<P> 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 . </P> <P> Ribosomal protein synthesis in eukaryotes is a major metabolic activity . It occurs, like most protein synthesis, in the cytoplasm just outside the nucleus . Individual ribosomal proteins are synthesized and imported into the nucleus through nuclear pores . See nuclear import for more about the movement of the ribosomal proteins into the nucleus . </P> <P> The rRNA is transcribed, at a high speed, in the nucleolus, which contains all 45S rRNA genes . The only exception is the 5S rRNA which is transcribed outside the nucleolus . After transcription, the rRNAs associate with the ribosomal proteins, forming the two types of ribosomal subunits (large and small). These will later assemble in the cytosol to make a functioning ribosome . See nuclear export for more about the movement of the ribosomal subunits out of the nucleus . </P> <P> Eukaryotic cells co-transcribe three of the mature rRNA species though a series of steps . The maturation process of the rRNAs and the process of recruiting the r - proteins happen in precursor ribosomal particles, sometimes called pre-ribosomes, and takes place in the nucleolus, nucleoplasm, and cytoplasm . The yeast, S. cerevisiae is the eukaryotic model organism for the study of ribosome biogenesis . Ribosome biogenesis starts in the nucleolus . There, the 18S, 5.8 S, and 25S subunits of the rRNA are cotranscribed from ribosomal genes as a polycistronic transcript by RNA polymerase I, and is called 35S pre-RNA . </P>

Where does the formation of ribosomal subunits occur