Eukaryotic RNA polymerase 18S+ 30 proteins= 40S

1. Initiation of rDNA transcription Two transcription factors, UBF and SL1, bind cooperatively to the rDNA promoter and recruit RNA polymerase I to form an initiation complex. One subunit of SL1 is the TATA-binding protein (TBP) and the other is TBP- associated Factor (TAF).

2. Transcription-control elements in genes transcribed by RNA polymerase I (a) and III (b,c). Assembly of transcription-initiation complexes on these genes begins with the binding of specific general transcription factors to the control elements

3. Initiation of transcription by RNA polymerase I. One possible model envisages initial binding of the two identical subunits of the upstream binding factor to the upstream control element and the core promoter element, and forcing these two sequences to come into close proximity, enabling their subsequent binding by the selectivity factor 1 (SL1) which consists of four subunits. The stabilized structure permits subsequent binding of other factors (not shown) and subsequently RNA polymerase I.

7. Processing of pre-rRNA and assembly of ribosomes in eukaryotes. (a) Major intermediates and times required for various steps in pre-rRNA processing in higher eukaryotes. Ribosomal and nucleolar proteins associate with 45S pre-rRNA soon after its synthesis, forming an 80S pre-rRNP. Synthesis of 5S rRNA occurs outside of the nucleolus. The extensive secondary structure of rRNAs is not represented here. Note that RNA constitutes about two-thirds of the mass of the ribosomal subunits, and protein about one-third. (b) Pathway for processing of 6.6-kb (35S) pre-rRNA primary transcript in S. cerevisiae. The transcribed spacer regions (tan), which are discarded during processing, separate the regions corresponding to the mature 18S, 5.8S, and 25S rRNAs. All of the intermediates diagrammed have been identified; their sizes are indicated in red type.

8. Pre-rRNA processing in Escherichia coli. The pre-rRNA, containing copies of the 16S, 23S and 5S rRNAs, is cleaved by ribonucleases III, P and F, and the resulting molecules trimmed by ribonucleases M16, M23 and M5 to give the mature rRNAs. Eukaryotic pre-rRNAs, which contain the rRNA sequences in the order 18S-5.8S-28S, are processed in a similar fashion. Note that a tRNA is located between the 16S and 23S sequences in the E. coli pre-rRNA.

9. Processing of ribosomal RNAs Prokaryotic cells contain three rRNAs (16S, 23S, and 5S), which are formed by cleavage of a pre-rRNA transcript. Eukaryotic cells (e.g., human cells) contain four rRNAs. One of these (5S rRNA) is transcribed from a separate gene; the other three (18S, 28S, and 5.8S) are derived from a common pre-rRNA. Following cleavage, the 5.8S rRNA (which is unique to eukaryotes) becomes hydrogen-bonded to 28S rRNA.

10. Figure 8.28. Ribosome assembly Ribosomal proteins are imported to the nucleolus from the cytoplasm and begin to assemble on pre-rRNA prior to its cleavage. As the pre-rRNA is processed, additional ribosomal proteins and the 5S rRNA (which is synthesized elsewhere in the nucleus) assemble to form preribosomal particles. The final steps of maturation follow the export of preribosomal particles to the cytoplasm, yielding the 40S and 60S ribosomal subunits.

12. Eukaryotic RNA polymerase 18S+ 30 proteins= 40S Poly merase I, II, III 5.8S, 5S, 28S+ 5 proteins = 60S Transcription Factors UCE (upstream control element) 16S + 21 proteins = 30S UBF (upstream binding factor) 5S, 23S + 34 protein = 50S SL1 (TBP= TATA binding protein) (TAF= TBP associated factor) Initiation complex Chemical modifications Methylation Pseudouridine Small nucleolar RNA (sno RNA) Sno RNP Prokaryotic pre-rRNA 16S, 23S, 5S rRNA Leader, Trailer, Spacer Rnase III M16, M23, M5 Eukaryotic pre-rRNA 18S, 5.8S, 28S rRNA