Ribosome biogenesis is the process of making ribosomes. In prokaryotic cells, it takes place in the cytoplasm with the transcription of many ribosome gene operons. In eukaryotes, it takes place both in the cell cytoplasm and in the nucleolus of eukaryotic cells. It involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins.
First, a reduction in aminoacyl-tRNA will cause the prokaryotic cell to respond by lowering transcription and translation. This occurs through a series of steps, beginning with stringent factor binding to ribosomes and catalyzing the reaction:
GTP + ATP --> pppGpp + AMP
The γ-phosphate is then removed and ppGpp will bind to and inhibit RNA polymerase. This binding causes a reduction in rRNA transcription. A reduced amount of rRNA means that ribosomal proteins (r-proteins) will be translated but will not have an rRNA to bind to. Instead, they will negatively feedback and bind to their own mRNA, repressing r-protein synthesis. Note that r-proteins preferentially bind to its complementary rRNA if it is present, rather than mRNA.
The ribosome operons also include the genes for RNA polymerase and elongation factors (used in RNA translation). Regulation of all of these genes at once illustrate the coupling between transcription and translation in prokaryotes.
Ribosomal protein synthesis in eukaryotes occurs, like most protein synthesis, in the cytoplasm just outside the nucleus. Individual large and small units are synthesized and imported into the nucleus through nuclear pores. These pores have a diameter of 120nm and import 560 000 ribosomal proteins per minute into the nucleus with active transport. See nuclear import for more about the movement of the ribosomal proteins into the nucleus.
Ribosomal RNA (rRNA) is transcribed at the nucleolus, at a high speed, which contains all 45s rRNA genes. After transcription, the rRNA is put together with the ribosomal subunits to make a functioning ribosome. See nuclear export for more about the movement of the ribosomal subunit out of the nucleus.
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