Bvg-independent virulence determinant

In gram-negative bacteria, high-affinity iron uptake requires the TonB/ExbB/ExbD envelope complex to release iron chelates from their specific outer membrane receptors into the periplasm. Based on sequence similarities, the Bordetella pertussis tonB exbB exbD locus was identified on a cloned DNA fragment. The tight organization of the three genes suggests that they are cotranscribed. A putative Fur-binding sequence located upstream from tonB was detected in a Fur titration assay, indicating that the tonB exbB exbD operon may be Fur-repressed in high-iron growth conditions. Putative structural genes of the beta-subunit of the histone-like protein HU and of a new two-component regulatory system were identified upstream from tonB and downstream from exbD, respectively. A B. pertussis DeltatonB exbB::Km(r) mutant was constructed by allelic exchange and characterized. The mutant was impaired for growth in low-iron medium in vitro and could not use ferrichrome, desferal, or hemin as iron sources. Levels of production of the major bacterial toxins and adhesins were similar in the TonB(+)/TonB(-) pair. The DeltatonB exbB mutant was still responsive to chemical modulators of virulence; thus, the BvgA/BvgS two-component system is not TonB dependent. Nevertheless, in vivo in the mouse respiratory infection model, the colonization ability of the mutant was reduced compared to the parental strain.

Role of the Haemophilus ducreyi Ton system in internalization of heme from hemoglobin

By cloning into Escherichia coli and construction of isogenic mutants of Haemophilus ducreyi, we showed that the hemoglobin receptor (HgbA) is TonB dependent. An E. coli hemA tonB mutant expressing H. ducreyi hgbA grew on low levels of hemoglobin as a source of heme only when an intact H. ducreyi Ton system plasmid was present. In contrast, growth on heme by the E. coli hemA tonB mutant expressing hgbA was observed only at high concentrations of heme, was TonB independent, and demonstrated that H. ducreyi HgbA was not sufficient to function as a typical TonB-dependent heme receptor in E. coli. Allelic replacement of the wild-type H. ducreyi exbB, exbD, and tonB loci with the exbB, exbD, and tonB deletion resulted in an H. ducreyi isogenic mutant unable to utilize hemoglobin but able to utilize hemin at the same levels as the parent strain to fulfill its heme requirement. This finding confirms the TonB dependence of HgbA-mediated hemoglobin utilization and suggests that uptake of hemin in H. ducreyi is TonB independent. Additionally, the H. ducreyi Ton system mutant synthesized increased amounts of HgbA and other heme-regulated outer membrane proteins, consistent with derepression of these proteins due to lower intracellular heme and/or iron concentrations in the mutant. Sequencing of the Ton system genes revealed that the arrangement of the genes was exbB exbD tonB. The proximity and structure of these genes suggested that they are transcribed as an operon. This arrangement, as well as the DNA and deduced amino acid sequences of these H. ducreyi genes, was most similar to those from other pasteurellae.

Unusual structure of the tonB-exb DNA region of Xanthomonas campestris pv. campestris: tonB, exbB, and exbD1 are essential for ferric iron uptake, but exbD2 is not

The nucleotide sequence of a 3.6-kb HindIII-SmaI DNA fragment of Xanthomonas campestris pv. campestris revealed four open reading frames which, based on sequence homologies, were designated tonB, exbB, exbD1, and exbD2. Analysis of translational fusions to alkaline phosphatase and beta-galactosidase confirmed that the TonB, ExbB, ExbD1, and ExbD2 proteins are anchored in the cytoplasmic membrane. The TonB protein of X. campestris pv. campestris lacks the conserved (Glu-Pro)n and (Lys-Pro)m repeats but harbors a 13-fold repeat of proline residues. By mutational analysis, the tonB, exbB, and exbD1 genes were shown to be essential for ferric iron import in X. campestris pv. campestris. In contrast, the exbD2 gene is not involved in the uptake of ferric iron.

Cloning and characterization of the exbB-exbD-tonB locus of Pasteurella haemolytica A1

A recombinant plasmid (pMG1) carrying Pasteurella haemolytica A1 DNA which complements a tonB mutation of Escherichia coli has been isolated. E. coli tonB metE which carries pMG1 exhibits growth kinetics in the presence of vitamin B12 similar to that of the wild-type host. In addition, the complemented E. coli is susceptible to killing by bacteriophage phi 80 and colicin B. Analysis of the nucleotide sequence in the complementing DNA showed that it codes for three genes in the order of exbB-exbD-tonB. This genetic organization has been reported in Haemophilus influenzae, H. ducreyi, Pseudomonas putida and Vibrio cholerae, and may represent a separate lineage of evolution from that of the Enterobacteriaceae in which tonB is unlinked with the accessory genes exbB and exbD. A comparison of the DNA flanking the exbB-exbD-tonB locus in P. haemolytica A1 and H. influenzae showed that the flanking regions are completely different between the two organisms.

Bacterial transferrin and lactoferrin receptors

Pathogenic members of the Neisseriaceae and Pasteurellaceae express outer-membrane receptor proteins involved in the direct assimilation of iron from the host glycoproteins transferrin and lactoferrin. The critical requirement of iron for growth suggests that this function is an important component of colonization and infection. A model describing this novel process is presented.

Identification of TonB homologs in the family Enterobacteriaceae and evidence for conservation of TonB-dependent energy transduction complexes

The transport of Fe(III)-siderophore complexes and vitamin B12 across the outer membrane of Escherichia coli requires the TonB-dependent energy transduction system. A set of murine monoclonal antibodies (MAbs) was generated against an E. coli TrpC-TonB fusion protein to facilitate structure and function studies. In the present study, the epitopes recognized by these MAbs were mapped, and their distribution in gram-negative organisms was examined. Cross-species reactivity patterns obtained against TonB homologs of known sequence were used to refine epitope mapping, with some epitopes ultimately confirmed by inhibition experiments using synthetic polypeptides. Epitopes recognized by this set of MAbs were conserved in TonB homologs for 9 of 12 species in the family Enterobacteriaceae (including E. coli), including previously unidentified TonB homologs in Shigella, Citrobacter, Proteus, and Kluyvera species. These homologs were also detected by a polyclonal alpha-TrpC-TonB serum that additionally recognized the known Yersinia enterocolitica TonB homolog and a putative TonB homolog in Edwardsiella tarda. These antibody preparations failed to detect the known TonB homologs of either Pseudomonas putida or Haemophilus influenzae but did identify potential TonB homologs in several other nonenteric gram-negative species. In vivo chemical cross-linking experiments demonstrated that in addition to TonB, auxiliary components of the TonB-dependent energy transduction system are broadly conserved in members of the family Enterobacteriaceae, suggesting that the TonB system represents a common system for high-affinity active transport across the gram-negative outer membrane.