Biomedical subjects
Publications and source records attributed to P F Johnson.
The prosthodontic rehabilitation of a patient is usually the culmination of a patient's dental treatment. In order to maximize the prognosis, it is necessary to understand the patient, to make a thorough diagnosis, to coordinate the restoration with the other disciplines of dentistry, and to be knowledgeable of the spectrum of treatment modalities available.
Precise deletion of the intervening sequence of a yeast tRNATyr ochre suppressor gene (SUP6) significantly reduced its suppressor activity relative to that of the unaltered gene. This is probably the result of the absence of the pseudouridine modification, normally present at the middle anticodon position of mature suppressor tRNA, in tRNA synthesized in vivo from the deleted gene.
Many eukaryotic genes contain intevening sequences, segments of DNA that interrupt the continuity of the gene. They are removed from RNA transcripts of the gene by a process known as splicing. The intervening sequence in a yeast tyrosine transfer RNA (tRNA Tyr) suppressor gene was deleted in order to test its role in the expression of the gene. The altered gene and its parent were introduced into yeast by transformation. Both genes exhibited suppressor function, showing that the intervening sequence is not absolutely essential for the expression of this gene.
Genes for yeast tRNATyr and tRNAPhe have been sequenced (Goodman, Olson and Hall, 1977; Valenzuela et al., 1978) which contain additional nucleotides (intervening sequences) within the middle of the gene that are not present in the mature tRNA. We have isolated precursors to rRNATyr and tRNAPhe from a yeast temperature-sensitive mutant (at the rna1 locus) which accumulates only certain precursor tRNAs at the nonpermissive temperature. The tRNATyr and tRNAPhe precursors were analyzed by oligonucleotide mapping; they each contain the intervening sequence and fully matured 5' and 3' termini. Furthermore, these precursors were used as substrates to search for an enzymatic activity which can remove the intervening sequences and religate the ends. We have shown that wild-type yeast contains such an activity, and that this activity specifically removes the intervening sequences to produce mature-sized RNAs.
Four thousand Escherichia coli clones containing yeast DNA inserted into the plasmid pBR313 have been isolated. Of these, 175 clones were identified as carrying yeast transfer RNA genes. The initial analysis of the inserted transfer RNA genes via the colony hybridization technique with individual radioactive transfer RNA species is reported. The data indicate that yeast transfer RNA genes are not highly clustered, although some clustering exists. In addition, it was observed that the reiteration number of different transfer RNA genes may vary extensively.
Oxidation-reduction reactions which can destroy high current-density metal-stimulating electrodes are avoided when using capacitive electrodes. The results of in vitro and in vivo testing of anodized, high surface area, sintered tantalum electrodes are presented. The corrosion response of the electrodes is excellent; there is no evidence of dissolution of the electrode. A deposit forms on the surface of the electrodes, but has little effect on the voltage response to constant current stimulation. The physiological and histopathological results indicate the capacitive tantalum electrode to be the safest yet tested.
The results of evaluation of the corrosion response of Pt, Au, Rh, Ir, Pt-10% Ir and Pt-10% Rh electrodes subjected to bipolar current pulses of 0.1 and 1.0 A/geom. cm2 in a simulated neural environment are reported. The criteria for evaluation include the extent and morphology of material removal and corrosion product formation. The chemical changes on the electrode surface and in the test electrolyte are also evaluated. Modified potentiographic methods were developed which monitor interface conditions of the electrodes during chronic, long-term biphasic current passage. Under the test conditions established by the neural model, evaluation of the corrosion response of the candidate electrode materials shows that Rh was moderately damaged at 1.0 A/geom. cm2, while the remaining test electrodes were severely damaged at this current density. While Rh, Ir and Au exhibit corrosion resistance at 0.1 A/geom. cm2, damage to all of the test materials preclude their use in any long-term chronic neural prosthetic implant under the simulated stimulation parameters of the tests. The relative corrosion resistance of the test electrode materials is related to the adherence and coherence of the corrosion product produced on the electrode surface during the first few hundred hours of stimulation. Analysis of the voltage response of the electrodes to current passage provides a means of monitoring the nucleation and growth of the corrosion product.
Stimulation of the cat striate cortex with polished 1 mm2 gold, platinum, rhodium or carbon electrodes for 40h, at 50 Hz, and 0.5 msec duration, results in tissue damage at current densities of 0.6 A/cm2. At 0.1 A/cm2 rhodium and carbon were toxic to brain. In vitro gold and platinum corroded after 1,500-2,300 h of testing in simulated cerebrospinal fluid. The materials utilized are not suitable for long-term chronic electrodes for neural prosthetics.
A model for in vitro evaluation of materials for use as neural stimulation electrodes is developed. Critical areas of concern in developing an in vitro test model discussed include: selection of environment, choice of material, design of stimulating equipment, and analytical procedures used to evaluate materials response. A method of providing quantitative analysis of materials response to stimulation conditions is presented. Evaluation techniques involve the use of scanning electron microscopy, x-ray spectroscopy, atomic absorption spectrometry, and potentiographic and dielectric analysis of the test electrodes. A diagnostics matrix is presented which assigns a scale factor to quantify the relative corrosion response of the candidate materials. The corrosion response of Pt electrodes is evaluated in terms of scale factors assigned through use of the diagnostics matrix.
An index of myocardial blood flow developed from studies with a computer and a mechanical circulatory model has been investigated in 18 dogs. An intravenous injection of 131I is used with external scintillation detection over the left ventricle and the lung. The index is given by the ratio of the half time of the downslope of the curve recorded from the left ventricle to that of the curve recorded from the lung. This index has been compared with that suggested by Mena et al in which the left ventricular half time is compared with the half time of the brain curve. The effect on these indices and on the clearance of 133Xe of clamping the anterior descending branch of the left coronary artery has been observed. Statistically significant differences in both indices and in Xenon clearance were seen to be induced by coronary artery clamping but there was overlap between the clamped and unclamped values. The method used for coronary clamping also affected the results.
Coordination of gene expression in the lac operon was compared in Escherichia coli and Salmonella typhimurium as an approach to detecting possible differences in protein synthesis or membrane structure between organisms. Either a wild-type F' lac pro(AB) episome or the same episome with a polar mutation in one of the lac genes was introduced into pro(-) derivatives of the two strains of bacteria. Activity assays showed that the beta-galactosidase levels were only slightly lower in the S. typhimurium cells than in E. coli cells, whereas the transacetylase levels were significantly higher in S. typhimurium for all of the lac markers tested. Galactoside transport activities were always comparable in the two strains of bacteria; this latter result indicates that the cell envelopes of E. coli and S. typhimurium do not differ sufficiently to affect the membrane-associated lac transport system. It was found, however, that the specific transport activity is very sensitive to culture age in both bacteria, and decreases rapidly in cultures past the mid-exponential phase of growth.
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