Ribosomes are complex ribonucleoprotein particles with a diameter of 25–30 nm which are located free or membrane-bound in the cytoplasm where they are engaged in protein synthesis. Ribosomes are made up of four types of ribosomal RNA (rRNA) molecules and around eighty different ribosomal proteins. Ribosome biogenesis occurs mainly in the nucleolus, being completed later in the nucleoplasm and cytoplasm. The transcription of ribosomal DNA (rDNA) occurs in the nucleolus where there are around 400 copies of ribosomal genes – and requires the assembly of a multiprotein complex which includes the RNA Polymerase I (Pol I) and a number of basal transcription initiation factors atthe rDNA promoter. In mammalian cells, at least three basal factors – the ribosomal DNA transcription factor Rrn3 (also referred to as Transcription Initiation Factor I (TIF-I) A, the Selectivity factor 1 (SL1), and the Upstream Binding Factor (UBF) – are necessary for ribosome gene transcription. 
The transcription of ribosomal genes produces the 47S rRNA precursor which is then processed to generate the mature 18S, 5.8S, and 28S rRNA. The fourth type of rRNA– the 5S rRNA– is transcribed in the nucleoplasm by RNA Polymerase III (Pol III), and imported to the nucleolus. These rRNAs are then assembled with ribosomal proteins (RPs) to form the large 60S and the small 40S subunits of mature ribosomes. The large 60S subunit contains one each of the 28S, 5.8S, and 5S RNAs, together with 47 ribosomal proteins, called RPLs, whereas the small 40S subunit contains only the 18S RNA and 32 ribosomal proteins, called RPSs. The RPs whose mRNA is transcribed by RNA Polymerase II (Pol II) are synthesized in the cytoplasm and imported to the nucleolus.


1.Derenzini M, et al. Acta Histochem. 2017;119(3):190–197.