RNA
Polymerase I Transcription
Enzyme - RNA polymerase I is a complex enzyme, containing 13 subunits totaling over 600,000 daltons. It is responsible for synthesizing the large 45S pre-rRNA transcript that is later processed into mature 28S, 18S, and 5.8S ribosomal RNAs (rRNAs). At least two transcription factors are known to be required, but there is no need for an elaborate transcriptional apparatus characteristic of pol II transcription (see here), because only a single kind of gene is transcribed.
Ribosome assembly - Eukaryotic ribosomes contain four rRNA molecules. The small ribosomal subunit has an 18S rRNA, whereas the large subunit contains 28S, 5.8S, and 5S rRNA molecules. The nucleolus is the site of ribosomal subunit assembly in eukaryotes. The gene for the 45S pre-rRNA is present in the nucleolus as multiple, tandemly arranged copies, as shown in Figure 28.20a. After transcription, the 45S pre-rRNA is processed to yield 18S, 5.8S, and 28S rRNA molecules. The rRNAs are then combined with 5S rRNA from other regions of the nucleus and ribosomal proteins synthesized in the cytosol. The resulting ribosomal subunits are exported from the nucleolus back into the cytosol.
Transcription of 45S rRNA - Transcription of the tandem copies of 45S pre-rRNA can be beautifully visualized in the electron microscope. It appears likely that nucleosomes are not present in nucleolar chromatin, at least in the transcribed regions. Absence of nucleosomes may be a specific chromatin modification to allow rapid and continuous transcription of these genes.
Autocatalysis by Intron - The 28S rRNA of the protozoan Tetrahymena, contains an intron near its 3' end. Excision of this intron and splicing of the RNA are carried out by a remarkable process in which the RNA itself acts as the catalyst, via the series of reactions shown in Figure 11.30.
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3. RNA Editing