The 28S and 5.8S rRNAs associated with the large (60S) ribosomal subunit and the 18S rRNA associated with the small (40S) ribosomal subunit in higher eukaryotes (and the functionally equivalent rRNAs in all other eukaryotes) are all encoded by a single pre-rRNA transcription unit. In human cells, its transcription by RNA polymerase I yields a 45S (~13.7-kb) primary transcript (pre-rRNA), which is cleaved and processed into the mature 28S, 18S, and 5.8S rRNAs found in cytoplasmic ribosomes. The fourth rRNA, 5S, is encoded separately and transcribed outside the nucleolus. Sequencing of the DNA encoding the 45S pre-rRNA from many species showed that this DNA shares several properties in all eukaryotes. First, the pre-rRNA genes are arranged in long tandem arrays separated by nontranscribed spacer regions ranging in length from 2 kb in frogs to 30 kb in humans (Figure 1). Second, the genomic regions corresponding to the three mature rRNAs are always arranged in the same 5′→3′ order: 18S, 5.8S, and 28S. Third, in all eukaryotic cells (and even in bacteria), the pre-rRNA gene codes for regions that are removed during processing and rapidly degraded. These regions probably contribute to proper folding of the rRNAs but are not required once that folding has occurred. The general structure of pre-rRNA transcription units is diagrammed in Figure2.

Fig1. Electron micrograph of pre-rRNA transcription units from the nucleolus of a frog oocyte. Each “feather” represents multiple pre-rRNA molecules associated with protein in a pre-ribonucleoprotein complex (pre-rRNP) emerging from a transcription unit. Note the dense “knob” at the 5′ end of each nascent pre-RNP, which is thought to be a processome. Pre-rRNA transcription units are arranged in tandem, separated by nontranscribed spacer regions of nucleolar chromatin. [Courtesy of Y. Osheim and O. J. Miller, Jr.]

Fig2. General structure of eukaryotic pre-rRNA transcription units. The three coding regions (red) encode the 18S, 5.8S, and 28S rRNAs found in ribosomes of higher eukaryotes, or their equivalents in other species. The order of these coding regions in the genome is always 5′→3′. Variations in the lengths of the transcribed spacer regions (blue) account for most of the difference in the lengths of pre-rRNA transcription units among different organisms.

The synthesis and most of the processing of pre-rRNA occurs in the nucleolus. When pre-rRNA genes were initially identified in the nucleolus by in situ hybridization, it was not known whether any other DNA was required to form the nucleolus. Subsequent experiments with transgenic Drosophila strains demonstrated that a single complete pre-rRNA transcription unit induces formation of a small nucleolus. Thus a single pre-rRNA gene is sufficient to be a nucleolar organizer, and all the other components of the ribosome diffuse to the newly formed pre-rRNA. The structure of the nucleolus observed by light and electron microscopy results from the processing of pre-RNA and the assembly of ribosomal subunits.

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مواضيع ذات صلة

Localization of mRNAs Permits Production of Proteins at Specific Regions Within the Cytoplasm

Surveillance Mechanisms Prevent Translation of Improperly Processed mRNAs

Sequence-Specific RNA-Binding Proteins Control Translation of Specific mRNAs

Protein Synthesis Can Be Globally Regulated

Cytoplasmic Polyadenylation Promotes Translation of Some mRNAs

RNA Interference Induces Degradation of Precisely Complementary mRNAs

Alternative Polyadenylation Increases miRNA Control Options

Micro-RNAs Repress Translation and Induce Degradation of Specific mRNAs

Degradation of mRNAs in the Cytoplasm Occurs by Several Mechanisms

RNA Processing Solves the Problem of Pervasive Transcription of the Genome in Metazoans

Nuclear Exoribonucleases Degrade RNA That Is Processed Out of Pre-mRNAs

Microchip Electrophoresis