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Biomedical subjects

H L Levin

Publications and source records attributed to H L Levin.

28 records · Page 2Linked to original sources
PubMed1990

Two related families of retrotransposons from Schizosaccharomyces pombe.

Two related families of transposons were isolated from schizosaccharomyces pombe, an organism which has been the object of extensive genetic studies which had previously produced no evidence for the existence of such elements. These two classes of repeated DNAs, dubbed Tf1 (transposon of fission yeast 1) and Tf2 have many properties of retrotransposons. Tf1 and Tf2 both possess long terminal repeats and predicted protein sequences that resemble the protease, reverse transcriptase, and integrase domains of retroviruses. The chromosomal locations and total numbers of Tf1 and Tf2 differ greatly in various isolates of S. pombe. The Tf elements are expressed in the form of 4.5-kb mRNAs. The complete sequence of Tf1 was determined and suggests that a novel mechanism for regulating its gene expression may be used.

Amino Acid Sequence↗
PubMed1989

Attenuation in the regulation of the pyrBI operon in Escherichia coli. In vivo studies of transcriptional termination.

The attenuation model for transcriptional regulation of the Escherichia coli pyrBI operon is based on the assumption that transcription terminates upstream of the structural genes at a rho-independent terminator when cells contain high levels of UTP. When, however, the cells are limited for pyrimidines, the presence of ribosomes translating the short leader peptide is presumed to cause an alteration in the secondary structure of the terminator in a way that allows RNA polymerase to transcribe the entire operon. These two premises of transcriptional regulation were tested by using exonuclease protection assays to map the 3' ends of transcripts extracted from cells containing either ample or depleted concentrations of pyrimidines. The results support the model since 99% of the pyrBI transcripts terminated at the (G + C)-rich region of dyad symmetry upstream of the structural genes when cells were grown in excess uracil. In addition, a significant portion (36%) of the pyrBI transcripts extracted from cells containing reduced pyrimidine concentrations extended past the dyad into the structural genes. This observation correlated with the amounts of aspartate transcarbamoylase synthesized in cells under the various conditions. The mapping technique was also used to determine the position of the 5' ends of the transcripts to measure contributions of two potential start sites (P1 and P2) to the pool of pyrBI transcripts. The results show that under all conditions no more than 3% of the total transcripts had 5' ends corresponding to the upstream promoter, P1. In cells lacking P1 virtually all transcripts from P2 terminated at the (G + C)-rich hairpin when the cellular level of pyrimidines was high. Conversely 57% of the transcripts extended past the terminator when cells were grown in UMP. The S1 nuclease technique also provided a measure of the steady state level of transcripts originating at P2. In cells depleted of pyrimidines there was a 5-10-fold increase in these transcripts depending on the number of copies of pyrBI. This increase, which is independent of attenuation, is caused by a different regulatory mechanism which as yet has not been identified.

Aspartate Carbamoyltransferase↗
PubMed1987

Laryngotracheal reconstruction. Sternohyoid myocutaneous rotary door flap.

The vascularized sternohyoid myocutaneous rotary door flap has been used for laryngotracheal reconstruction in ten patients in the past two years. Nine were reconstructed for laryngotracheal stenosis and one for immediate reconstruction after conservation laryngeal surgery for carcinoma. Before reconstruction, the nine patients with stenosis had each undergone a mean of eight surgical attempts at correction. Eight patients were tracheostomy dependent, and only two patients had effective voices. To date, seven patients have been decannulated and all ten have effective voices. No significant complications have been noted. This flap can provide a readily applicable, dependable technique that is useful in management of difficult laryngotracheal stenosis and for reconstruction after conservation laryngeal surgery.

Adult↗
PubMed1985

Regulation of aspartate transcarbamoylase synthesis in Escherichia coli: analysis of deletion mutations in the promoter region of the pyrBI operon.

The catalytic and regulatory polypeptide chains of Escherichia coli aspartate transcarbamoylase are encoded by the pyrB and pyrI genes, respectively, which constitute a single transcriptional unit in the pyrBI operon. The DNA sequence immediately preceding the first structural gene, pyrB, contains a short open reading frame that could encode a 44-amino acid leader peptide and a (G+C)-rich region of dyad symmetry followed by eight thymidine residues. Synthesis of the enzyme is negatively controlled at the level of transcription depending on the cellular level of UTP, and an attenuation mechanism has been proposed to account for the 70-fold increase in pyrBI expression on pyrimidine starvation. The potential role of the dyad and eight thymidines as an attenuator was tested with a plasmid containing the promoter region of the pyrBI operon upstream of the galK coding sequence. When cells containing this plasmid, pPYRB10, were grown in a medium low in uracil, there was an 83-fold increase in galactokinase activity compared with the same cells grown at high uracil levels. This regulation is similar to that for aspartate transcarbamoylase synthesis in cells depleted of pyrimidines. Deletions constructed in the promoter region of pPYRB10 from the 3' side produced one plasmid that retained normal control of galK expression and five that exhibited greatly reduced regulation. Nucleotide sequence determination showed that the one deletion mutation that was functionally similar to the wild-type plasmid contained the entire region of dyad symmetry, including the eight thymidines. The plasmids with more extensive deletions lacked the region with dyad symmetry and the eight thymidines. One of the deletion mutants that exhibited very low levels of regulation lacks the entire sequence coding for the putative leader peptide up to the major promoter. The results demonstrating the crucial role of a 19-nucleotide sequence (from -33 to -15) support an attenuation model but indicate that other mechanisms also contribute to the regulation of the pyrBI operon.

Aspartate Carbamoyltransferase↗
PubMed1980

Transverse sinus anomaly simulating a meningioma on CT scanning.

A rare, anomalously dilated right transverse sinus with involvement of the torcular herophili strongly suggested the appearance of a meningioma on CT scanning. Carotid and vertebral arteriography did not reveal the nature of the abnormality, since there was only faint opacification of the anomalous vessel, which was interpreted as a faint tumor stain. Transjugular venography was necessary to establish this diagnosis. When a similar lesion is encountered on CT scanning and the transverse sinus is not clearly seen on arteriography, venography should be carried out to determine if the appearance is due to occlusion by a mass or, as in this case, anomalous dilatation of the sinus.

Adult↗
PubMed1975

Short-term changes in incubation temperature: behavioral and physiological effects in the chick embryo from 6 to 20 days.

In 3 experiments we have attempted to determine the extent to which the chick embryo is behaviorally and physiologically responsive to short term changes (.5-2 hr) in the normal ambient incubation temperature. Embryos ranging in age from 6-20 days of incubation have been examined after exposure to temperatures ranging from 30.5 degrees to 44.4 degrees C (normal incubator and nesting temperatures are 37-38 degrees C). At 6,9 and 12 days of age the heart rate and the duration of amnion contractions were significantly altered by exposure to temperature either higher or lower than normal although overt neuromuscular activity (motality) remains unchanged. At 15 and 20 days, however, (hatching occurs on Day 21) the rate of neuromusclar activity is altered at both low and high temperatures. At 20 days, beak-clapping, vocalization, and respiration rates also change reliably upon short-term exposure to both high and low temperatures. The possibility is discussed that embryonic responsiveness to temperature changes in the environment during natural incubation may play some role in later behavioral capabilities.

Amnion↗