The genes that code for 45S rRNA, the precursor of 18S, 5.8S and 28S rRNA, are transcribed by RNA polymerase I. In many eukaryotes the genes are arranged as tandem repeats in discrete chromosomal clusters. rDNA transcription and rRNA processing occur in the nucleolus. In vertebrates, at least two factors, SL-1 and UBF, specific for transcription by RNA polymerase I cooperate in the formation of the initiation complex. Interestingly, there are proteins analogous to SL-1 in unicellular eukaryotes, but the requirement for a UBF-like factor appears to vary. Recent advances in our understanding of the rDNA transcription system and its regulation have demonstrated overlap with the other nuclear transcription systems (RNA polymerase II and III). This is exemplified by the utilization of TBP as a component of SL-1 and the role of Rb in regulatory rDNA transcription.

Transcription by RNA polymerase I.

A plasmid (pRSE562) containing the metE and metR genes of Escherichia coli was used to study the expression of these genes and the role of the MetR protein in regulating metE expression. DNA sequence analysis of the 236-base-pair region separating these genes showed the presence of seven putative met boxes. When this plasmid was used to transform either wild-type E. coli, metE mutant, or metR mutant, MetE enzyme activity increased 5- to 7-fold over wild-type levels. The metR gene was subcloned from pRSE562, and this plasmid, pMRIII, relieved the methionine auxotrophy of a metR mutant after transformation. The metR gene was also cloned into a vector containing the lambda PL promoter, and the MetR protein was overexpressed and purified to near homogeneity. This protein, when added to an in vitro DNA-dependent protein synthesis system in which the MetE and/or MetR proteins were synthesized, caused a large increase in the expression of the metE gene but a decrease in the expression of the metR gene. The in vitro expression of both genes was inhibited by the MetJ protein and S-adenosylmethionine in the presence or absence of MetR protein. These results provide evidence that the product of the metR gene is a trans-activator of the expression of the metE gene and that the expression of the metR gene is under autogenous regulation and is repressed by the MetJ protein.

https://www.pnas.org/content/pnas/86/1/85.full.pdf

Regulation of methionine synthesis in Escherichia coli: effect of the MetR protein on the expression of the metE and metR genes.

Publisher Summary This chapter outlines the early consequences of surface receptor stimulation, and summarizes the information about the structure of nuclear pores and the transport of macromolecules across the nuclear envelope. It is assumed that the signal transduction between cytoplasm and nucleus involves primarily the translocation of proteins between the two compartments. According to the classic two-step model of steroid hormone action, the receptor activation is accompanied by translocation of ligand–receptor complexes from cytoplasm to nucleus. The steroid-free receptors form the macromolecular complexes through association with a 90-kDa heat shock protein, and hormone-dependent release from this complex may be required for receptor function. Protein phosphorylation is known to represent a major mechanism for controlling the nuclear activities and gene transcription. Immunocytochemical studies suggest that nuclear effects of cAMP are mediated by the C subunit. In response to the cAMP elevation, the catalytic subunit of protein kinases (PK-A) translocates from a perinuclear location to the nucleus.

Mechanisms of signal transduction to the cell nucleus.

Publisher Summary This chapter discusses the contributions of immunochemical approaches to the study of nuclear structure and activity. By segregating the nuclear compartment from the cytoplasm, the nuclear envelope represents the most distinctive feature of a eukaryotic cell. It consists of a double membrane enclosing a 20- to 50-nm-wide perinuclear space. The two membranes are frequently joined and penetrated by nuclear pore complexes, that is, elaborate supramolecular assemblies through which most, if not all, macromolecules are exchanged between nucleus and cytoplasm. The outer nuclear membrane is frequently contiguous with the endoplasmic reticulum and studded with ribosomes, while in most cells the inner membrane is closely associated with chromatin. The nuclear lamina is a fibrillar protein meshwork apposed to the nucleoplasmic surface of the inner nuclear membrane. It is presumed to be important for envelope structure and stability; moreover, it may play a role in organizing the spatial arrangement of interphase chromosomes. Immunochemical studies have revealed that gametogenesis and embryonic development of different organisms are accompanied by profound changes in the polypeptide composition of the nuclear lamina. It is possible that these changes might relate to alterations in the functional organization of nuclei in differentiating cells.

Nuclear Function and Organization: The Potential of Immunochemical Approaches

The Tax protein of human T-cell lymphotropic virus type 1 (HTLV-1) is a 40-kDa transcriptional activator which is critical for HTLV-1 gene regulation and virus-induced cellular transformation. Tax is localized to the DNA through its interaction with the site-specific activators cyclic AMP-responsive element-binding protein, NF-kappaB, and serum response factor. It has been suggested that the recruitment of Tax to the DNA positions Tax for interaction with the basal transcriptional machinery. On the basis of several independent assays, we now report a physical and functional interaction between Tax and the transcription factor, TFIIA. First, Tax was found to interact with the 35-kDa (alpha) subunit of TFIIA in the yeast two-hybrid interaction system. Importantly, two previously characterized mutants with point mutations in Tax, M32 (Y196A, K197S) and M41 (H287A, P288S), which were shown to be defective in Tax-activated transcription were unable to interact with TFIIA in this assay. Second, a glutathione-S-transferase (GST) affinity-binding assay showed that the interaction of holo-TFIIA with GST-Tax was 20-fold higher than that observed with either the GST-Tax M32 activation mutant or the GST control. Third, a coimmunoprecipitation assay showed that in HTLV-1-infected human T lymphocytes, Tax and TFIIA were associated. Finally, TFIIA facilitates Tax transactivation in vitro and in vivo. In vitro transcription studies showed reduced levels of Tax-activated transcription in cell extracts depleted of TFIIA. In addition, transfection of human T lymphocytes with TFIIA expression vectors enhanced Tax-activated transcription of an HTLV-1 long terminal repeat-chloramphenicol acetyltransferase reporter construct. Our study suggests that the interaction of Tax with the transcription factor TFIIA may play a role in Tax-mediated transcriptional activation.

Interaction of the human T-cell lymphotropic virus type 1 tax transactivator with transcription factor IIA.

This book examines an array of topics on the regulation of gene expression, including an examination of DNA-protein interactions and the role of oncogene proteins in normal and abnormal cellular responses. The book focuses on the control of mRNA transcription in eykaryotes and delineates other areas including gene regulation in prokaryotes and control of stable RNA synthesis.

Mechanisms of control of gene expression

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Mechanisms of control of gene expression