Nuclear rRNA

The gene encoding the smallest structural RNA component of the nuclear ribosome is the 5S rRNA gene. This gene is not co-located with the remaining nuclear rRNA genes. Neither sequence nor chromosomal location of the 5S gene is known for any neodermatan and it will not be discussed further here. 

The remaining three nuclear rRNA genes, those for the nSSU, 5.8S and nLSU rRNAs, are arranged in a linear manner within an operon. 

Fig. 4.1. Lengths of various components (diagram not to scale) of the nuclear rRNA operon in selected neodermatans. Mature rRNA molecules are shown as shaded boxes. Literature sources are listed. Lengths of some nSSU and nLSU are slightly underestimated because conserved primer sites at each end are not always included in the GenBank accessions.

Nuclear rRNA precursors and cytoplasmic rRNA formed by RNA polymerase I account for some 70+% of mammalian cell RNA and 35-40% of total RNA synthesis. The products of RNA polymerase II, hnRNA and mRNA, comprise some 10% of the total RNA and 55-60% of total synthesis. The hnRNA and mRNA transcripts are unstable and have half-lives in higher eukaryotes of about 24 h, compared with an average half-life in bacteria of 90 s. Pre-tRNAs, synthesized along with 5 S rRNA by RNA polymerase III, are relatively stable, with an average half-life of 4-6 days. 

Although we will stick to the IL-6 gene, it should be mentioned at the side that two other RNA polymerases exist in mammalian cells responsible for the synthesis of RNA molecules, which are not translated into proteins ribosomal (rRNA), transfer (tRNA), small nuclear (snRNA), small nucleolar (snoRNA), and some of the recently discovered microRNAs and piRNAs. These RNA molecules act in the process of translation and mRNA turnover. Micro and piRNAs are probably extremely important in the definition of stem cells and of differentiation programs. Some of them are synthesized by RNA polymerase II. 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

All eukaryotic cells have four major classes of RNA ri-bosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). The first three are involved in protein synthesis, and snRNA is involved in mRNA splicing. As shown in Table 37-1, these various classes of RNA are different in their diversity, stability, and abundance in cells. 

There are three distinct nuclear DNA-dependent RNA polymerases in mammals RNA polymerases I, II, and III. These enzymes control the transcriptional function—the transcription of rRNA, mRNA, and small RNA (tRNA/5S rRNA, snRNA) genes, respectively. 

S, 5S and 4.5S rRNAs) and a small 30S subunit (containing 16S rRNA). Chloro-plast ribosomal proteins are encoded by both nuclear and chloroplast genes. 

The Xenopus transcription factor IIIA not only acts as an essential RNA polymerase transcription factor for the expression of the 5S rRNA gene, it also binds to the 5S rRNA to form a 7S ribonucleoprotein particle that stabilizes the RNA until it is required for ribosome assembly and facilitates nuclear export of the 5S rRNA. Indeed, it was originally shown to be the protein component associated with 5S rRNA in the 7S particle in Xenopus oocytes before it was recognized as a transcription factor. How, we may ask, can this protein not only recognize specific DNA sequences in the 5S rRNA gene upstream region, but also recognize different, but equally specific, sequences in 5S rRNA ... 

The nucleus is not capable of synthesizing proteins. All of the nuclear proteins therefore have to be imported—the histones with which DNA is associated in chromatin, and also the so-called non-histone proteins (DNA polymerases and RNA polymerases, auxiliary and structural proteins, transcription factors, and ribosomal proteins). Ribosomal RNA (rRNA) already associates with proteins in the nucleolus to form ribosome precursors. 

All eukaryotic cells in our bodies contain the same 23 chromosomes with the same DNA base sequences. The lone differences are the mitochondria. The mitochondria in typical somatic cells contain less than 0.1% of the cell s DNA but in fertilized and dividing egg cells this number is greater. mtDNA is much smaller, often containing fewer than 20,000 base pairs. The value for humans is 16,569 base pairs. The mtDNA is a circular duplex. mDNA codes for the mitochondrial tRNAs and rRNAs but only a fraction of the mitochondrial proteins. Over 95% of the mitochondrial proteins are encoded by nuclear DNA. The mitochondria divide when the cell divides. 

The RNA product may encode transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), or small nuclear RNAs (snRNAs) that have end point functions in the cell. 

Mitochondrial DNA codes for the mitochondrial tRNAs and rRNAs and for a few mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nuclear DNA. Mitochondria and chloroplasts divide when the cell divides. Their DNA is replicated before and during division, and the daughter DNA molecules pass into the daughter organelles. 

There are three major types of RNA that participate in the process of protein synthesis ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). They are unbranched polymers of nucleotides, but differ from DNA by containing ribose instead of deoxyribose and uracil instead of thymine. rRNA is a component of the ribosomes. tRNA serves as an adaptor molecule that carries a spe dfic amino acid to the site of protein synthesis. mRNA carries genetic information from the nuclear DNA to the cytosol, where it is used as the template for protein synthesis. The process of RNA synthesis is called transcription, and its substrates are ribonucleoside triphosphates. The enzyme that synthesizes RNA is RNA polymerase, which is a multisub-irit enzyme. In prokaryotic cells, the core enzyme has four subunits—... 

A mutation in any of the 13 protein subunits, the 22 tRNAs, or the two rRNAs whose genes are carried in mitochondrial DNA may possibly cause disease. The 13 protein subunits are all involved in electron transport or oxidative phosphorylation. The syndromes resulting from mutations in mtDNA frequently affect oxidative phosphorylation (OXPHOS) causing what are often called "OXPHOS diseases."3-6 Mitochondrial oxidative phosphorylation also depends upon 100 proteins encoded in the nucleus. Therefore, OXPHOS diseases may result from defects in either mitochondrial or nuclear genes. The former are distinguished by the fact that they are inherited almost exclusively maternally. Most mitochondrial diseases are rare. However, mtDNA is subject to rapid mutation, and it is possible that accumulating mutants in mtDNA may be an important component of aging.h k... 

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