Ribosomal subunit assembly pathway in S. cerevisiae. In the nucleolus, the 35S pre-rRNA associates with many early-associating 40S and 60S r-proteins to form a 90S preribosome. The 5S rRNA forms a stable RNP with Rpl5p (L5, red dot). The 5S rRNA is probably already present in 90S preribosomes; therefore, Rpl5p may belong to the group of early-assembling 60S r-proteins. From the 90S particle, 66S and 43S preribosomes containing the 27S and 20S pre-rRNAs, respectively, are formed. The 66S and the 43S preribosomes contain the late-associating r-proteins. Preribosomes are most likely actively exported through nuclear pores. It has been demonstrated that export of 60S ribosomal subunits requires the nuclear or cytoplasmic Ran cycle and distinct nucleoporins (not shown; for details, see reference 70). It is believed that the 66S particle remains in the nucleus until the 7S pre-rRNA is processed to the mature 5.8S rRNA, while the 43S preribosome is rapidly exported to the cytoplasm, where the final maturation step in the synthesis of the 18S rRNA takes place. Assembly of 60S ribosomal subunits is also completed in the cytoplasm by the final incorporation of some r-proteins. At least three of them (also depicted as red dots), including the P2B and Rpl10p/Qsr1p (L10) r-proteins, can exchange on mature 60S ribosomal subunits in vivo. Preribosomes are drawn as balloons, and the nuclear envelope is depicted as pink rods. Protein trans-acting factors with likely involvement in the assembly of ribosomal subunits are colored as follows: red, putative ATP-dependent RNA helicases; black, others. Only those r-proteins for which an indication about the timing of their association with preribosomal particles is known are shown. See the text for further details.

Pre-rRNA processing in S. cerevisiae. (A) Structure of an rDNA repeat unit. Each unit contains a large operon encoding the 18S, 5.8S, and 25S rRNAs, which is transcribed by RNA pol I (pr, promoter; tr, terminator; eh, enhancer), and an RNA pol III-transcribed 5S rRNA gene. Mature sequences of the RNA pol I transcript are separated by ITS1 and ITS2 and flanked by a 5′ ETS and a 3′ ETS. The 5S rRNA gene is located between two nontranscribed spacers (NTS1 and NTS2). Black boxes represent mature rRNAs, white thin bars represent the transcribed spacers, and lines represent the nontranscribed spacers. Processing sites are also indicated. (B) Pre-rRNA processing pathways. The primary RNA pol I transcript is processed at its 3′ end to yield the 35S pre-rRNA, which is the longest detectable pre-rRNA. The 35S pre-rRNA is first processed at sites A₀, A₁, and A₂, which results in the separation of the pre-rRNAs destined for the small and large ribosomal subunits. The 20S pre-rRNA is matured by endonucleolytic cleavage at site D. The 27SA₂ precursor is processed by two alternative pathways. In the major pathway, about 85% of 27SA₂ pre-rRNA is cleaved at site A₃ and then 5′→3′ exonucleolytically digested to site B_1S. In the minor pathway, about 15% of the 27SA₂ molecules are processed at site B_1L. While processing at site B₁ is completed, the 3′ end of mature 25S rRNA is generated by processing at site B₂. The subsequent ITS2 processing of both 27SB species appears to be the same. Cleavage at sites C₁ and C₂ releases the mature 25S rRNA and the 7S pre-rRNA. The latter undergoes 3′→5′ exonucleolytic digestion to the 3′ end of the mature 5.8S rRNA. The pre-5S rRNA is processed at its 3′ end to generate the mature 5S rRNA. Pseudouridylation and 2′-O-ribose methylation take place on the primary transcript. Note that except for Dim1p, the timing of base methylation and the protein trans-acting factors involved therein have not yet been uncovered. Factors involved in pre-rRNA processing and modification are shown. Colors were assigned to each class of protein trans-acting factors as follows: blue, exo- and endonucleases; red, putative ATP-dependent RNA helicases; green, components involved in pre-rRNA modification; black, others. See the text for further details.