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  • 1
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Nuclear Physics, Section B 296 (1988), S. 961-990 
    ISSN: 0550-3213
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-072X
    Keywords: Streptomyces niveus ; Plasmid pSN2 ; Plasmid instability ; In vivo deletions ; Broad host range
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract A plasmid designated pSN2 (molecular size 32.0 kb) was isolated from the wild type of Streptomyces niveus ATCC 19793. To permit phenotypic identification of pSN2 the 1.9 kb BclI fragment was replaced in vitro by the 1.1 kb BclI fragment of pIJ702 carrying the thiostrepton resistance (tsr) gene to form the plasmid pSN3. pSN3 transforms S. lividans to thiostrepton resistance at high frequency and is stably maintained. However, when used to transform S. niveus pSN3 was unstable and produced a 5.5 kb thiostrepton resistant deletion derivative pLG5. pLG5 is also stable and expresses thiostrepton resistance in S. lividans but on transformation of S. niveus was unstable and produced a further thiostrepton resistant derivative, pLG10, of 6.5 kb. pLG5 and pLG10 like pSN3 transform S. lividans at high frequency and produce pocks. DNA hybridizations with a probe derived from pLG5 confirm that pLG5 is derived from DNA sequences present on pSN2 and pSN3.
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  • 3
    ISSN: 1617-4623
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Extracts derived from E. coli cells infected non-permissively with phage T1 amber mutants were used in an in vitro system to investigate the packaging of T1 DNA into phage heads. The standard extract used infections with amber mutants in genes 1 and 2 (g1-g2-) which are defective in T1 DNA synthesis but can synthesis the proteins required for particle morphogenesis. g1-g2- extracts packaged T1+ virion DNA molecules with an efficiency of 3×105 pfu/μg DNA. Extracts from cells infected with phage also defective in DNA synthesis but carrying additional mutations in genes 3.5 or 4 which are required for concatemer formation in vivo (g1-g3.5- and g1-g4- extracts) package T1 virion DNA at substantially lower efficiencies. Analysis of the DNA products from these in vitro reaction showed that concatemeric DNA is formed very efficiently by g1-g2- extracts but not by g1-g3.5- or g1-g4- extracts. These results are interpreted as evidence that the T1 in vitro DNA packaging system primarily operates in a similar manner to the in vivo headful mechanism. This is achieved in vitro by the highly efficient conversion of T1 virion DNA into concatemers which are then packaged with a much lower efficiency into heads to form infectious particles. A secondary pathway for packaging T1 DNA into heads and unrelated to the headful mechanism may also exist.
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  • 4
    ISSN: 1617-4623
    Keywords: Tellurium resistance ; Inducible ; Plasmid ; Escherichia coli
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary A large (〉250 kb) conjugative plasmid, pMER610, specifying resistance to tellurium and mercury was isolated from an Alcaligenes strain and transferred by conjugation to Escherichia coli AB1157. The acquisition of pMER610 by AB1157 increased the resistance to both tellurite and tellurate by 100-fold. Expression of tellurite resistance by pMER610 and the cloned Ter determinant was inducible by prior exposure to tellurite at levels sub-toxic to the sesitive AB1157. Physical analysis of the cloned Ter fragment located the resistance determinant to a 3.55 kb region. Insertion of Tn 1000 (γδ) into this region produced two classes of sensitive mutations, fully sensitive and intermediate or hyposensitive, which map in adjacent regions and form two complementation groups. Maxicell analysis identified four polypeptides (15.5, 22, 23 and 41 kDa) expressed by the Ter clone. The 23 kDa polypeptide may not be required for resistance since tellurium-sensitive γδ insertion mutations were not detected in the 23 kDa coding region.
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  • 5
    ISSN: 1617-4623
    Keywords: Pseudomonas ; Mercury resistance ; incJ plasmids ; R391 ; pMERPH
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary HgCl2 resistance (Hgr) in a strain of Pseudomonas putrefaciens isolated from the River Mersey was identified as plasmid-borne by its transfer to Escherichia coli in conjugative matings. This plasmid, pMERPH, could not be isolated and was incompatible with the chromosomally integrated IncJ Hgr plasmid R391. pMERPH and R391 both express inducible, narrow-spectrum mercury resistance and detoxify HgCl2 by volatilization. The cloned mer determinants from pMERPH (pSP100) and R391 (pSP200) have very similar restriction maps and express identical polypeptide products. However, these features show distinct differences from those of the Tn501 family of mer determinants. pSP100 and pSP200 failed to hybridize at moderate stringency to merRTPA and merC probes from Tn501 and Tn21, respectively. We conclude that the IncJ mer determinants are only distantly related to that from Tn501 and its closely homologous relatives and that it identifies a novel sequence which is relatively rare in bacteria isolated from natural environments.
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  • 6
    ISSN: 1617-4623
    Keywords: Streptomyces hygroscopicus ; Geldanamycin biosynthesis ; Ansamycins ; Polyketide antibiotics ; Insertional mutagenesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract A gene library constructed from large (∼20 kb) fragments of total DNA from the geldananmycin-producing strain Streptomyces hygroscopicus 3602 cloned in the plasmid vector pIJ61 were used to transform S. lividans TK24. Three transformants of about 800 tested were found to have acquired the ability to produce an antibiotic lethal to a geldanamycin-sensitive strain of Bacillus subtilis. The plasmids isolated from these transformants, pIA101, pIA102 and pIA103, each contained an insert of ∼15 kb. A 4.5 kb DNA fragment from the insert in pIA102 hybridised to DNA from S. hygroscopicus 3602 and to DNA encoding part of the erythromycin polyketide synthase but not to S. lividans TK24 DNA. The integration-defective phage vector ϕC31 KC515 containing this 4.5 kb fragment was able to lysogenise S. hygroscopicus 3602 to produce lysogens defective in geldanamycin production. Loss of the prophage restored the ability to produce geldanamycin. Extracts of fermentation broth cultures of S. lividans containing pIA101, pIA102 and pIA102 and pIA103 analysed by thin-layer chromatography (TLC) contained compounds identical or very similar to purified geldanamycin, which were not present in S. lividans. These compounds showed a mass spectrum indistinguishable from geldanamycin. The evidence suggests that the clones contain DNA sequences encoding functions required for geldanamycin biosynthesis including components of the polyketide synthase.
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  • 7
    ISSN: 1617-4623
    Keywords: Telluriur resistence ; Constitutive transcription pMER610 ; Mini Mu-lac fusions ; Northern blotting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The transpositional phage MudI 1734 lacZ was used to construct transcriptional fusions within the plasmid pMJ611, which contains the cloned tellurite resistance (TeR) determinant of the IncHI-2 plasmid pMER610. A series of 70 MudI insertions, in both orientations, causing loss of tellurite resistance in pMJ611, mapped within a 4.3 kb region which included the genes terA-terD and a 0.4 kb region upstream of the site previously reported as the 5′ limit of the TeR determinant. Expression of β-galactosidase from these transcriptional fusions, including those involving the 5′ upstream region, occurred only from inserts transcribed in the direction terA-terD, confirming the transcriptional orientation of the TeR determinant deduced from DNA sequence analysis. Sixteen of the tellurite-sensitive MudI fusions, distributed over the entire determinant and in both orientations, showed the same pattern of expression when transferred by conjugation and homologous recombination to pMER610, except that the β-galactosidase levels were consistently 2- to 3-fold higher in the parent plasmid. Northern analysis with a DNA probe spanning the TeR determinant identified five transcripts of 4.8, 4.0, 2.7, 1.5 and 1.0 kb synthesised by pMER610. Further hybridisations with DNA probes defining sub-sections of the TeR determinant, together with DNA sequence analysis, suggested the presence of three transcriptional start sites, at approximately 0.9 and 0.1 kb upstream of terA, and near the junction between terC and terD. Three transcriptional termination sites, located within terA, near the terC-terD junction and at the 3′ end of terE are also indicated. Both the expression of β-galactosidase from the MudI fusions and the synthesis of ter gene transcripts are constitutive and were not affected by prior exposure of cultures to sub-toxic levels of tellurite. Further DNA sequence analysis reveals that the extensive homology between terD and terE extends to a section of terA.
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  • 8
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 301 (1983), S. 264-266 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] We have shown that amber mutants in 8 of the 10 known Tl head genes accumulate concatemeric DNA during non-permissive infection7. One of the two exceptions, aw37 (gene 12), efficiently degraded concatemers to unit-length molecules in the same manner as wild type. The other, am283 (gene 13.3), ...
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  • 9
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have developed a technique for the fabrication of high-mobility electron gases formed in undoped GaAs/AlGaAs heterostructures. The use of an insulated gate allows independent control over the carrier density in the Hall bar and ohmic contact regions of the device. This unique design eliminates difficulties in obtaining reliable ohmic contacts, particularly in the low carrier density regime. In the absence of remote ionized impurity scattering, extremely high transport mobilities are obtained at low carrier densities (1×106 cm2 V−1 s−1 at 1×1010 cm−2). This design has been adapted to the formation of undoped one-dimensional electron gases that show clean and reproducible conductance plateau at 1.5 K. © 1999 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 49-51 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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