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排序方式: 共有647条查询结果,搜索用时 15 毫秒
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John R. Lloyd Ill Y. Lee Vinod Nair Elizabeth Fischer Helena I. M. Boshoff Clifton E. Barry III 《Molecular microbiology》2012,86(2):367-381
Carbapenems such as meropenem are being investigated for their potential therapeutic utility against highly drug‐resistant tuberculosis. These β‐lactams target the transpeptidases that introduce interpeptide cross‐links into bacterial peptidoglycan thereby controlling rigidity of the bacterial envelope. Treatment of Mycobacterium tuberculosis (Mtb) with the β‐lactamase inhibitor clavulanate together with meropenem resulted in rapid, polar, cell lysis releasing cytoplasmic contents. In Mtb it has been previously demonstrated that 3‐3 cross‐linkages [involving two diaminopimelate (DAP) molecules] predominate over 4‐3 cross‐linkages (involving one DAP and one D‐alanine) in stationary‐phase cells. We purified and analysed peptidoglycan from Mtb and found that 3‐3 cross‐linkages predominate throughout all growth phases and the ratio of 4‐3/3‐3 linkages does not vary significantly under any growth condition. Meropenem treatment was accompanied by a dramatic accumulation of unlinked pentapeptide stems with no change in the tetrapeptide pools, suggesting that meropenem inhibits both a D,D‐carboxypeptidase and an L,D‐transpeptidase. We purified a candidate D,D‐carboxypeptidase DacB2 and showed that meropenem indeed directly inhibits this enzyme by forming a stable adduct at the enzyme active site. These results suggest that the rapid lysis of meropenem‐treated cells is the result of synergistically inhibiting the transpeptidases that introduce 3,3‐cross‐links while simultaneously limiting the pool of available substrates available for cross‐linking. 相似文献
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Tedmund J. Swiecki Anton G. Endress O. Clifton Taylor 《American journal of botany》1982,69(1):141-149
Primary leaves of Phaseolus vulgaris L. (pinto bean), 9 or 12 days from sowing, were exposed to aqueous acids, chloride salts, or hydrogen chloride gas. Leaves were examined for the presence and severity of resultant visible injury and sampled for light and scanning electron microscopy. Exposure to 0.06 n HCl, HNO3, H2SO4 or 14.5–19.0 mg m-3 gaseous HCl for 20 min evoked similar foliar injury including glazing and necrosis of the laminas. This injury appeared to result initially from plasmolysis and collapse of the epidermis and subsequently of the underlying mesophyll. Cellular injury was accompanied by various cytoplasmic alterations. Microscopic symptoms observed in leaves exposed to gaseous HCl or aqueous acids included vesicles and particulates within the larger vacuoles. Similar symptoms were present in leaves exposed to polyethylene glycol 6000. Differential effects included formation of necrotic “pits” and preferential injury to paravascular tissues in leaves treated with aqueous acids and crystalline chloroplast inclusions in gaseous HCl-treated and water-stressed leaves. The visible and microscopic appearances of leaves exposed to aqueous acids or gaseous HCl were compared and related to injury stemming from acid precipitation and a possible mechanism of action for gaseous HCl phytotoxicity. 相似文献
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Donia D. McLemore Adel El Naggar L. Clifton Stephens John H. Jardine 《Biotechnic & histochemistry》1990,65(6):279-291
Flow Cytometric (FCM) DNA content analysis of paraffin-embedded tissues has become a widely accepted procedure in assessing the biologic course in some tumors from archival material. Difficulty in interpreting histograms is frequently due to high levels of debris, and wide coefficients of variation (CV). These may lead to underestimating near-diploid abnormality. Although the clinical significance of low-degree aneuploidy has yet to be established, the procedure reported here improved our endeavor to detect DNA Indices (DI) of at least 1.1%. Experience has shown that careful technique can result in overall improvement of DNA histograms by lowering levels of debris and % CV, dramatically so in some cases. Archival histograms can be generated that rival in quality fresh tissue results. Archival material is currently employed for diagnostic and research purposes. 相似文献
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