Pre rRNA, processing

The most abundant protein in the fibrillar regions of the nucleus, where the early stages of pre-rRNA processing occur, is fibrillarin.541,548,549 Many of the snoRNAs are closely associated with this protein. Fibrillarin is also well known as an autoantigen, which can induce formation of destructive antibodies that cause scleroderma (Chapter 31, Section F). 

The most abundant protein in the fibrillar regions of the nucleus, where the early stages of pre-rRNA processing occur, is Many of fhe... 

Prokaryotic pre-rRNA processing The E. coli primary rRNA transcript contains 16S rRNA, 1 or 2tRNA, 23S rRNA and 5S rRNA and some with 1 or 2 more tRNA (in order from 5 to 3 ends). The trimming and processing of pre-rRNA to mature rRNA involves various RNases such as RNase HI, RNase P, RNase E, RNase F and RNase M at different stages. 

Eukaryotic pre-rRNA processing The primary rRNA transaipt is 45S RNA containing 18S, 5.8S and 28S rRNA separated by spacer sequences (in order from 5 to 3 ends). An initial methylation of -110 sites yields O -methyribose residues (-80% of methylated sites) and methylated bases (-20% of methylated sites) such as N , N -dimethy-ladenine and 2-methylguanine. Subsequent trimming and processing by RNases produces mature rRNA. 

Posttranscriptional processing is not limited to mRNA. Ribosomal RNAs of both prokaryotic and eukaryotic cells are made from longer precursors called preribosomal RNAs, or pre-rRNAs, synthesized by Pol I. In bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded elsewhere) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing (Fig. 26-21). 

FIGURE 26-22 Processing of pre-rRNA transcripts in vertebrates. In step (T), the 45S precursor is methylated at more than 100 of its 14,000 nucleotides, mostly on the 2 -OH groups of ribose units retained in the final products. (5) A series of enzymatic cleavages produces the 18S, 5.8S, and 28S rRNAs. The cleavage reactions require RNAs found in the nucleolus, called small nucleolar RNAs (snoRNAs), within protein complexes reminiscent of spliceosomes. The 5S rRNA is produced separately. 

In eukaryotes, a 45S pre-rRNA transcript is processed in the nucleolus to form the 18S, 28S, and 5.8S rRNAs characteristic of eukaryotic ribosomes (Fig. 26-22). The 5S rRNA of most eukaryotes is made as a completely separate transcript by a different polymerase (Pol III instead of Pol I). 

In at least one eukaryote, Tetmhymem, the pre-rRNA molecule contains an intron. Removal of the intron during processing of the pre-rRNA does not require the assistance of any protein Instead, in the presence of guanosine, GMP, GDP or GTP, the intron excises itself, a phenomenon known as selfsplicing. This was the first demonstration of ribozymes, that is, catalytic RNA molecules that catalyze specific reactions. The list of ribozymes is growing. For example, self-splicing introns have been discovered in some eukaryotic mRNAs and even peptidyl transferase, a key enzyme activity in protein synthesis, is now known to be a ribozyme (see Topic H2). 

The cleavage of the precursor into three separate rRNAs is actually the final step in its processing. First, the nucleotides of the pre-rRNA sequences destined for the ribosome undergo extensive modification, on both ribose and base components, directed by many small nucleola.r ribonucleo-proteins (snoRNPs), each of which consists of one snoRNA and several proteins. The pre-rRNA is assembled with ribosomal proteins, as guided by... 

Transcription of rRNA and its assembly into precursor-ribosomes can be visualized by electron microscopy. The structures resemble Christmas trees the trunk is the rDNA and each branch is a pre-rRNA transcript. Transcription starts at the top of the tree, where the shortest transcripts can be seen, and progresses down the rDNA to the end of the gene. The terminal knobs visible at the end of some pre-rRNA transcripts likely correspond to the SSU processome. a large ribonucleoprotein required for processing the pre-rRNA. 

Small Nucleolar RNAs Assist in Processing Pre-rRNAs and Assembling Ribosome Subunits... 

A FIGURE 12-34 Processing of pre-rRNA and assembly of ribosomes in higher eukaryotes. Ribosomal and nucleolar proteins associate with 45S pre-RNA as it is synthesized, forming an SOS pre-rRNR Sites of cleavage and chemical modifications are determined by small nucleolar RNAs (not shown). Note that synthesis of 5S rRNA occurs outside the nucleolus. 

Synthesis and processing of pre-rRNA occur in the nucleolus. The 5S rRNA component of the large ribosomal subunit is synthesized in the nucleoplasm by RNA polymerase III and is not processed. 

Enzyme - RNA polymerase I is a complex enzyme, containing 13 subunits totaling over 600,000 daltons. It is responsible for synthesizing the large 45S pre-rRNA transcript that is later processed into mature 28S, 18S, and 5.8S ribosomal RNAs (rRNAs). At least two transcription factors are known to be required, but there is no need for an elaborate transcriptional apparatus characteristic of pol II transcription (see here), because only a single kind of gene is transcribed. 

Ribosome assembly - Eukaryotic ribosomes contain four rRNA molecules. The small ribosomal subunit has an 18S rRNA, whereas the large subunit contains 28S, 5.8S, and 5S rRNA molecules. The nucleolus is the site of ribosomal subunit assembly in eukaryotes. The gene for the 45 S pre-rRNA is present in the nucleolus as multiple, tandemly arranged copies, as shown in Figure 28.20a. After transcription, the 45S pre-rRNA is processed to yield 18S, 5.8S, and 28S rRNA molecules. The rRNAs are then combined with 5S rRNA from other regions of the nucleus and ribosomal proteins synthesized in the cytosol. The resulting ribosomal subunits are exported from the nucleolus back into the cytosol. 

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