create the pre-60S ribosomal particles. 40S subunit The transcriptional assembly of the 40S subunit precursor, sometimes referred to as the small subunit processome (SSU) or 90S particle happens in a hierarchical fashion – essentially a stepwise incorporation of the UTP-A, UTP-B, and UTP-C subcomplexes. These subcomplexes are made up of over 30 non ribosomal protein factors, the U3 snoRNP particle, a few Rps proteins, and the 35S pre-rRNA. Their exact role, though has not been discovered. The composition of the pre-40S particle changes drastically once cleavage at the U3 snoRNPA dependent sites (sites A0, A1, and A2) are made. This cleavage event creates the 20S pre-rRNA and causes ribosomal factors to dissociate from the pre-40S particle. U3 is displaced from the nascent 40S by the helicase Dhr1. At this point in the ribosome biogenesis process, the 40S pre-ribosome already shows the “head” and “body” structures of the mature 40S subunit. The 40S pre-ribosome is transported out of the nucleolus and into the cytoplasm. The cytoplasmic 40S pre-ribosome now contains ribosomal proteins, the 20s rRNA and a few non-ribosomal factors. The final formation of the 40S subunit “beak” structure occurs after a phosphorylation and dephosphorylation event involving the Enp1-Ltv1-Rps3 complex and the kinase, Hrr25. Cleavage of the 20S pre-rRNA at the D-site creates the mature 18s rRNA. This cleavage event is dependent on several non-ribosomal factors such as Nob1, Rio1, Rio2, Tsr1 and Fap7. 60S subunit The maturation of the pre-60S subunit into a mature 60S subunit requires many biogenesis factors that associate and disassociate. In addition, some assembly factors associate with the 60S subunit while others only interact with it transiently. As an overall trend, the maturation of the pre-60S subunit is marked a gradual decrease in complexity. The subunit matures as it moves from the nucleolus to the cytoplasm and gradually the number of trans-acting factors are reduced. The maturation of the 60S subunit requires the help of about 80 factors. Eight of these factors are directly involved with the processing of the 27S A3 pre-rRNA, which actually completes the formation of the mature 5’end of the 5.8S rRNA. The A3 factors bind to distant sites on the pre-RNA as well as to each other. Subsequently, they bring areas of rRNA close together and promote the processing of pre-rRNA and the recruitment of ribosomal proteins. Three AAA-type ATPases work to strip the factors from the maturing 60S pre-ribosome. One