Detalles del recurso
At high concentration, actinomycin D kills the protozon
Chlamydomonas reinhardi with exponential kinetics. The rate of killing
is dependent upon the temperature of incubation. This dependence
is partially a function of the increased extent of binding of the
drug at the higher temperature (33°C), but sensitivity of the cell
must also be stimulated by the higher temperature. While actinomycin
D probably kills cells in a reaction which requires binding to DNA,
there is no correlation between the lethal event and the inhibition
of macromolecular synthesis or the breakdown of macromolecules.
I have developed a simple model to explain the difference in
sensitivity of various species of RNA to actinomycin D inhibition.
This model predicts that frequently transcribed genes will be much
more sensitive to the drug than infrequently transcribed genes.
Mutants of Chlamydomonas reinhardi have also been isolated which are both temperature sensitive in growth and resistant to killing by actinomycin D. These mutants, unlike other actinomycin D resistant cell lines, are neither impermeable to the drug nor do they excrete it at an accelerated rate. The mutants are partially temper ature sensitive in their ability to synthesize RNA. In the presence of actinomycin D, however, RNA synthesis is partially protected at the nonpermissive temperature (and, in some cases, at the permissive temperature also) when compared to the inhibition of wild type cells.
Extraction and examination of RNA from these mutants reveals
that actinomycin D inhibits different species of RNA to different extents.
I propose that the mutants have an altered chromosomal constituent,
which impedes the binding to the genome. At the nonpermissive
temperature the alteration is postulated to partially interfere with
In the course of these experiments it became necessary to determine
the maturation pathways of the ribosomal RNA species of Chlamydomonas
reinhardi. Cytoplasmic rRNAs of C. reinhardi are cleaved from a
single precursor of molecular weights 2.4 · 106 to a mature rRNA
(0.69 · 106mol.wt) and a 1.4 · 106-mol. wt precursor of a mature 1.3 · 106-mol. wt rRNA. The kinetics of incorporation of radioactive label into the rRNAs suggest that the 0.69 · 106-mol. wt rRNA gene is located closer to the promotor than is the gene for the 1.4 · 106-mol. wt rRNA. The synthesis of cytoplasma rRNAs is extremely sensitive to camptothecin, an inhibitor of nuclear rRNA synthesis, but synthesis of
chloroplast rRNA is quite resistant to the inhibitor. This has allowed
us to demonstrate that chloroplast rRNAs are processed from precursors
which resemble those of blue-green algae. A 1.14 · 106-mol. wt precursor
is processed to the 1.07 · 106-mol. wt mature chloroplast rRNA, and
a 0.64 · 106-mol. wt precursor is cleaved to a 0.56 · 106-mol. wt species and then to the mature 0.54 · 106-mol. wt rRNA. This study
demonstrates two new ways in which the function of the chloroplast genome resembles those of prokaryotes more than those of the nucleus.
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Fecha de contribución: 08-sep-2017
* Miller, Mark James (1976) Investigations of the Mechanism of Cell Killing Induced by Actinomycin D. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:09072017-112605896