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L Simpson

Publications and source records attributed to L Simpson.

12 recordsLinked to original sources
PubMed1979

Evidence for a charge-shift electrochromic mechanism in a probe of membrane potential.

Extrinsic optical probes have become important tools for monitoring membrane potential, with probes now available for many tissue or cell suspension systems. In each case that has been studied in detail, it seems that the mechanism involves a shift in the equilibrium population of the probe from one chemical environment to another in response to the transmembrane potential; the environments perturb the probe's spectrum differently. As this indirect mechanism involves a redistribution of dye between chemical environments that are likely to vary if a given probe is transferred from one membrane to another, a potential probe that is effective and calibrated for all membrane systems has not been realised. We present here evidence for a direct response of a probe chromophore to the electric field across membrane systems. The results suggest it might be possible to develop a universal set of membrane probes.

Cholesterol↗
PubMed1979

Restriction map, partial cloning and localization of 9S and 12S kinetoplast RNA genes on the maxicircle component of the kinetoplast DNA of Leishmania tarentolae.

We have constructed a restriction map of the maxicircle component of the kinetoplast DNA of Leishmania tarentolae for the enzymes EcoRI, Bam HI, HaeIII, HpaII, SalI, BglII and HindIII. The 9 and 12S kinetoplast RNAs were localized on this map. Two fragments of this maxicircle molecule were cloned in the bacterial plasmid, pBR322, including a 4.4 . 10(6) dalton EcoRI/BamHI fragment which contains the 9 and 12S RNA genes.

Animals↗
PubMed1979

Isolation of maxicircle component of kinetoplast DNA from hemoflagellate protozoa.

Maxicircle DNA has been isolated from the kinetoplast DNA of Leishmania tarentolae culture forms by bouyant separation in CsCl in the presence of the A + T-binding dye Hoechst 33258, after liberation from the kinetoplast DNA network by cleavage with the single-hit restriction endonuclease EcoRI. The purified linearized maxicircle DNA has a density in CsCl of 1.681 g/cm3 (79% A + T) and a molecular weight of approximately 18-20 x 10(6). The maxicircle molecule exhibited intramolecular base composition heterogeneity ranging from 85% A + T to 74% A + T.

Animals↗
PubMed1978

Kinetoplast RNA of Leishmania tarentolae.

RNA has been isolated from highly purified kinetoplast-mitochondrial fractions of Leishmania tarentolae, and shown to consist of two major species that sediment at 9S and 12S in sucrose and also several additional low molecular weight species which were visualized by gel electrophoresis. The in vivo transcription of 9S and 12S RNAs was inhibited by ethidium bromide and rifampin, and was fairly insensitive to low actinomycin D and camptothecin. The 9S and 12S RNAs were isolated by acrylamide gel electrophoresis or by sedimentation in sucrose. Both RNAs contained approximately 80% A + U and did not contain long stretches of poly(A). The 9S and 12S RNAs were found to hybridize selectively to the maxicircle sequences of the kinetoplast DNA, implying that the maxicircle, and not the minicircle, represents the informational mitochondrial DNA in the kinetoplast.

Animals↗
PubMed1977

Radiation treatment planning.

Radiation treatment planning has developed into a substantial and effective component of the entire radiation treatment approach. Over the last 2 decades, with the development of high energy electron and x-ray sources, and also with the availability of new radionuclides and techniques for internally applied radiation, all of which permit a high degree of concentration of radiation, treatment planning has been developed to make best use of these modalities. The use of automatic computation has proved necessary in order to handle the large amounts of radiation data involved in treatment dose calculation. Tumor and anatomical localization has been carried out with increasing precision. Several cases are described which illustrate planning and in particular the use of computerized transverse tomography. The use of moving shadow-shields is also described. For internally applied radiation, the features of iodine-125 are illustrated.

Adult↗
PubMed1976

Pulse-labeling of kinetoplast DNA: localization of 2 sites of synthesis within the networks and kinetics of labeling of closed minicircles.

Short pulse-labeling of log phase Crithidia fasciculata cells with [3H]thymidine allowed the autoradiographic visualization of 2 sites of replication of kinetoplast DNA situated at the periphery of the networks and separated by 180 degrees. Longer pulse-labeling led to the previously reported total peripheral labeling pattern. Pulse-labeled networks possess an intermediate density in ethidium bromide-CsCl equilibrium gradients between the densities characteristic of closed networks and open or linear DNA. Removal of ethidium bromide by several methods and treatment of intermediate band networks with RNase and pronase had no effect on the equilibrium rebanding pattern. Closed minicircles of Leishmania tarentolae are not labeled by a short pulse of intact cells with [3H]thymidine. A chase of approximately 3-4 hr is required for the appearance of radioactivity in closed minicircles, a time delay which implies the existence of intermediate events between replication and eventual covalent closure of the minicircles.

Animals↗