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Biomedical subjects

S N Cohen

Publications and source records attributed to S N Cohen.

At least 19 recordsLinked to original sources
PubMed1992

Propagation of pSC101 plasmids defective in binding of integration host factor.

Integration host factor (IHF), a multifunctional protein of E. coli, normally is required for the replication of plasmid pSC101. T. T. Stenzel, P. Patel, and D. Bastia (Cell 49:709-717, 1987) have reported that IHF binds to a DNA locus near the pSC101 replication origin and enhances a static bend present in this region; mutation of the IHF binding site affects the plasmid's ability to replicate. We report here studies indicating that the requirement for IHF binding near the pSC101 replication origin is circumvented partially or completely by (i) mutation of the plasmid-encoded repA (replicase) gene or the chromosomally encoded topA gene, (ii) the presence on the plasmid of the pSC101 partition (par) locus, or (iii) replacement of the par locus by a strong transcriptional promoter. With the exception of the repA mutation, the factors that substitute for a functional origin region IHF binding site are known to alter plasmid topology by increasing negative DNA supercoiling, as does IHF itself. These results are consistent with the proposal that IHF binding near the pSC101 replication origin promotes plasmid replication by inducing a conformational change leading to formation of a repA-dependent DNA-protein complex. A variety of IHF-independent mechanisms can facilitate formation of the putative replication-initiation complex.

Bacterial Proteins↗
PubMed1992

Random-choice replication of extrachromosomal bovine papillomavirus (BPV) molecules in heterogeneous, clonally derived BPV-infected cell lines.

Using fluorescence in situ hybridization and Southern blot analysis, we show that three clonally derived cell lines transformed with bovine papillomavirus (BPV), including ID13, the cell line commonly employed for BPV replication studies, are heterogeneous populations having extensive cell-to-cell variation in both the distribution and amount of BPV DNA. Different subclones of ID13 were found to differ in the form and amount of BPV DNA they contain. Most subclones showed no detectable BPV sequences; some contained either extrachromosomal BPV molecules distributed throughout the nucleus or BPV sequences integrated at discrete chromosomal sites, while others contained both integrated and plasmid forms. The results of density gradient analysis of BPV DNA from individual homogeneous subclones showed replication of the extrachromosomal BPV plasmids in a random-choice mode. In all cell lines studied, the presence after one round of chromosomal DNA replication of unreplicated BPV DNA and of BPV DNA having two postreplicative strands was independent of the presence of high-BPV-copy-number ("jackpot") cells. Our results substantiate the earlier conclusion that extrachromosomal BPV molecules replicate randomly and not according to a once-per-cell-cycle mechanism.

Animals↗
PubMed1979

Antimicrobial selection by a computer. A blinded evaluation by infectious diseases experts.

An evaluation of a computer-based consultation system called MYCIN was made. Eight independent evaluators with special expertise in the management of meningitis compared MYCIN's choice of antimicrobials with the choices of nine human prescribers for ten test cases of meningitis. MYCIN received an acceptability rating of 65% by the evaluators; the corresponding ratings for acceptability of the regimen prescribed by the five faculty specialists ranged from 42.5% to 62.5%. The system never failed to cover a treatable pathogen while demonstrating efficiency in minimizing the number of antimicrobials prescribed. The study design may be useful in assessing the performance of other computer-based clinical decision-making systems.

Adult↗
PubMed1979

Genetic analysis of the inter-relationship between plasmid replication and incompatibility.

The relationship between replication control and plasmid incompatibility has been investigated using a composite replicon, pPM1, which consists of the pSC101 plasmid ligated to another small multicopy plasmid, RSF1050. Since pPM1 can utilise the replication system of either of the two functionally distinct components, propagation of the composite plasmid can occur in the presence of a mutation of one of its moieties. Such mutants are detected by their inability to rescue the composite plasmid under conditions not permissive for replication of the other moiety. Mutations in incompatibility functions can be detected by the failure of the composite replicon to exclude co-existing plasmids carrying a replication system identical to the one on pPM1. The inability of the composite plasmid to replicate at 42 degrees in a host synthesizing temperature-sensitive DNA polymerase I, which is required by the RSF1050 replication system, was used to isolate pPM1 mutants defective in replication of the pSC101 component. Mutants defective in the incompatibility functions of pSC101 were obtained by selecting derivatives that allow the stable coexistence of a second pSC101 replicon in the same cell. Analysis of these two classes of mutants indicates that plasmids selected for defective pSC101 replication ability nervertheless retain pSC101 incompatibility. In contrast, plasmid mutants that have lost incompatibility functions were found always to be defective in replication ability.

DNA Replication↗
PubMed1979

Nucleotide sequence of cloned cDNA for bovine corticotropin-beta-lipotropin precursor.

The nucleotide sequence of a 1,091-base pair cloned cDNA insert encoding bovine corticotropin-beta-lipotropin precursor mRNA is reported. The corresponding amino acid sequence indicates that the precursor protein consists of repetitive units and includes a third melanotropin sequence in its cryptic portion. Pairs of lysine and arginine residues separate the component peptides of the precursor.

Adrenocorticotropic Hormone↗
PubMed1979

High frequency transformation of Bacillus subtilis protoplasts by plasmid DNA.

A highly efficient method for transformation of Bacillus subtilis by plasmid DNA is reported. The procedure, which involves polyethylene glycol-induced DNA uptake by protoplasts and subsequent regeneration of the bacterial cell wall, yields up to 80% transformants with an efficiency of 4 x 10(7) transformants per microgram of supercoiled DNA. Plasmids constructed by in vitro ligation or endonuclease-generated fragments of linear plasmid DNA can also transform PEG-treated protoplasts, but at a lower frequency.

Bacillus subtilis↗
PubMed1979

Evaluating the performance of a computer-based consultant.

The performance of a computer-based clinical consultation system is evaluated. The program, called MYCIN, is designed to function as an aid for infectious disease diagnosis and therapy selection, with an initial emphasis on bacteremias. The evaluation methodology is discussed, as well as the difficulties encountered in attempting to evaluate clinical judgments. Specialists in infectious diseases judged MYCIN's final therapy recommedation, and intermediate conclusions about the significance of the infection and identity of infecting organisms. The evaluation techniques described may be useful in assessing the performance of other clinical decision aids. Results of the evaluation show that the program's therapy recommedations meet Stanford experts' standards of acceptable practice 90.9% of the time (table 2), with some variation noted both among individual experts and between Stanford experts and others (tables 1, 2).

Animals↗
PubMed1979

Identification and characterization of a self-regulated repressor of translocation of the Tn3 element.

Gene fusions that bring expression of the lacZ gene under control of transcriptional and trnaslational signals within the transposable element Tn3 have been used to study regulation of Tn3-specified proteins. A gene encoding a 21,355-Mr peptide that represses translocation of Tn3 and acts at the level oquenced; amber, missense, and cis-dominant (operator-constitutive) point mutations in this gene have been isolated and characterized.

Bacterial Proteins↗
PubMed1979

Lactose genes fused to exogenous promoters in one step using a Mu-lac bacteriophage: in vivo probe for transcriptional control sequences.

The lactose structural genes, without the lactose promoter, have been incorporated into the bacteriophage Mu genome to form a Mu-lac specialized transducing phage. This phage also carries a gene encoding resistance to ampicillin (Ap)[Mu(Ap, lac)]. After infection and upon establishment of lysogeny, the Mu(Ap, lac) genome can integrate into apparently random sites in the Escherichia coli chromosome. When integration occurs within a gene in the orientation of its transcription, the lactose structural genes are so situated that they become expressed solely from the promoter of that gene. Thus, expression of the lactose genes of Mu(Ap, lac) can be used as an assay for transcription of that gene and for functional and mutational studies of gene regulation.

Arabinose↗
PubMed1979

Structural and functional analysis of cloned DNA segments containing the replication and incompatibility regions of a miniplasmid derived from a copy number mutant of NR1.

A 1.45-megadalton segment of DNA cloned from a miniplasmid derived in vivo from a copy number mutant of the R plasmid NR1 has been shown to contain all functions essential for incompatibility and autonomous plasmid replication in Escherichia coli. Specific endonuclease cleavage sites within this DNA segment that localize functions required for replication have been mapped. A 0.45-megadalton fragment that specifies the FII incompatibility of NR1 has been identified within the replication region, and DNA fragments containing this incompatibility region, but lacking other functions required for replication, have been cloned.

DNA Replication↗
PubMed1979

Effect of temperature on translocation frequency of the Tn3 element.

The effect of temperature on the translocation frequency of the Tn3 element was investigated. The temperature optimum for translocation of Tn3 was in the range from 26 to 30 degrees C. At temperatures above 30 degrees C, the translocation frequency decreased rapidly and linearly; at 36 degrees C it was only 5% of the frequency observed at 30 degrees C. The duration and reversibility of the temperature effect were utilized to demonstrate a requirement for protein synthesis in the translocation process.

Bacterial Proteins↗
PubMed1979

Teaching clinical problem solving in preclinical occlusion courses.

The teaching of the basic principles and concepts of occlusion can be enhanced. Preclinical occlusion courses are made relevant to the clinical experience by introducing clinically related problem-solving exercises. These exercises develop a student's clinical problem-solving skill and create an excitement for active learning in the preclinical laboratory. Examples of these exercises are presented.

Clinical Competence↗