Dihydrostreptomycin Directly Binds to, Modulates, and Passes through the MscL Channel Pore.

The primary mechanism of action of the antibiotic dihydrostreptomycin is binding to and modifying the function of the bacterial ribosome, thus leading to decreased and aberrant translation of proteins; however, the routes by which it enters the bacterial cell are largely unknown. The mechanosensitive channel of large conductance, MscL, is found in the vast majority of bacterial species, where it serves as an emergency release valve rescuing the cell from sudden decreases in external osmolarity. While it is known that MscL expression increases the potency of dihydrostreptomycin, it has remained unclear if this effect is due to a direct interaction. Here, we use a combination of genetic screening, MD simulations, and biochemical and mutational approaches to determine if dihydrostreptomycin directly interacts with MscL. Our data strongly suggest that dihydrostreptomycin binds to a specific site on MscL and modifies its conformation, thus allowing the passage of K+ and glutamate out of, and dihydrostreptomycin into, the cell.

Streptomycin potency is dependent on MscL channel expression.

The antibiotic streptomycin is widely used in the treatment of microbial infections. The primary mechanism of action is inhibition of translation by binding to the ribosome, but how it enters the bacterial cell is unclear. Early in the study of this antibiotic, a mysterious streptomycin-induced potassium efflux preceding any decrease in viability was observed; it was speculated that this changed the electrochemical gradient such that streptomycin better accessed the cytoplasm. Here we use a high-throughput screen to search for compounds targeting the mechanosensitive channel of large conductance (MscL) and find dihydrostreptomycin among the 'hits'. Furthermore, we find that MscL is not only necessary for the previously described streptomycin-induced potassium efflux, but also directly increases MscL activity in electrophysiological studies. The data suggest that gating MscL is a novel mode of action of dihydrostreptomycin, and that MscL's large pore may provide a mechanism for cell entry.

Confirmatory method for the determination of streptomycin and dihydrostreptomycin in honey by LC-MS/MS.

A method was specifically developed for the determination and confirmation of streptomycin and dihydrostreptomycin in different types of honey. The method is simple, rapid, sensitive and was validated for streptomycin and dihydrostreptomycin in accordance with Commission Decision 2002/657/EC. After extraction with phosphate buffer and a pH change, clean-up was performed via SPE with polymeric phase. LC-MS/MS analysis was carried out using two different HILIC columns for the separation of the analytes and using a triple quadrupole mass spectrometer in positive ESI mode to measure the transitions of the substances in MRM mode. For the quantification of both substances, matrix calibration curves in a linear range of 5-80 g kg(-1) were used. The validation parameters established for streptomycin and dihydrostreptomycin, CCα (11.8 and 11.5 µg kg(-1), respectively), CCß (18.9 and 19.9 µg kg(-1), respectively), recovery (97 and 101%, respectively) and the relative within-laboratory reproducibility RSD(wR) (16.4 and 20.8%, respectively) at the recommended concentration of 40 µg kg(-1), fulfil the requirements of Commission Decision 2002/657/EC.

Isolation and characterization of bluensomycin biosynthetic genes from Streptomyces bluensis.

The biosynthetic gene cluster for bluensomycin, a member of the aminoglycoside family of antibiotics, was isolated and characterized from the bluensomycin producing strain, Streptomyces bluensis ATCC27420. PCR primers were designed specifically to amplify a segment of the dTDP-glucose synthase gene based on its conserved sequences among several actinomycete strains. By screening a cosmid library using amplified PCR fragments, a 30-kb DNA fragment was isolated. Sequence analysis identified 15 open reading frames (ORFs), eight of which had previously been identified by Piepersberg et al. But seven are novel to this study. We demonstrated that one of these ORFs, blmA, confers resistance against the antibiotic dihydrostreptomycin, and another, blmD, encodes a dTDP-glucose synthase. These findings suggest that the isolated gene cluster is very likely to be responsible for the biosynthesis of bluensomycin.

In vitro selection and characterization of streptomycin-binding RNAs: recognition discrimination between antibiotics.

As pathogens continue to evade therapeutical drugs, a better understanding of the mode of action of antibiotics continues to have high importance. A growing body of evidence points to RNA as a crucial target for antibacterial and antiviral drugs. For example, the aminocyclitol antibiotic streptomycin interacts with the 16S ribosomal RNA and, in addition, inhibits group I intron splicing. To understand the mode of binding of streptomycin to RNA, we isolated small, streptomycin-binding RNA aptamers via in vitro selection. In addition, bluensomycin, a streptomycin analogue that does not inhibit splicing, was used in a counter-selection to obtain RNAs that bind streptomycin with high affinity and specificity. Although an RNA from the normal selection (motif 2) bound both antibiotics, an RNA from the counter-selection (motif 1) discriminated between streptomycin and bluensomycin by four orders of magnitude. The binding site of streptomycin on the RNAs was determined via chemical probing with dimethylsulfate and kethoxal. The minimal size required for drug binding was a 46- and a 41-mer RNA for motifs 1 and 2, respectively. Using Pb2+ cleavage in the presence and absence of streptomycin, a conformational change spanning the entire mapped sequence length of motif 1 was observed only when both streptomycin and Mg2+ were present. Both RNAs require Mg2+ for binding streptomycin.

Identification of stsC, the gene encoding the L-glutamine:scyllo-inosose aminotransferase from streptomycin-producing Streptomycetes.

Eight new genes, strO-stsABCDEFG, were identified by sequencing DNA in the gene cluster that encodes proteins for streptomycin production of Streptomyces griseus N2-3-11. The StsA (calculated molecular mass 43.5 kDa) and StsC (45.5 kDa) proteins - together with another gene product, StrS (39.8 kDa), encoded in another operon of the same gene cluster - show significant sequence identity and are members of a new class of pyridoxal-phosphate-dependent aminotransferases that have been observed mainly in the biosynthetic pathways for secondary metabolites. The aminotransferase activity was demonstrated for the first time by identification of the overproduced and purified StsC protein as the L-glutamine:scyllo-inosose aminotransferase, which catalyzes the first amino transfer in the biosynthesis of the streptidine subunit of streptomycin. The stsC and stsA genes each hybridized specifically to distinct fragments in the genomic DNA of most actinomycetes tested that produce diaminocyclitolaminoglycosides. In contrast, only stsC, but not stsA, hybridized to the DNA of Streptomyces hygroscopicus ssp. glebosus, which produces the monoaminocyclitol antibiotic bluensomycin; this suggests that both genes are specifically used in the first and second steps of the cyclitol transamination reactions. Sequence comparison studies performed with the deduced polypeptides of the genes adjacent to stsC suggest that the enzymes encoded by some of these genes [strO (putative phosphatase gene), stsB (putative oxidoreductase gene), and stsE (putative phosphotransferase gene)] also could be involved in (di-)aminocyclitol synthesis.

The 5' proximal helix of 16S rRNA is involved in the binding of streptomycin to the ribosome.

Single mutations at the end of the 5' proximal helix and in the 915 region (13U-->A or C; 914A-->U or G), and double mutations (13U-->A and 914A-->U; 13U-->C and 914A-->G) were constructed into Escherichia coli 16S ribosomal RNA. The mutations were introduced into an expression plasmid containing the rrnB operon under the transcriptional control of the temperature-inducible lambda PL promoter. None of the mutant 16S rRNAs affected cell growth when expressed. Ribosomes extracted after induction of expression of the mutant 16S rRNAs were assayed for their capacity to bind the error-inducing drug streptomycin and for translational misreading in the presence of streptomycin. All mutations impaired the binding of streptomycin, and consequently its capacity to stimulate misreading. Our results demonstrate the involvement of the 5' proximal helix of 16S rRNA in the binding of streptomycin and confirm the participation of the 915 region. They do not support a previous suggestion [Leclerc, D. and Brakier-Gingras, L. (1991) FEBS Lett., Vol. 279, pp. 171-174] that base pairing between nucleotides 13 and 914 stabilizes the binding of streptomycin.

Streptomycin biosynthesis and its regulation in Streptomycetes.

New insights into the gene orders, structures, evolution, and functions of streptomycin (Sm) biosynthetic genes (str) were gained via hybridization studies, determination of nucleotide sequences, and measurement of expression in the str gene clusters of Streptomyces griseus and S. glaucescens. Both str clusters showed considerable divergence in macro and micro structure. Genes putatively involved in pathways leading to the (dihydro-)streptose and N-methyl-L-glucosamine moieties of Sm were identified. Additional regulatory elements, such as gene strS and conserved TTA codons in the N-terminal sections of reading frames, are reported. Evidences for the involvement of physiological state, signal transduction, and activators in the control of Sm production are presented.

The bactericidal action of streptomycin: membrane permeabilization caused by the insertion of mistranslated proteins into the cytoplasmic membrane of Escherichia coli and subsequent caging of the antibiotic inside the cells due to degradation of these proteins.

The mechanism by which the aminoglycoside antibiotic streptomycin permeabilizes the cytoplasmic membrane of Escherichia coli cells was reinvestigated. For this purpose, the extent of streptomycin-induced K+ loss from cells growing at low external K+ concentrations was taken as a measure of membrane permeabilization. Experiments with different K(+)-uptake mutants showed that the antibiotic specifically increased the passive permeability of the cell membrane to K+ and other ions. These permeability changes were small and the membrane potential of the treated cells remained high. The membrane permeabilization was not due to a direct interaction of the antibiotic with the cell membrane, since cells that carry an rpsL mutation and synthesize proteins in a streptomycin-insensitive way did not lose K+ after the addition of the antibiotic. Due to misreading and premature termination of translation the cells synthesized aberrant proteins under the conditions where membrane permeabilization occurred. Two conditions are described under which the cells both degraded these mistranslated proteins rapidly and reaccumulated K+, lending support to the hypothesis that membrane permeabilization is due to the presence of the mistranslated proteins in the cell membrane. Evidence is presented that the irreversibility of (dihydro)streptomycin uptake by cells washed free from the antibiotic might also be due to rapid degradation of the mistranslated proteins, leading to 'caging' of the antibiotic inside the cells.

The interaction between streptomycin and ribosomal RNA.

The present study shows that a mutation in the 530 loop of 16S rRNA impairs the binding of streptomycin to the bacterial ribosome, thereby restricting the misreading effect of the drug. Previous reports demonstrated that proteins S4, S5 and S12 as well as the 915 region of 16S rRNA are involved in the binding of streptomycin, and indicated that the drug not only interacts with the 30S subunit but also with the 50S subunit. The relationship between the target of streptomycin and its known interference with the proofreading control of translational accuracy is examined in light of these results.

Mutations in the 915 region of Escherichia coli 16S ribosomal RNA reduce the binding of streptomycin to the ribosome.

The nine possible single-base substitutions were produced at positions 913 to 915 of the 16S ribosomal RNA of Escherichia coli, a region known to be protected by streptomycin [Moazed, D. and Noller, H.F. (1987) Nature, 327, 389-394]. When the mutations were introduced into the expression vector pKK3535, only two of them (913A----G and 915A----G) permitted recovery of viable transformants. Ribosomes were isolated from the transformed bacteria and were assayed for their response to streptomycin in poly(U)- and MS2 RNA-directed assays. They were resistant to the stimulation of misreading and to the inhibition of protein synthesis by streptomycin, and this correlated with a decreased binding of the drug. These results therefore demonstrate that, in line with the footprinting studies of Moazed and Noller, mutations in the 915 region alter the interaction between the ribosome and streptomycin.

Possible evolutionary relationships between streptomycin and bluensomycin biosynthetic pathways: detection of novel inositol kinase and O-carbamoyltransferase activities.

Bluensomycin (glebomycin) is an aminocyclitol antibiotic that differs structurally from dihydrostreptomycin in having bluensidine (1D-1-O-carbamoyl-3-guanidinodeoxy-scyllo-inositol) rather than streptidine (1,3-diguanidino-1,3-dideoxy-scyllo-inositol) as its aminocyclitol moiety. Extracts of the bluensomycin producer Streptomyces hygroscopicus form glebosus ATCC 14607 (S. glebosus) were found to have aminodeoxy-scyllo-inositol kinase activity but to lack 1D-1-guanidino-3-amino-1,3-dideoxy-scyllo-inositol kinase activity, showing for the first time that these two reactions in streptomycin producers must be catalyzed by different enzymes. S. glebosus extracts therefore possess the same five enzymes required for synthesis of guanidinodeoxy-scyllo-inositol from myo-inositol that are found in streptomycin producers but lack the next three of the four enzymes found in streptomycin producers that are required to synthesize the second guanidino group of streptidine-P. In place of a second guanidino group, S. glebosus extracts were found to catalyze a Mg2(+)-dependent carbamoylation of guanidinodeoxy-scyllo-inositol to form bluensidine, followed by a phosphorylation to form bluensidine-P. The novel carbamoyl-P:guanidinodeoxy-scyllo-inositol O-carbamoyltransferase and ATP:bluensidine phosphotransferase activities were not detected in streptomycin producers or in S. glebosus during its early rapid growth phase. Free bluensidine appears to be a normal intermediate in bluensomycin biosynthesis, in contrast to the case of streptomycin biosynthesis; in the latter, although exogenous streptidine can enter the pathway via streptidine-P, free streptidine is not an intermediate in the endogenous biosynthetic pathway. Comparison of the streptomycin and bluensomycin biosynthetic pathways provides a unique opportunity to evaluate those proposed mechanisms for the evolutionary acquisition of new biosynthetic capabilities that involve gene duplication and subsequent mutational changes in one member of the pair. In this model, there are at least five pairs of enzymes catalyzing analogous reactions that can be analyzed for homology at both the protein and DNA levels, including two putative pairs of inositol kinases detected in this study.

Uptake of antibiotics by Plasmodium falciparum in culture.

The ability of Plasmodium falciparum to accumulate 3H-clindamycin, 3H-tetracycline, and 3H-dihydrostreptomycin was determined in synchronized trophozoites and schizonts in 87% parasitemia 5% hematocrit cultures. Accumulation was time-dependent and modestly concentration-dependent. At 0.1 mM initial external drug concentration, the ratio of drug concentration in cells:medium after 120 min was 5.5 for 3H-clindamycin, 2.8 for 3H-tetracycline, and 0.95 for 3H-dihydrostreptomycin. Corresponding values for uninfected erythrocytes were 3.2, 2.3, and 0.78, respectively. These data may explain the lack of antimalarial activity of the aminoglycosides and may partially explain the slow onset of therapeutic action of clindamycin and tetracycline. The low intracellular concentrations attained for the latter two drugs have important implications for understanding the kinetics and mechanism of their antiplasmodial effects.

Bacteriostatic action of streptomycin on ribosomally resistant mutants (rpsL) of Salmonella typhimurium.

Incubation of streptomycin-resistant (rpsL) mutants of Salmonella typhimurium in alkaline nutrient medium containing streptomycin brought about an inhibition of cell growth that was readily reversed by removing the antibiotic or neutralizing the medium. Growth inhibition was maximal at pH 8.2 and a streptomycin concentration of 800 micrograms/ml. A similar amount of dihydrostreptomycin had a negligible effect, and 10-times-higher concentrations of this antibiotic were required to reproduce the streptomycin action. Addition of streptomycin (400 micrograms/ml) to rpsL cells in alkaline (pH 8.2) nutrient medium caused inhibition of protein and DNA synthesis and also, but to a lower degree, of RNA synthesis. This effect on macromolecular synthesis was not due to ATP deprivation, since ATP content rose after addition of the antibiotic. At pH 8.2, the rate of entrance of streptomycin increased fourfold with respect to the rate at pH 7.0, leading to a large accumulation of streptomycin into rpsL cells. Uptake of the antibiotic was halted by addition of KCN or chloramphenicol. Equal uptake was obtained with 800 micrograms of dihydrostreptomycin or 400 micrograms of streptomycin per ml, yet the former did not affect cell growth at that concentration. It is concluded that high pH stimulates streptomycin and dihydrostreptomycin uptake by rpsL strains but only streptomycin accumulation causes growth inhibition in cells lacking the high-affinity ribosomal site.

Cross-linking of streptomycin to the 16S ribosomal RNA of Escherichia coli.

[3H]Dihydrostreptomycin was cross-linked to the 30S ribosomal subunit from Escherichia coli with the bifunctional reagent nitrogen mustard. The cross-linking primarily involved the 16S RNA. To localize the site of cross-linking of streptomycin to the 16S RNA, we hybridized RNA labeled with streptomycin to restriction fragments of the 16S RNA gene. Labeled RNA hybridized to DNA fragments corresponding to bases 892-917 and bases 1394-1415. These two segments of the ribosomal RNA must be juxtaposed in the ribosome, since there is a single binding site for streptomycin. This region has been implicated both in the decoding site and in the binding of initiation factor IF-3, indicating its functional importance.

Does streptomycin cause an error catastrophe?

We have examined the interpretation that streptomycin kills a bacterial culture by initiating the so-called error catastrophe. In particular, we asked whether the increased translational error rate induced by the antibiotic gives rise to an autocatalytic loss of functional fidelity of the devices responsible for gene expression, which ultimately causes the death of the culture. We have analyzed the performance characteristics of one of these devices, namely the ribosome in streptomycin-treated bacteria. We find that, although the treated ribosomes are constructed from error-containing proteins, they are not significantly different in elongation rate and fidelity from those ribosomes taken from untreated bacteria. We conclude that the bacteriocidal effect of streptomycin is not due to the initiation of an error catastrophe.

On the mechanism of translocation of dihydrostreptomycin across the bacterial cytoplasmic membrane.

This review examines two mechanisms, the channel and the uniport, proposed to explain the rapid, energy-dependent (EDP-II) phase of transport of dihydrostreptomycin (and streptomycin) across the bacterial cytoplasmic membrane. Bioenergetic and kinetic predictions are made from these two mechanisms and compared with available experimental data. Both the above mechanisms would be expected to lead to reversible transport kinetics, and to observable uptake of dihydrostreptomycin by respiring cytoplasmic membrane vesicles. However, transport is kinetically irreversible and is not observed in membrane vesicles (although the membrane vesicle findings need further confirmation), so the author rejects the proposed channel and uniport mechanisms. A possible mechanism of dihydrostreptomycin transport that would be consistent with the above experimental data, would be one in which a chemical reaction occurred as an obligatory part of the translocation cycle. Such a mechanism could be classified as primary translocation. The author emphasizes that this hypothesis is put forward to stimulate further experimental testing; it is not proposed to be a definitive explanation of the mechanism of energy-dependent dihydrostreptomycin transport.

Whole-body autoradiography of [3H]dihydrostreptomycin in guinea pigs and rats: the labelling of the inner ear in relation to other tissues.

[3H]Dihydrostreptomycin was given intramuscularly to young pigmented guinea pigs and rats. Whole-body autoradiography, combined with densitometric measurement of the blackening of the autoradiograms, and liquid scintillation counting were used to determine the levels of radioactivity in the inner ear in relation to blood and other tissues. It was found that there was an accumulation of radioactivity in the perilymph of both the cochlear and vestibular parts of the labyrinth. The labelling of the endolymph was weak. The levels of radioactivity in the membranous linings of the labyrinth including the areas covered with the neuroepithelial structures, were about the same as in the perilymph. The general distribution pictures were characterized by a localization of radioactivity in extracellular tissues, such as cartilages and connective tissues and by strong labelling of the kidney cortex. The preferential uptake in the perilymph indicates a route by which the inner ear hair cells can be exposed to high levels of aminoglycoside antibiotics.