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Biomedical subjects

A D Johnson

Publications and source records attributed to A D Johnson.

At least 19 recordsLinked to original sources
PubMed1992

Ssn6-Tup1 is a general repressor of transcription in yeast.

The homeodomain protein alpha 2 and the SRF-like protein Mcm1 are required to establish cell type in the yeast Saccharomyces cerevisiae. Together, these regulatory proteins recognize a specific DNA operator, marking a set of genes for transcriptional repression. In this paper, we show that occupancy of the operator by alpha 2-Mcm1 is not sufficient to bring about repression. Rather, repression is effected only when Ssn6 (a TPR protein) and Tup1 (a beta-transducin repeat protein) are also present in the cell. We show that Ssn6 represses transcription when brought to a promoter by a bacterial DNA-binding domain and that Tup1 is required for this repression. Based on these and other results, we propose that Ssn6-Tup1 is a general repressor of transcription in yeast, recruited to target promoters by a variety of sequence-specific DNA-binding proteins.

Immunoblotting↗
PubMed1992

A molecular mechanism for combinatorial control in yeast: MCM1 protein sets the spacing and orientation of the homeodomains of an alpha 2 dimer.

DNA recognition sequences for dimeric proteins typically contain two types of information. The first is the DNA sequence of each half-site, and the second is the arrangement of these half-sites. We show that dimers of the yeast homeodomain protein alpha 2, although able to read the first type of information, lack the ability to assess the second type. Rather, alpha 2 dimers bind with equal affinity to artificial operators in which the two half-sites are arrayed as inverted repeats, as direct repeats, or as everted (inside-out) repeats. We show that a second protein-MCM1-sets the exact spacing and orientation of the homeodomains in the alpha 2 dimer so that they accommodate only the geometry of the naturally occurring operators. These experiments show directly how the target specificity of a homeodomain protein is raised by an auxiliary protein, allowing it to distinguish the biologically correct operators from closely related sequences in the cell.

Base Sequence↗
PubMed1992

Meiotic induction of the yeast HOP1 gene is controlled by positive and negative regulatory sites.

The process of meiosis and sporulation in the yeast Saccharomyces cerevisiae is a highly regulated developmental pathway dependent on genetic as well as nutritional signals. The HOP1 gene, which encodes a component of meiotic chromosomes, is not expressed in mitotically growing cells, but its transcription is induced shortly after yeast cells enter the meiotic pathway. Through a series of deletions and mutations in the HOP1 promoter, we located two regulatory sites that are essential for proper regulation of HOP1. One site, called URS1H, brings about repression of HOP1 in mitotic cells and functions as an activator sequence in cells undergoing meiosis. The second site, which we designated UASH, acts as an activator sequence in meiotic cells and has similarity to the binding site of the mammalian CCAAT/enhancer binding protein (C/EBP). Both sites are required for full meiotic induction of the HOP1 promoter. We conclude that in mitotic yeast cells, the URS1H site maintains the repressed state of the HOP1 promoter, masking the effect of the UASH site. Upon entry into meiosis, repression is lifted, allowing the URS1H and UASH sites to activate high-level transcription.

Base Sequence↗
PubMed1991

Crystal structure of a MAT alpha 2 homeodomain-operator complex suggests a general model for homeodomain-DNA interactions.

The MAT alpha 2 homeodomain regulates the expression of cell type-specific genes in yeast. We have determined the 2.7 A resolution crystal structure of the alpha 2 homeodomain bound to a biologically relevant DNA sequence. The DNA in this complex is contacted primarily by the third of three alpha-helices, with additional contacts coming from an N-terminal arm. Comparison of the yeast alpha 2 and the Drosophila engrailed homeodomain-DNA complexes shows that the protein fold is highly conserved, despite a 3-residue insertion in alpha 2 and only 27% sequence identity between the two homeodomains. Moreover, the orientation of the recognition helix on the DNA is also conserved. This docking arrangement is maintained by side chain contacts with the DNA--primarily the sugar-phosphate backbone--that are identical in alpha 2 and engrailed. Since these residues are conserved among all homeodomains, we propose that the contacts with the DNA are also conserved and suggest a general model for homeodomain-DNA interactions.

Base Sequence↗
PubMed1991

Secondary structure of the homeo domain of yeast alpha 2 repressor determined by NMR spectroscopy.

The yeast alpha 2 protein is a regulator of cell type in Saccharomyces cerevisiae. It represses transcription of a set of target genes by binding to an operator located upstream of each of these genes. The alpha 2 protein shares weak sequence similarity with members of the homeo domain family; the homeo domain is a 60-amino-acid segment found in many eukaryotic transcriptional regulators. In this paper we address the question of whether alpha 2 is structurally related to prototypical members of the homeo domain family. We used solution 1H and 15N nuclear magnetic resonance [NMR] spectroscopy to determine the secondary structure of an 83-amino-acid residue fragment of alpha 2 that contains the homeo domain homology. We have obtained resonance assignments for the backbone protons and nitrogens of the entire 60-residue region of the putative homeo domain and for most of the remainder of the alpha 2 fragment. The secondary structure was determined by using NOE connectivities between backbone protons, 3JHN-H alpha coupling constants, and dynamical information from the hydrogen exchange kinetics of the backbone amides. Three helical segments exist in the alpha 2 fragment consisting of residues 11-23, 32-42, and 46-60 (corresponding to residues 138-150, 159-169, and 173-187 of the intact protein). The positions of these three helices correspond extremely well to those of the Drosophila Antennapedia (Antp) and engrailed (en) homeo domains, whose three-dimensional structures have recently been determined by NMR spectroscopy and X-ray crystallography, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1991

Histochemistry and ultrastructure of the metacercarial cysts of blackspot trematodes Uvulifer ambloplitis and Neascus pyriformis.

Cysts of Uvulifer ambloplitis from green sunfish, Lepomis cyanellus, and Neascus pyriformis from red shiners, Notropis lutrensis, were studied with light-level histochemistry and scanning and transmission electron microscopy. Cysts of both species are bilayered, consisting of an outer host capsule and an inner parasite cyst; the space between these layers is filled with a viscous material. The outer portion of the host capsule of both species is composed of fibrocytes, melanin granules, and collagen fibrils, and the inner portion of layers of flattened fibrocytes. The parasite cyst of U. ambloplitis is formed of 2 layers, an outer dense layer and an inner light layer, whereas the parasite cyst of N. pyriformis is made of 3 layers. A thin outer light-staining layer is present in addition to the 2 layers observed in U. ambloplitis. Results of histochemical staining were the same for both species. The host capsule stained positively for proteins and neutral and acid mucopolysaccharides. The viscous material was positive for neutral and acid mucopolysaccharides but not for proteins. The parasite cyst gave a strong positive reaction for neutral mucopolysaccharides but was negative for acid mucopolysaccharides and proteins.

Animals↗
PubMed1979

Distribution and severity of coronary artery disease in 500 patients with angina pectoris.

The distribution and severity of coronary disease in 500 patients with angina pectoris and at least one area of 50% or greater reduction of luminal diameter in a major coronary artery were compared with respect to patients' age and coronary arterial pattern. The coronary arterial patterns were separated into right (360 patients), mixed (89 patients), and left (51 patients) systems, depending upon the blood supply to the inferior surface of the left ventricle. The following relationships were noted: 1) In patients with angina pectoris, the distribution and severity of coronary artery disease is similar from the third to eighth decade. 2) Coronary arterial stenoses of 50-70% of greater reduction of luminal diameter involve most frequently the proximal portion of the major vessels. Coronary artery disease is multivessel in nature in 80% of cases. In single vessel disease the left anterior descending artery is involved most frequently. 3) The left main coronary artery is moderately to severely obstructed less frequently in individuals with left (2%) as compared to right (8%) and mixed (10%) systems. Otherwise, the distribution of coronary artery disease is similar in right, mixed, and left systems. 4) Coronary artery disease is a diffuse rather than a focal process. As demonstrated by coronary arteriography, patients with coronary artery disease have smaller vessels throughout the arterial tree as compared with individuals free of evident coronary atherosclerosis.

Adult↗
PubMed1979

Transseptal and retrograde left ventricular catheterization in patients with the stent-mounted porcine xenograft valve prosthesis in the aortic position.

Cardiac catheterization techniques for measuring the systolic pressure gradient across the stent-mounted porcine xenograft in the aortic position and accomplishing left ventriculography are described. The transseptal technique is a rapid and predictable means of entering the left ventricle but requires a highly skilled operator. Retrograde left ventricular catheterization via the femoral artery is a technique familiar to all invasive cardiologists. Usually a pitfall catheter is used. In patients with the stent-mounted procine xenograft in the aortic position, we have found the A2 Multipurpose catheter to be the fastest and most predictable means of entering the left ventricle from the groin. Potential complications of retrograde left ventricular catheterization in patients with aortic valve prostheses are discussed.

Aortic Valve↗
PubMed1979

Interactions between DNA-bound repressors govern regulation by the lambda phage repressor.

The lambda phage repressor binds cooperatively to the three sites in the right operator (O(R)) according to the following pattern. If the DNA is wild type, O(R)1 and O(R)2 are filled coordinately because of interactions between repressor dimers bound to these two sites. Site O(R)3 is filled only at higher repressor concentrations. In contrast, if O(R)1 is mutant, O(R)2 and O(R)3 are filled coordinately because of interactions between repressors bound to these sites. In this case, the affinity of O(R)3 is increased and that of O(R)2 is decreased relative to the wild type. We infer that a repressor dimer bound to the middle site O(R)2 can interact either with another repressor dimer bound to O(R)1 (wild-type case) or, alternatively, with one bound to O(R)3 (mutant O(R)1 case). We argue that these repressor interactions are mediated by protein-protein contacts between adjacent repressor dimers, because the isolated amino-terminal domains of repressor bind to the operator sites noncooperatively. The cro protein of phage lambda, a second regulatory protein, which recognizes the same three sites in O(R) as does repressor, binds non-cooperatively. Experiments performed in vivo show that regulation of gene expression by repressor can be influenced critically by cooperative interactions. We demonstrate that the effect of repressor in a lysogen on the activity of the promoter P(RM) can be changed from activation to repression by deletion of O(R)1. We explain this effect in terms of the alternative cooperative interactions described above.

Bacteriophage lambda↗
PubMed1979

Purification and characterization of different types of exfoliative toxin from Staphylococcus aureus.

Exfoliative toxin was isolated from strain DI of Staphylococcus aureus using carboxymethyl cellulose and hydroxylapatite chromatography. This purified toxin was compared with that produced by strain TA. The specific biological activity of the two toxins was the same, but they were serologically distinct. These strains have different loci (plasmid or chromosomal) for toxin production; differences were seen in molecular weight and amino acid composition. N-terminal amino acid sequences from the two strains showed significant homology using a single unit alignment shift.

Amino Acid Sequence↗
PubMed1979

Instantaneous transmitral blood flow and anterior mitral leaflet motion in man.

Transmitral blood flow was measured in man by numerical differentiation of left ventricular volume as a function of time in 11 patients undergoing cardiac catheterization. Using this technique, transmitral blood flow may be studied in a variety of pathologic states without the need for surgically introduced flowmeters. Just before left ventriculography, echocardiography of the mitral valve was performed. The pattern of transmitral blood flow was strikingly similar to the diastolic movement of the anterior mitral leaflet. At any equivalent diastolic filling time, the percent of the integrated area beneath the curve inscribed by the diastolic anterior mirtal leaflet echoes closely approximated the percent of stroke volume which had entered the left ventricle. This observation supports the hypothesis that mitral leaflet motion accurately reflects transmitral flow. Consequently, at a given time during diastole, the relative velocity of transmitral flow and the percent of the stroke volume which has entered the left ventricle may be approximated noninvasively from the anterior mitral leaflet echogram.

Adult↗
PubMed1979

Measurement of left ventricular ejection fraction by mechanical cross-sectional echocardiography.

Cross-sectional echocardiography is a new noninvasive technique for imaging the heart. We developed a method for using mechanical cross-sectional echocardiograms (sector scans) to determine left ventricular volumes and ejection fraction. Using left ventricular cineangiography as a standard, sector scan ejection fraction correlated better (r = 0.93) than M-mode echocardiography by any of three established methods, and the sector scan regression line did not differ from the line of identity (p greater than 0.33). Interobserver variability for sector scan ejection fraction was 2.3 +/- 1.2% (mean +/- SD). Variation between two studies performed within 24 hours and analyzed by the same observer was 1.4 +/- 1.5%. However, the sector scans consistently underestimated left ventricular end-diastolic volume. We conclude that sector scan echocardiography is more reliable than conventional M-mode technique for estimating left ventricular ejection fraction, but estimation of left ventricular end-diastolic volume is unreliable with the methods currently available.

Blood Volume↗