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Journal ArticleDOI

Evolution of the Transcription Unit of Ribosomal RNA

TL;DR: It is suggested 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, while birds, marsupials and placental mammals, exhibit a seemingly less economical form of r RNA synthesis.
Abstract: 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.
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Journal ArticleDOI
TL;DR: Over the range of actinomycin concentrations employed cellular uptake of the drug is proportional to external concentration, and maximal inhibition of RNA synthesis is generally achieved within one hour after exposure.
Abstract: A quantitative dose-response study was made of the inhibition by actinomycin D of the synthesis of various RNA species in cultured L cells. After a suitable preincubation with various concentrations of actinomycin, synthesis was measured by a brief incorporation of radioactive uridine. The 45S precursor of ribosomal RNA, 5S ribosomal RNA, transfer RNA and various sized fractions of heterogeneous nuclear RNA were extracted from the appropriate subcellular fractions and resolved by acrylamide gel electrophoresis. Polyribosomal messenger RNA was also studied. Over the range of actinomycin concentrations employed cellular uptake of the drug is proportional to external concentration, and maximal inhibition of RNA synthesis is generally achieved within one hour after exposure. For most of the RNA species studied the dose-response data appear to follow an exponential inactivation function with some “tailing” evident at the higher doses. Sensitivities, defined as the reciprocal of the dose necessary to reduce synthesis to 1/e of the control level, were compared and examined with respect to the molecular weight of the RNA species under consideration. Among the heterogeneous nuclear RNA's there is a good correlation between RNA size and sensitivity, such that the sensitivity per unit molecular weight is relatively constant. A similar relationship is roughly applicable to the ribosomal and transfer RNA's when taken as a group, although in this case the sensitivity per unit molecular weight is 50 to 100 fold greater. A model is proposed in which this marked difference in actinomycin sensitivity arises as a consequence of the transcription of stretches of contiguous repetitive genes by RNA polymerase molecules which attach to the DNA only at a limited number of entry points, the highest probability of attachment being at the beginning of a stretch.

506 citations

Journal ArticleDOI
TL;DR: It is suggested that any further testing of the legitimacy of this taxon should, at the least, include data from opisthokont protists, and the results underline the critical position of these "animal-fungal allies" with respect to the origin and early evolution of animals and fungi.
Abstract: A close evolutionary relationship between Metazoa (animals) and Fungi was proposed over 20 years ago. The name Opisthokonta reflects the presence of a single, posteriorly directed flagellum found on the reproductive cells of most metazoans and some fungi. Subsequent molecular work confirmed this relationship and also identified several protistan groups (including the choanoflagellates, ichthyosporeans and nucleariids) belonging to this clade. In this chapter we review the literature on this group in order to describe the opisthokont lineages and the current thinking about how they are related to each other. The phylogeny of the opisthokonts is far from complete and we will discuss the areas that need to be addressed, as well as current evidence on the possible sister-groups of the opisthokonts.

287 citations


Cites background from "Evolution of the Transcription Unit..."

  • ...However, the two genes are together in the genome, transcribed as a single unit (Perry et al. 1970) and function in the same biosynthetic pathway....

    [...]

Journal ArticleDOI
01 Feb 1977-Cell
TL;DR: R loop mapping of a population of long units obtained directly from the rDNA has demonstrated that this interruption of the sequence coding for the 28S rRNA is a characteristic of the long class.

253 citations

Journal ArticleDOI
01 Feb 1977-Cell
TL;DR: Mapping of the 18S and 28S rDNA in Dm103 was accomplished by quantitative determination of the amount of these rDNAs in each member of an ordered set of restriction fragments obtained by Hind III and Eco R1 ccleavage of pDm103.

228 citations

Book ChapterDOI
TL;DR: The chapter presents a picture of the detailed physical structure of the ribosomal cistron and how it is integrated into the genomic DNA, both of which have important implications for cell differentiation and for evolutionary theory.
Abstract: Publisher Summary This chapter reviews that the ribosomal cistrons of both bacteria and eukaryotes, with their unique and well-defined characteristics, have increasingly attracted the attention of biochemists and geneticists. It discusses that the arrangement of the ribosomal cistrons on the chromosomal DNA, and they have been mapped in some detail by means of biophysical techniques. Ribosoinal cistrons were the first Mendelian genes isolated in pure form free from other DNA and it has now become possible to investigate their organization and composition and to study their biological activity in vitro . The chapter focuses on the organizational features and primary structure of DNA. It presents a picture of the detailed physical structure of the ribosomal cistron and how it is integrated into the genomic DNA. It has considered the divergence of ribosomes in different species and the variation of base sequence within the ribosomal cistrons of a single organism, both of which have important implications for cell differentiation and for evolutionary theory.

228 citations