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41.
42.
Although bacterial species display wide variation in their overall GC contents, the genes within a particular species' genome
are relatively similar in base composition. As a result, sequences that are novel to a bacterial genome—i.e., DNA introduced
through recent horizontal transfer—often bear unusual sequence characteristics and can be distinguished from ancestral DNA.
At the time of introgression, horizontally transferred genes reflect the base composition of the donor genome; but, over time,
these sequences will ameliorate to reflect the DNA composition of the new genome because the introgressed genes are subject
to the same mutational processes affecting all genes in the recipient genome. This process of amelioration is evident in a
large group of genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the amount of time required
after transfer for foreign DNA to resemble native DNA. Furthermore, models of amelioration can be used to estimate the time
of introgression of foreign genes in a chromosome. Applying this approach to a 1.43-megabase continuous sequence, we have
calculated that the entire Escherichia coli chromosome contains more than 600 kb of horizontally transferred, protein-coding DNA. Estimates of amelioration times indicate
that this DNA has accumulated at a rate of 31 kb per million years, which is on the order of the amount of variant DNA introduced
by point mutations. This rate predicts that the E. coli and Salmonella enterica lineages have each gained and lost more than 3 megabases of novel DNA since their divergence.
Received: 7 July 1996 / Accepted: 27 September 1996 相似文献
43.
44.
45.
The effect of inhibitors and uncouplers on the osmotic shock-sensitive transport systems for glutamine and galactose (by the β-methyl galactoside permease) was compared to their effect on the osmotic shock-resistant proline and galactose permease systems in cytochrome-deficient cells of Salmonella typhimurium SASY28. Both osmotic shock-sensitive and -resistant systems were sensitive to uncouplers and to inhibitors of the membrane-bound Ca2+, Mg2+-activated adenosine triphosphatase. This suggests that uptake by both types of systems is energized in these cells by an electrochemical gradient of protons formed by ATP hydrolysis through the ATPase. 相似文献
46.
Alexander P. Tulloch 《Phytochemistry》1978,17(9):1613-1615
Leaf wax of a glaucous variety of Poa ampla contains hydrocarbons (5%, C23–C35), esters (9%, C36–C56), free acids (3%, C16–C34), free alcohols (6%, mainly C26); hentriacontane-14,16-dione (14%), 5-oxohentriacontane-14,16-dione (1%); hydroxy β-diketones (56%) and unidentified material (6%). The hydroxy β-diketones, which are more abundant in this wax than in others, were shown by 13C NMR to consist of 4-hydroxy (15%), 5-hydroxy (70%) and 6-hydroxy (15%) hentriacontane-14,16-diones. 相似文献
47.
E Melloni B Sparatore F Salamino M Michetti S Pontremoli 《Biochemical and biophysical research communications》1982,107(3):1053-1059
The soluble neutral proteinase of human erythrocytes dissociates into constituent subunits of 80k and 30k in the presence of mM concentrations of Ca2+. Similarly the soluble natural inhibitor of this proteinase, of approximate molecular weight 240k, is dissociated into 60k subunits by mM concentrations of Ca2+. Removal of Ca2+ restores the native oligomeric structure of the proteinase and of the natural inhibitor. The formation of the native active enzyme or of the inactive enzyme-inhibitor complex depends on reversible association-dissociation processes mediated by Ca2+ concentration. 相似文献
48.
Nuñas (Thaseolus vulgaris, Fabaceae), commonly called popping beans, are traditionally grown in the Andean highlands of South America, and are consumed as a snack food after a quick toasting process. Proximate analysis of their nutritive value revealed that nunas have a higher content of starch, amylose, and copper than four dry bean varieties and a lower mean content of protein, phosphorous, iron, and boron. The unique texture and taste of nuñas may be related to their high starch content. Antinutritional factors such as lectins were higher in raw and boiled nuña samples than in toasted nuñas, while tannin levels did not change from raw to toasted treatments. Overall in-vitro digestibility was slightly lower for toasted nunas than boiled dry bean. 相似文献
49.
Four naturally occurring compounds, indole-3-carbinol (I3C), apigenin (Api), ellagic acid (EA) and tannic acid (TA), were tested for their inhibitory effects against 1-nitropyrene- (1-NP) or 1,6-dinitropyrene (1,6-DNP)-induced genotoxicity in Salmonella tester strains and Chinese hamster ovary (CHO) cells. Api and TA strongly inhibited the bacterial mutagenesis induced by nitropyrenes, while 13C and EA had little or no effect. For example, in TA98, 0.2 μmole Api resulted in 48% and 56% inhibition of the mutagenicity induced by 4 nmole 1-NP and 0.035 nmole 1,6-DNP, respectively. With an equal dose, expected, a good correlation was observed between the antimutagenicity of nitropyrenes and their inhibitory effect on nitroreductase activity. This indicated that one of the possible antimutagenic mechanisms of Api or TA was to inactivate the metabolism of nitropyrenes. Two biological end-points, cytotoxicity and sister-chromatid exchange (SCEs), were used to screen the antigenotoxic effects of these compounds in CHO cells. At the sub-cytotoxic dose, 13C, Api and TA all protected against the cytotoxicity induced by 1-NP and 1,6-DNP, but only TA and Api gave a significant reduction of the frequency of SCEs. Moreover, this reduction was found to be highly dose-dependent. 相似文献
50.
Hiroaki Nakayama Koji Nakayama Ritsuko Nakayama Yasuhiko Kato 《Archives of microbiology》1982,131(4):308-312
Hydrophobic and charge-charge interactions of Salmonella typhimirium and Serratia marcescens were determined and related to their content of fimbriae and lipopolysaccharide (LPS). The cell surface structures were characterized with hydrophobic interaction chromatography (HIC), electrostatic interaction chromatography (ESIC) and particle electrophoresis measurements. The degree of interaction at the air-water interface was tested using a monolayered lipid film applied to an aqueous surface. The cell surface hydrophobicity of S. typhimurium in the presence of fimbriae was less in smooth than in rought bacteria. Examination of a series of rough mutants of S. typhimurium indicates that reduction of the O-side chain and core oligosaccharides was correlated with increased cell hydrophobicity. The enrichment factors at the air-water interface were significantly higher for fimbriated than for non-fimbriated S. typhimurium cells. Fimbriated S. marcescens cells were less hydrophobic and adhered to a lesser degree at the air-water surface than non-fimbriated counterparts. Electrophoretic measurements and adsorption to ion exchangers gives different information about the surface charge of bacteria. The latter technique gives the interaction between localized charged surfaces.Abbreviations HIC
hydrophobic interaction chromatography
- ESIC
electrostatic interaction chromatography
- LPS
lipopolysaccharide
- PBS
phosphate buffered saline solution 相似文献