Chapter category: Gene Expression
Regulation of RNA Polymerase III Transcription
RNA Polymerase III Transcription
Second Edition
Edited by: Robert J. WhiteISBN: 1-57059-482-1
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Chapter authors:
Robert J. White
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There are two families of active 5S genes in Xenopus laevis. One consists of the somatic 5S genes, of which there are 400 copies per haploid genome, organized in a single cluster.1,2 The other is divided into two classes, the 20,000 major oocyte and the 1,300 trace oocyte 5S genes.3,4 There are only 6 nucleotides different between the 120 bp coding regions of the somatic and major oocyte types, but the flanking sequences are completely divergent.1,3,4
Each of the ribosomal RNAs is synthesized at greatly elevated rates in oocytes, but this is achieved by differing mechanisms. The genes encoding 18S and 28S rRNAs are amplified specifically in oocytes.5,6 In contrast, the large auxiliary oocyte 5S rRNA gene family is present in all cell types but is active only in oocytes.7-9 Both somatic and oocyte 5S genes are actively expressed during oogenesis, resulting in a massive accumulation of 5S rRNA for subsequent incorporation into ribosomes.10 Following fertilization and meiosis there is a generalized repression of transcription that continues through the first 12 cleavage divisions of the fertilized egg, until the mid-blastula transition (MBT). This repression appears to result primarily from a large excess of chromatin proteins.11-13 The oocyte accumulates sufficient core histones to assemble 13,000-16,000 nuclei.14 These gain access to the chromosomes when the nucleus (germinal vesicle) breaks down during oocyte maturation. Titrating away the excess of histones by injecting nonspecific DNA at a concentration comparable to the normal DNA mass present at the MBT is sufficient to activate tRNA expression precociously.11,13 It therefore seems that the class III transcription apparatus remains functional prior to MBT, but cannot gain access to appropriate templates. When transcription resumes at the MBT, approximately equal amounts of somatic and oocyte 5S RNA are produced, representing a 50-fold transcriptional preference for somatic over oocyte 5S genes.10,15 Two or three cell divisions later, the final state of differential gene expression is established as the oocyte 5S genes are inactivated and the transcriptional preference reaches 1000.10,15,16 This process is not irreversible, since the oocyte 5S genes can be reactivated if somatic cell nuclei are transferred into oocytes.16
Considerable effort has been invested in trying to understand the differential regulation of the two 5S gene families.17-20 The system provides an important paradigm for what may be a common developmental mechanism, where two or more gene families have similar but not identical cis-acting sequences recognized by the same transcription factors, but are nonetheless controlled differentially.
Additional chapters from this book:
Perspective
Robert J. White
Our understanding of the details of pol III transcription has increased substan-tially in recent years. The characterization and cloning of many of the components of the system have been ac...
Regulation of RNA Polymerase III Transcription
Robert J. White
There are two families of active 5S genes in Xenopus laevis. One consists of the somatic 5S genes, of which there are 400 copies per haploid genome, organized in a single cluster.
Proteins that Modulate the Rate of RNA Polymerase III Transcription
Robert J. White
So far I have described the basal pol III transcription apparatus and how this functions to allow expression of class III genes. The level of transcription can be modulated in either a posi...
Chromatin Structure of Class III Genes
Robert J. White
The chromatin structure of a gene can be a major determinant of its transcrip-tional activity (reviewed in refs. 1–8). In chromatin, 146 bp of DNA is wrapped approximately twice around...
Transcription
Robert J. White
Once a preinitiation complex has formed on a yeast tRNA gene, RNA chain ini-tiation requires a further 5 min at 22°C (half-life ~2 min).1 During this period, three successiv...
Transcription Complex Formation on Class III Genes
Robert J. White
The formation of transcription complexes, composed of factors bound to DNA, was initially investigated by means of the template exclusion assay. This approach monitors the ability of a gene...
Transcription Factors Utilized by RNA Polymerase III
Robert J. White
Purified pol III initiates transcription randomly.1–4 Accurate and specific initia-tion requires the assistance of transcription factors in order to recruit the polymerase t...
RNA Polymerase III
Robert J. White
Pol III is the largest of the nuclear RNA polymerases, with an aggregate molecular weight of 600700 kD (reviewed by Thuriaux and Sentenac1–3). This is, perhaps, surprising s...
Promoter Structure of Class III Genes
Robert J. White
The promoters of most class III genes include discontinuous intragenic structures, termed internal control regions (ICRs), that are composed of essential sequence blocks separated by noness...
Class III Genes
Robert J. White
The genes transcribed by pol III encode a variety of small RNA molecules. (Table 1) Many of these have essential functions in cellular metabolism, such as tRNA and 5S rRNA, which are required ...
