Properties and composition of the isolated ribosomal DNA satellite of Xenopus laevis

Kinetics of RNA synthesis in toad bladder epithelium: action of aldosterone during the latent period

Processing of RNA in the toad bladder was analyzed by polyacrylamide-gel electrophoresis to determine whether aldosterone causes any changes in the 1 hr before it potentiates transport of sodium ion. No change was found in the quantity or in the specific activity of bulk RNA labeled with uridine-5-(3)H. In vivo and in vitro with either uridine-5-(3)H or with methionine-(methyl)-(3)H as precursors, processing of RNA was extremely slow. Heterodisperse RNA was obvious after 30 min of continuous labeling, but labeling of the 40S precursor of ribosomal RNA was not apparent for 60 min. Labeling of mature 28S and 18S RNA first became apparent after 8 hr. approximately 7S RNA was the principal fastmigrating species labeled at 30 min, and 4S RNA was not heavily labeled until 1 hr. Aldosterone (5 x 10(-7) mole/liter) produced no changes. If care were not taken to inhibit metabolism of native bacteria colonizing the bladder, bacterial RNA of high specific activity predominated. We conclude that RNA metabolism in the toad bladder is extraordinarily slow, that a major acceleration of de novo synthesis in response to physiologic doses of aldosterone was not demonstrable, and that some reports to the contrary may have been influenced by artifacts from bacterial RNA metabolism. Earlier evidence for obligatory alterations in RNA metabolism during the latent period is not strong.

Biochemical studies on adenovirus multiplication, XIX. Resolution of late viral RNA species in the nucleus and cytoplasm

Late after infection of cultured human cells (KB) with adenovirus type 2, the nucleus contains heterogeneous viral RNA species ranging in size from 10 to 43 S. Four viral RNA species found in the nucleus (36, 38, 40, and 43 S) are synthesized predominantly during a 15-min labeling period with [(3)H]uridine, while smaller RNA species accumulate when labeling is continued for longer periods. In contrast, 6-8 viral RNA species, of sedimentation coefficient from 10 to 29 S, are found in the cytoplasm after a 30-min pulse label and a 2-hr chase. DNA-RNA hybridization-competition experiments demonstrate that viral RNA sequences present in nuclear 36-43S RNA are also present in cytoplasmic and polyribosomal RNA, suggesting that at least some of the cytoplasmic viral-specific RNA molecules are derived by cleavage of high molecular weight precursors from the nucleus.

Evolution of higher-organism DNA

A great deal of information about evolutionary events and processes has been inferred from careful studies of fossil records. Other forms of evidence have also contributed greatly to the understanding of evolution. Comparative biochemistry (Florkin, 1949), immunology (Boyden, 1942), protein sequencing (Dayoff, 1969; Anfinsen, 1959), and early DNA studies (McCarthy & Bolton, 1963; Schildkraut, Marmur & Doty, 1961) have for the most part corroborated earlier evolutionary findings, and at the same time provided new understanding of molecular processes in evolution. Of these approaches the comparison of DNA seems most promising since a relatively precise quantitative comparison can be made of all of the genetic material of different species.

Evolution of the transcription unit of ribosomal RNA

In eukaryotes the two principal RNA components of the ribosomes are initially synthesized as a large complex precursor molecule, which may be thought of as a transcription unit. The precursor is converted, via intermediates, to the mature forms of ribosomal RNA (rRNA). In order to assess the extent of variation in the size of this rRNA transcription unit among different organisms, and to infer its possible mode of evolution, we have determined its molecular weight in several selected species. Pulse-labeled and long-term labeled RNA's were extracted from various types of cells, and analyzed by electrophoresis on acrylamide gels. Identification of particular components as rRNA precursors was made according to several stated criteria. Our results, together with an analysis of previously published data, suggest that in plants and lower animals, up to and including reptiles, the unit of transcription of rRNA is a 2.7-2.8 million dalton molecule, which is only about 25 per cent larger than its combined rRNA products. In contrast, birds, marsupials and placental mammals, exhibit a seemingly less economical form of rRNA synthesis. Their transcription units are 4.0-4.2 million daltons, about 80 per cent larger than the rRNA products. In the organisms with the smaller transcription unit the major intermediate precursor of rRNA is 1.5-1.6 million daltons, as compared to 2.0-2.2 million daltons in birds and mammals. The significance of these findings is discussed in relation to evolutionary changes in the base composition of the ribosomal RNA genes.