Role of Aromatic and Negatively Charged Residues of DrrB in Multisubstrate Specificity Conferred by the DrrAB System of Streptomyces peucetius

The human ABCG2 transporter engages three gates to control multidrug extrusion

The human ABCG2 transporter plays roles in physiological detoxification across barriers and in anticancer multidrug resistance. The translocation pathway for drug extrusion and its gating mechanism remains elusive. Here, we demonstrate that the ABCG2 multidrug transporter holds two cavities that are delineated by three regulatory gates, indicating a substrate translocation channel. Drugs are trapped in the central cavity after entering through the pivotal intracellular entry gate. This flexible cavity is surrounded by a cluster of three highly conserved phenylalanines. Their aromatic side chains enact a "clamp-push-seal" motion to ensure unidirectional substrate movement. The unique residues T435 and N436 act as critical selectors for ligands, determining the broad substrate specificity. The upper cavity is covered by the lid architecture, constituting the final gate before multidrug extrusion. This work unravels deep mechanistic details on how the translocation channel utilizes pivotal gating steps, including the sequence of events that drive ABCG2-mediated multidrug efflux.

Deep mutational scan of a drug efflux pump reveals its structure-function landscape

Drug efflux is a common resistance mechanism found in bacteria and cancer cells, but studies providing comprehensive functional insights are scarce. In this study, we performed deep mutational scanning (DMS) on the bacterial ABC transporter EfrCD to determine the drug efflux activity profile of more than 1,430 single variants. These systematic measurements revealed that the introduction of negative charges at different locations within the large substrate binding pocket results in strongly increased efflux activity toward positively charged ethidium, whereas additional aromatic residues did not display the same effect. Data analysis in the context of an inward-facing cryogenic electron microscopy structure of EfrCD uncovered a high-affinity binding site, which releases bound drugs through a peristaltic transport mechanism as the transporter transits to its outward-facing conformation. Finally, we identified substitutions resulting in rapid Hoechst influx without affecting the efflux activity for ethidium and daunorubicin. Hence, single mutations can convert EfrCD into a drug-specific ABC importer.

Physiological Functions of Bacterial "Multidrug" Efflux Pumps

Bacterial multidrug efflux pumps have come to prominence in human and veterinary pathogenesis because they help bacteria protect themselves against the antimicrobials used to overcome their infections. However, it is increasingly realized that many, probably most, such pumps have physiological roles that are distinct from protection of bacteria against antimicrobials administered by humans. Here we undertake a broad survey of the proteins involved, allied to detailed examples of their evolution, energetics, structures, chemical recognition, and molecular mechanisms, together with the experimental strategies that enable rapid and economical progress in understanding their true physiological roles. Once these roles are established, the knowledge can be harnessed to design more effective drugs, improve existing microbial production of drugs for clinical practice and of feedstocks for commercial exploitation, and even develop more sustainable biological processes that avoid, for example, utilization of petroleum.

Antibiograms and risk factors of Salmonella isolates from laying hens and eggs in Jimma Town, South Western Ethiopia

Objectives

Salmonella is the most important causes of foodborne illness especially from poultry and poultry products. So the aim of this study was to carryout phenotypic characterization, antimicrobials susceptibility pattern and risk factors of Salmonella isolates from farms and markets eggs, cloacae swabs of chickens and stool of egg collectors. A cross-sectional study was conducted from January 2018 to September 2018. Samples were, processed; Salmonella was isolated, phenotypically identified by OmniLog and antimicrobials susceptibility were carried out.

Result

Over all; 11 (2.65%) of Salmonella enterica were phenotypically characterized out of 415 samples from farms egg content (n = 83), farms eggshell (n = 83), cloacae (n = 83), market eggshell (n = 83) and market egg contents (n = 83) with 2.4%, 0%, 2.4%, 4.8% and 3.6% prevalence, respectively. Out of isolates, 8 (72.72%) displayed multidrug resistance. All isolates showed susceptibility to Gentamicin, Kanamycin and Streptomycin. Lack of separating cracked eggs, washing hand, eggs stay longer unsold, and mixing excreta with feed were associated risk factors for Salmonella presence (P-value < 0.05). The presence of drug resistant Salmonella enterica within egg/and chicken can pose serious health problem. Good hygienic practices are important to reduce risk factors of Salmonella contamination.

Antibiotic Resistance Mechanisms in Bacteria: Relationships Between Resistance Determinants of Antibiotic Producers, Environmental Bacteria, and Clinical Pathogens

Emergence of antibiotic resistant pathogenic bacteria poses a serious public health challenge worldwide. However, antibiotic resistance genes are not confined to the clinic; instead they are widely prevalent in different bacterial populations in the environment. Therefore, to understand development of antibiotic resistance in pathogens, we need to consider important reservoirs of resistance genes, which may include determinants that confer self-resistance in antibiotic producing soil bacteria and genes encoding intrinsic resistance mechanisms present in all or most non-producer environmental bacteria. While the presence of resistance determinants in soil and environmental bacteria does not pose a threat to human health, their mobilization to new hosts and their expression under different contexts, for example their transfer to plasmids and integrons in pathogenic bacteria, can translate into a problem of huge proportions, as discussed in this review. Selective pressure brought about by human activities further results in enrichment of such determinants in bacterial populations. Thus, there is an urgent need to understand distribution of resistance determinants in bacterial populations, elucidate resistance mechanisms, and determine environmental factors that promote their dissemination. This comprehensive review describes the major known self-resistance mechanisms found in producer soil bacteria of the genus Streptomyces and explores the relationships between resistance determinants found in producer soil bacteria, non-producer environmental bacteria, and clinical isolates. Specific examples highlighting potential pathways by which pathogenic clinical isolates might acquire these resistance determinants from soil and environmental bacteria are also discussed. Overall, this article provides a conceptual framework for understanding the complexity of the problem of emergence of antibiotic resistance in the clinic. Availability of such knowledge will allow researchers to build models for dissemination of resistance genes and for developing interventions to prevent recruitment of additional or novel genes into pathogens.

Conformational changes in a multidrug resistance ABC transporter DrrAB: Fluorescence-based approaches to study substrate binding

Bacterial multidrug transporter DrrAB exhibits overlapping substrate specificity with mammalian P-glycoprotein. DrrA hydrolyzes ATP, and the energy is transduced to carrier DrrB resulting in export of drugs. Previous studies suggested that DrrB contains a large and flexible drug-binding pocket made of aromatic residues contributed by several transmembrane helices with different drugs binding to both specific and shared residues in this pocket. However, direct binding of drugs to DrrAB or the mechanism of substrate-induced conformational changes between DrrA and DrrB has so far not been investigated. We used two fluorescence-based approaches to determine substrate binding to purified DrrAB. Our analysis shows that DrrB binds drugs with variable affinities and contains multiple drug binding sites. This work also provides evidence for two asymmetric nucleotide binding sites in DrrA with strikingly different binding affinities. Using targeted fluorescence labeling, we provide clear evidence of long-range conformational changes occurring between DrrA and DrrB. It is proposed that the transduction pathway from the nucleotide-binding DrrA subunit to the substrate binding DrrB subunit includes Q-loop and CREEM motifs in DrrA and EAA-like motif in DrrB. This study lays a solid groundwork for examining roles of various conserved regions of DrrA and DrrB in transduction of conformational changes.

Genome-guided exploration of metabolic features of Streptomyces peucetius ATCC 27952: past, current, and prospect

Streptomyces peucetius ATCC 27952 produces two major anthracyclines, doxorubicin (DXR) and daunorubicin (DNR), which are potent chemotherapeutic agents for the treatment of several cancers. In order to gain detailed insight on genetics and biochemistry of the strain, the complete genome was determined and analyzed. The result showed that its complete sequence contains 7187 protein coding genes in a total of 8,023,114 bp, whereas 87% of the genome contributed to the protein coding region. The genomic sequence included 18 rRNA, 66 tRNAs, and 3 non-coding RNAs. In silico studies predicted ~ 68 biosynthetic gene clusters (BCGs) encoding diverse classes of secondary metabolites, including non-ribosomal polyketide synthase (NRPS), polyketide synthase (PKS I, II, and III), terpenes, and others. Detailed analysis of the genome sequence revealed versatile biocatalytic enzymes such as cytochrome P450 (CYP), electron transfer systems (ETS) genes, methyltransferase (MT), glycosyltransferase (GT). In addition, numerous functional genes (transporter gene, SOD, etc.) and regulatory genes (afsR-sp, metK-sp, etc.) involved in the regulation of secondary metabolites were found. This minireview summarizes the genome-based genome mining (GM) of diverse BCGs and genome exploration (GE) of versatile biocatalytic enzymes, and other enzymes involved in maintenance and regulation of metabolism of S. peucetius. The detailed analysis of genome sequence provides critically important knowledge useful in the bioengineering of the strain or harboring catalytically efficient enzymes for biotechnological applications.

Antibiotic Resistance Mechanisms in Bacteria: Relationships Between Resistance Determinants of Antibiotic Producers, Environmental Bacteria, and Clinical Pathogens

Emergence of antibiotic resistant pathogenic bacteria poses a serious public health challenge worldwide. However, antibiotic resistance genes are not confined to the clinic; instead they are widely prevalent in different bacterial populations in the environment. Therefore, to understand development of antibiotic resistance in pathogens, we need to consider important reservoirs of resistance genes, which may include determinants that confer self-resistance in antibiotic producing soil bacteria and genes encoding intrinsic resistance mechanisms present in all or most non-producer environmental bacteria. While the presence of resistance determinants in soil and environmental bacteria does not pose a threat to human health, their mobilization to new hosts and their expression under different contexts, for example their transfer to plasmids and integrons in pathogenic bacteria, can translate into a problem of huge proportions, as discussed in this review. Selective pressure brought about by human activities further results in enrichment of such determinants in bacterial populations. Thus, there is an urgent need to understand distribution of resistance determinants in bacterial populations, elucidate resistance mechanisms, and determine environmental factors that promote their dissemination. This comprehensive review describes the major known self-resistance mechanisms found in producer soil bacteria of the genus Streptomyces and explores the relationships between resistance determinants found in producer soil bacteria, non-producer environmental bacteria, and clinical isolates. Specific examples highlighting potential pathways by which pathogenic clinical isolates might acquire these resistance determinants from soil and environmental bacteria are also discussed. Overall, this article provides a conceptual framework for understanding the complexity of the problem of emergence of antibiotic resistance in the clinic. Availability of such knowledge will allow researchers to build models for dissemination of resistance genes and for developing interventions to prevent recruitment of additional or novel genes into pathogens.

The effect of drug binding on specific sites in transmembrane helices 4 and 6 of the ABC exporter MsbA studied by DNP-enhanced solid-state NMR

MsbA, a homodimeric ABC exporter, translocates its native substrate lipid A as well as a range of smaller, amphiphilic substrates across the membrane. Magic angle sample spinning (MAS) NMR, in combination with dynamic nuclear polarization (DNP) for signal enhancement, has been used to probe two specific sites in transmembrane helices 4 and 6 of full length MsbA embedded in lipid bilayers. Significant chemical shift changes in both sites were observed in the vanadate-trapped state compared to apo state MsbA. The reduced spectral line width indicates a more confined conformational space upon trapping. In the presence of substrates Hoechst 33342 and daunorubicin, further chemical shift changes and line shape alterations mainly in TM6 in the vanadate trapped state were detected. These data illustrate the conformational response of MsbA towards the presence of drugs during the catalytic cycle. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.

DrrC protein of Streptomyces peucetius removes daunorubicin from intercalated dnrI promoter

DrrC is a DNA-binding protein of Streptomyces peucetius that provides self-resistance against daunorubicin, the antibiotic produced by the organism. DrrC was expressed in E.coli and purified by using N-terminal MBP-tag which retained DNA-binding property in spite of the tag. Mobility shift assay confirmed the interaction of 313bp DNA that has the dnrI promoter, daunorubicin and MBP-DrrC in the presence of ATP. Biotinylated and immobilized 313bp DNA was intercalated with daunorubicin to observe the release of the drug when MBP-DrrC is allowed to act on the DNA. The release of daunorubicin was recorded by absorption and fluorescence spectroscopy. The experiments proved that daunorubicin was released from DNA in the presence of MBP-DrrC. Fluorescence emission of daunorubicin had a maximum peak at 591nm. However, emission spectrum of released daunorubicin showed hypochromism with a maximum peak at 584nm that is possibly because it is in complex with MBP-DrrC. We propose that DrrC naturally binds at intercalated sites to eject daunorubicin; in the process both drug and protein are dislodged from DNA. Like UvrA, DrrC possibly scans the DNA for intercalated daunorubicin. When it encounters daunorubicin, DrrC dislodges it, thereby allowing DNA replication and transcription to go on unhindered. Thus a novel self resistance mechanism by DNA repair is mediated by DrrC.