RNA - DNA hybridization on membrane filters with fragmented mammalian DNA

Comparative study of the ribosomal RNA from Leishmania and Trypanosoma

The ribosomal RNA from several stocks of the genera Leishmania and Trypanosoma were studied by gel electrophoresis, sedimentation on sucrose density gradients and RNA/DNA hybridization experiments. Three major components were observed after electrophoresis in polyacrylamide gels (PAGE-SDS), the relative molecular masses being respectively: X1 = 0.83 megadaltons, X2 = 0.63 megadaltons and X3 = 0.54 megadaltons for Leishmania RNA; and X1 = 0.86 megaldaltons, X2 = 0.78 megadaltons, and X3 = 0.58 megadaltons for Trypanosoma RNA. Depending upon the isolation procedure, a fourth component, X0 = 1.2 megadaltons (26S), became evident. The later component was purified from Leishmania brasiliensis (Y) by centrifugation on a linear 15-30% sucrose density gradient. This component, after heat denaturation and PAGE-SDS, gave rise to two bands coinciding in molecular mass with those of X2 and X3, indicating that these components are part of the large ribosomal subunit whereas X1 belongs to the small one. The above mentioned differences in mobilities of components X1 and X2 between the two genera were no longer observed after electrophoresis in denaturing agarose-formaldehyde gels, suggesting secondary structural differences among these RNA species. Hybridization experiments with L. brasiliensis (Y) DNA showed that both RNA types compete equally well for the ribosomal sites in this DNA, and that L. brasiliensis (Y) rRNA recognizes the ribosomal sites in DNA of Trypanosoma cruzi (EP), thus indicating that no gross changes occurred in their nucleotide sequences during evolution.

Ribosomal DNA of Leishmania brasiliensis: number of ribosomal copies and gene isolation

The DNA content of the hemoflagellate Leishmania brasiliensis, strain Y, has been determined by colorimetric reactions and found to be nearly 0.226 pg/cell. When this DNA is bound to filters and hybridized with labeled rRNA from the same organism, saturation is reached at 0.47% of the DNA, corresponding to an estimated 160 ribosomal gene copies. When the DNA is sheared and centrifuged to equilibrium in CsCl gradients, two major satellites of the main band (rho = 1.712 g/cm3) are observed: a heavy one (1.720 g/cm3), which hybridizes with labeled rRNA, and a light one (1.699 g/cm3) with the electron microscopic characteristics of the kinetoplast DNA network.

Azocarcinogen-induced release of chromatin-associated RNA into liver cytoplasm: the possible role of reiterated RNA sequences in the control of RNA processing

In the liver of rats fed the azocarcinogen 3'-methyl-4-dimethylaminoazobenzene (3'MeDAB) reiterated RNA sequence transcribed from middle repetitive DNA are released into the cytoplasm. The same repetitive nucleotide sequences can be isolated from the chromatin of the liver of control animals in the form of metabolically highly active, 13 000 daltons RNA. This small, chromatin-associated RNA originates from nuclear RNA larger than 10 S. The discontinuation of the feeding of the azocarcinogen will not stop the release of the nuclear reiterated RNA sequences into the cytoplasm. The repetitive sequences of nuclear RNA which are released into the cytoplasm in animals fed the azocarcinogen can no longer be found in the chromatin in the form of small RNA molecules. The results can be explained by the assumption that the reiterated RNA sequences are involved in the upholding of RNA processing. A cell-specific processing of RNA will be maintained by the interaction of reiterated RNA fragments from already processed RNA with the reiterated complementary sequences on RNA yet to be processed. Existence of such a feed-back circuit would make it possible to explain how a temporary interference of the azocarcinogen with RNA processing will result in the disappearance of specific reiterated RNA sequences from the chromatin. It could also explain the continuation of the release of the same repeated RNA sequences into the cytoplasm as part of larger RNA molecules even after the removal of the carcinogen.

Physical and functional mapping of rat-liver mitochondrial DNA

Rat-liver mitochondrial DNA (mtDNA) contains 2 cleavage sites of the restriction endonuclease Xbal. The molecular sizes of restriction fragments are 6.6 X 10(6) and 3.7 X 10(6) D. The results of partial cleavage of mtDNA with EcoRI allow the fragment F (0.32 X 10(6) D) to be localized in the sequence ABCEGFHDA. The functional map of mtDNA is constructed for two genes of the ATP-ase mRNAs from rat-liver mitochondria. Molecular hybridization shows that the ATPase genes are located in fragment B and in the GEHD area of mtDNA EcoRI cleavage.