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Biomedical subjects

S F Li

Publications and source records attributed to S F Li.

13 recordsLinked to original sources
PubMed1992

Upstream sequence elements required for NarL-mediated activation of transcription from the narGHJI promoter of Escherichia coli.

Transcription of the narGHJI operon (encoding nitrate reductase) in Escherichia coli is primarily dependent on the activation of the pleiotropic transacting factor Fnr, which interacts with the promoter through a cis element (Fnr box) located near the transcription start site. Further stimulation of transcription occurs in the presence of nitrate and is dependent on activation of the transacting factor NarL and a cis-acting sequence (NarL box) located approximately 200 base pairs upstream from the transcription start site. To define the structure of the NarL box, alterations in the NarL box region, generated by saturation mutagenesis of the sequence from positions -184 to -202 in the narGHJI promoter of a narG::lacZ fusion-bearing plasmid, were analyzed for their effects on NarL-mediated stimulation of transcription. Single base substitutions that significantly reduced the NarL-mediated stimulation were restricted to a 6-base sequence, TACTCC, located at positions -193 to -198 in the narGHJI promoter. When 2 bases were modified, NarL-mediated stimulation was severely reduced when one or both alterations were located within the 6-base sequence. Attempts to restore NarL-mediated stimulation with an inverted NarL box were not successful. Although previous studies suggested that NarL-mediated stimulation of transcription may occur by a DNA looping mechanism, the results presented here demonstrate that it does not involve the passive formation of a simple DNA loop. Replacement of 94 or 108 bases of the approximately 150 base sequence between the Fnr box and the NarL box with an unrelated sequence resulted in elimination of NarL-mediated stimulation of transcription. Furthermore, shifting of most of the intervening sequence or defined segments of the sequence by 4 bases while maintaining the position of the NarL box relative to sequences required for Fnr-dependent, anaerobic transcription also eliminated the NarL-mediated stimulation. We conclude that in addition to the 6-base NarL box located on a specific face of the promoter DNA, the stimulation of transcription by NarL requires some specific sequences and/or higher order structure specified by the DNA that separates the NarL box from the Fnr box.

Bacterial Proteins↗
PubMed1992

Analysis of food additives by ion-pairing electrokinetic chromatography.

A mixture of food additives is separated using ion-pairing electrokinetic chromatography with on-column ultraviolet detection at 190 nm. Tetrabutyl ammonium hydrogen sulfate (TBA) is used as modifier. The effect of the concentration of TBA on the migration behavior of the solutes is investigated. This method is used for the determination of the amount of additives in food samples. The percent relative standard deviation (%RSD) for the migration time is found to be less than 0.9% and that of the peak area is less than 2.5%.

Benzoates↗
PubMed1991

Determination of antihistamines in pharmaceuticals by capillary electrophoresis.

Capillary electrophoresis with on-column UV detection at 214 nm was used to separate a group of nine antihistamines. All these compounds were satisfactorily separated within ca. 6 min using a mixed carrier system containing sodium dodecyl sulphate with beta-cyclodextrin and tetrabutylammonium hydrogensulphate as modifiers. The application of the method to the determination of the amount of antihistamines present in commercial pharmaceutical samples was demonstrated. In addition, the migration behaviour of the antihistamines in the mixed carrier system was examined.

Capillary Action↗
PubMed1991

[Spectrophotometric determination of multicomponent vitamins by flexible tolerance simplex method].

Based on the brief discussion of error propagation in multivariate calibration and analysis, the flexible tolerance simplex method (FTSM) was applied to analyze vitamin B1, B2, B6 and niacinamide simultaneously. The results obtained were comparatively satisfactory both in precision and in accuracy. The average recoveries of vitamin B1, B2, B6 and niacinamide were 99.8 +/- 0.9% (CV), 100. 1 +/- 0.8% (CV), 100.2 +/- 2.1% (CV) and 100.1 +/- 0.7% (CV) respectively. All these show that FTSM, with calibration matrix K calculated from K = ASCST (CSCST)-1, is one of the most efficient procedures for improving accuracy, especially when there are interactions between variables and the problem is ill-conditioned.

Mathematics↗
PubMed1990

Retention of eleven priority phenols using micellar electrokinetic chromatography.

The use of micellar electrokinetic chromatography (MECC) for the separation of eleven substituted phenols listed by the United States Environmental Protection Agency as priority pollutants was investigated. Solutions of potassium and sodium dodecyl sulphate in phosphate-borate buffer of pH 6.6, 7.0 and 7.5 were used as the electrophoretic media. Satisfactory separation of the eleven phenols was obtained using a 180-microns capillary at 10 kV and pH 6.6 with a solution containing both sodium and potassium dodecyl sulphate. Observations on the retention behaviour of the phenols in MECC were related to their physico-chemical properties.

Chromatography↗
PubMed1990

Supercritical fluid extraction and chromatography of cholesterol in food samples.

A method based on supercritical fluid chromatography is presented which can be used for the determination of cholesterol in certain foods. The method involves the extraction with supercritical carbon dioxide and analysis of the extracts using a capillary column with supercritical carbon dioxide as mobile phase and flame ionization detection. Quantification is achieved using cholesteryl chloroacetate as an internal standard.

Carbon Dioxide↗
PubMed1989

[Intraoperative hemodynamics of pyrolytic carbon cambered bileaflet valve replacement in animals].

Pyrolytic carbon cambered bileaflet valve developed by Chengdu University of Technology and Sciences was evaluated for its hemodynamics. The curved surface of the valve leaflet is cylindrical. Under anaesthesia, cardiopulmonary bypass and chemical cardioplegia, a cambered bileaflet valve (i.d. = 16 mm) was replaced at mitral position in each of 12 goats. Before valve replacement, mitral flow rate (MFR) was 74.9 +/- 12.3 ml.sec-1, average valvular pressure drop delta P) 0.56 +/- 0.17 kPa (4.2 +/- 1.3 mmHg) and effective opening area (EOA) of the valve 1.20 +/- 0.18 cm2. After valve replacement, corresponding hemodynamic parameters were 49.3 +/- 12.7 ml.sec-1, 0.46 +/- 0.17 kPa (3.8 +/- 1.3 mmHg) and 0.81 +/- 0.12 cm2 respectively. In our previous report on yak pericardiac heart valve (i.d. = 20 mm) in goats, corresponding hemodynamic parameters after valve replacement were 45.6 +/- 11.8 ml.sec-1, 0.86 +/- 0.32 kpa (6.4 +/- 2.4 mmHg) and 0.65 +/- 0.26 cm2. With comparable mitral flow rate and heart rate (113.6 +/- 19.6 min-1 vs 119.1 +/- 17.1 min-1), the average valvular pressure drop of pyrolytic carbon cambered bileaflet valve is significantly smaller than that of the yak pericardiac valve (P less than 0.001), and the effective opening area is marginally larger than that of the yak pericardiac valve (0.05 less than P less than 0.1). If the same-sized valves of this two kinds are used, the hemodynamic parameter of cambered bileaflet valve would be better than those of yak pericardiac valve. Since the latter valve has been successfully used in patients, the present valve might be applicable in clinical situation.

Animals↗
PubMed1988

Location of sequences in the nar promoter of Escherichia coli required for regulation by Fnr and NarL.

Nitrate reductase, encoded by the nar operon in Escherichia coli, is produced only under anaerobic conditions and induced to its maximum level in the presence of nitrate. The anaerobic expression of the nar operon depends on the fnr gene product (Fnr), and the stimulation of anaerobic expression by nitrate requires the narL gene product (NarL). Distinct regulatory domains within the nar promoter are involved in these two responses. The specific locations of the sequences required for these two regulatory mechanisms were identified by analysis of a detailed set of deletions extending into the regulatory region of the nar operon from the 5' end. A region located around -55 base pairs (bp) from the transcriptional start site and immediately upstream from the presumed RNA polymerase binding site was required for the response to Fnr and anaerobic conditions. A base sequence no longer than 27 bp, located at about -200 bp, was essential for the stimulation by nitrate coupled with NarL. This NarL-specific sequence was equally effective if positioned 10 or 11 bp further upstream or downstream from its wild type position. However, it was ineffective if positioned 4, 6, or 14 bp or greater distances either upstream or downstream. Apparent autoregulation by active nitrate reductase occurred in all 5'-deletion constructions which retained the Fnr response, indicating that this regulatory phenomenon involves sequences located no further than -64 bp from the transcription start site.

Anaerobiosis↗
PubMed1988

Site-specific integration and expression of a developmental promoter in Myxococcus xanthus.

A series of intercellular signals are involved in the regulation of gene expression during fruiting body formation of Myxococcus xanthus. Mutations which block cell interactions, such as csgA (formerly known as spoC), also prevent expression of certain developmentally regulated promoters. csgA+ cells containing Tn5 lac omega DK4435, a developmentally regulated promoter fused to lacZ, began synthesizing lacZ mRNA 12 to 18 h into the developmental cycle. beta-Galactosidase specific activity increased about 12 h later. Neither lacZ mRNA nor beta-galactosidase activity was detected in a developing csgA mutant containing omega DK4435. The developmental promoter and its fused lacZ reporter gene were cloned into a pBR322-derived plasmid vector containing a portion of bacteriophage Mx8. These plasmids preferentially integrated into the M. xanthus chromosome by site-specific recombination at the bacteriophage Mx8 attachment site and maintained a copy number of 1 per chromosome. The integrated plasmids were relatively stable, segregating at a frequency of 0.0007% per generation in the absence of selection. The cloned and integrated promoter behaved like the native promoter, expressing beta-galactosidase at the proper time during wild-type development and failing to express the enzyme during development of a csgA mutant. The overall level of beta-galactosidase expression in merodiploid cells containing one native promoter and one promoter fused to lacZ was about half that of cells containing a single promoter fused to lacZ. These results suggest that the timing of developmentally regulated gene expression is largely independent of the location of this gene within the chromosome. Furthermore, they show that site-specific recombination can be a useful tool for establishing assays for promoter or gene function in M. xanthus.

Blotting, Southern↗
PubMed1987

Promoter region of the nar operon of Escherichia coli: nucleotide sequence and transcription initiation signals.

The nar operon, which encodes the three subunits of nitrate reductase in Escherichia coli, is fully induced under anaerobic conditions with nitrate. Two distinct regulatory domains have been delineated in the 5' region of the operon which respond respectively to positive induction by the fnr gene product under anaerobic conditions and to positive induction by the narL gene product in the presence of nitrate (S.F. Li, T. Rabi, and J.A. DeMoss, J. Bacteriol. 164:25-32). To characterize these two regulatory regions, we determined the DNA sequence for a 500-base-pair (bp) region extending upstream from the first structural gene of the nar operon. Analysis of subsequent subclones of the operon established that the 5' limit of the nar operon lies between 215 and 260 bp upstream from the translational start site of the first structural gene. The region required for induction by the fnr gene product is located within 160 bp from the translation start site, while the region responding to induction by nitrate extends an additional 100 bp upstream. Protein fusions of lacZ with the N-terminal sequence of the narG gene were constructed so that beta-galactosidase formation was under the control of the nar promoter and one or both regulatory domains. Analysis of strains bearing these fusion plasmids indicated that the expression of the hybrid proteins paralleled that of nitrate reductase by the parent plasmids, demonstrating that the regulatory signals did not extend significantly into the first structural gene. The transcriptional start site and the level of the transcription were determined by the S1 mapping procedure. One major transcript was identified which initiated -50 bp from the translational start site of the first structural gene. The synthesis of the transcript was repressed aerobically, was fully induced by nitrate anaerobically, and was greatly reduced in an Fnr- mutant. Possible regulatory sequences were identified in the 200-bp regulatory region extending upstream from the transcription start site.

Aerobiosis↗