Nucleoli multiplication

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 45 S 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. 

However, in eukaryotic cells at least one variation of this pattern is suggested by the presence of multiple forms of RNA polymerase which are associated with different regions of the cell nucleus. Each form may even show different initiator site specificities (Roeder and Rutter, 1969 1970 Kedinger et ah, 1970). One form (I or A) is located in the nucleolus (Roeder and Rutter, 1970) and is involved in the synthesis of the GC-rich (ribosomal) RNA at low ionic strength by isolated nuelei. This form is insensitive to the toxin a-amanitin. The seeond form (II or B) is less dependent on the presence of Mg and is found predominantly in the nucleoplasm, presumably in association with the euchro-matin. In the in vitro synthesis of RNA by isolated nuclei in high ionic strength in the presence of Mn, the newly synthesized product is predominately DNA-like RNA. The synthesis of this RNA is sensitive to low concentrations of a-amanitin. A third form has been detected in rat liver and sea urchins (Roeder and Rutter, 1969, 1970), but the function of this form has not yet been delineated. 

Biogenesis of ribosomes occurs in the nucleus where the rRNA genes are transcribed by RNA polymerase I to form 45S primary transcripts (Fig. 69.2). The first 45S primary transcript functions as a nucleolar organiser and induces the formation of a nucleolus where multiple copies of the ribosomal RNA genes are transcribed into rRNA. 

RNA polymerase has now been purified in various laboratories from various mammalian sources [188], and RNA polymerase of eukaryotic nuclei always exists in multiple forms that can be distinguished by the cation requirement, their sensitivity to a-amanitin, and their ability to react with specific templates. One enzyme (polymerase I) is found in the nucleolus, the others in the nucleoplasm. All mammalian ribonucle-ases are complex proteins formed of several subunits. Three types of RNA polymerases have been purified from ascites tumor cells by chromatography on car-boxymethyl-cellulose. Two are nucleolar and one is nucleoplasmic. Protein factors of unknown nature that stimulate all three enzymes have been found in calf thymus, rat liver, and ascites cells. These factors can be separated into two classes heat stable and heat labile. Both types stimulate the activity of the Novikoff RNA polymerase several-fold, but only with native DNA as templates. The factors have no effect on E. coli RNA polymerase [266-267]. For further information, refer to the review of Jacobs [189]. 

Both prokaryotic and eukaryotic RNA polymerases are composed of several subunits. Although only one form of RNA polymerase predominates in bacteria and blue-green algae, in eukaroytes there are multiple forms of RNA polymerase which have different nuclear and organelle compartmentalization, function, and subunit composition. Polymerase I (or A) is located in the nucleolus and transcribes 18 S and 28 S ribosomal RNA genes. By contrast, polymerase II (or B) and III (or... 

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