RRNA transcription

A strong link between the phosphorylation of nucleolin, its proteolysis and the production of ribosomal RNA has been observed (Bouche et al, 1984 Bourbon et al, 1983 Warrener and Petryshyn, 1991). The inhibition of proteolysis using leupeptin leads to a lower rRNA transcription in an in vitro transcription system (Bouche et al, 1984). In another series of experiments, the injection of nucleolin antiserum leads to 2-3.5 fold stimulation of pre-rRNA synthesis in Chironomus tentans salivary glands (Egyhazi et al, 1988), although it was not clearly demonstrated that these antibodies blocked specifically the homolog of nucleolin in this species. A model was proposed based on these observations where nucleolin was... 

A recent study in the carp Cyprinus carpio) shows that level of nucleolin is up regulated in cold-acclimatized carp with a concomitant nucleolar segregation and depression in rRNA transcription (Alvarez et al, 2003). 

On the other side, it has been demonstrated that nucleolin phosphorylation and rRNA transcription go hand in hand. Nucleolin phosphorylation could be triggered by a variety of stimuli like androgens and growth factors (Bonnet et al, 1996 Bouche et al, 1987 Issinger et al, 1988 Suzuki et al, 1985, 1991 Tawfic et al, 1994) and the phosphorylation is invariably accompanied by increased rRNA transcription and cell proliferation. All these observations suggest that indeed nucleolin could have regulatory role in RNA polymerase I transcription. However, further experiments are clearly required to clarify the function of nucleolin in rRNA transcription. 

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). 

Condon, C., Squires, C., Squires, C.L. (1995) Control of rRNA transcription in Escherichia coli. Microbiol. Rev. 59, 623-645. 

Gourse, R.L., Gaal, T., Bartlett, M.S., Appleman, J.A., Ross, W. (1996) rRNA transcription and growth rate-dependent regulation of ribosome synthesis in Escherichia coli. Annu. Rev. Microbiol. 50, 645-677. 

The primary eukaryotic rRNA transcripts extend several hundred nucleotides past the 3 termini of the... 

E. coli has seven rRNA transcription units, each containing one copy each of the 23S, 16S and 5S rRNA genes as well as one to four tRNA genes. Transcription produces a 30S pre-rRNA transcript. This folds up to form stem-loop structures, ribosomal proteins bind, and a number of nucleotides become methylated. The modified pre-rRNA transcript is then cleaved at specific sites by RNase III and the ends are trimmed by ribonucleases M5, M6 and M23 to release the mature rRNAs. 

Fig. 4. (a) rRNA transcription units (b) transcription of a single transcription unit by RNA... 

The rRNA transcription units in E. coli contain some tRNA genes that are transcribed and processed at the time of rRNA transcription (Topic G9). Other tRNA genes occur in clusters of up to seven tRNA sequences separated by spacer regions. Following transcription by the single prokaryotic RNA polymerase, the primary RNA transcript folds up into the characteristic stem-loop structures (Fig. 2) and is then processed in an ordered series of cleavages... 

As mentioned above, one consequence of stalled RNA polymerase II at a DNA adduct is activation of transcription-coupled repair [27], This effect may depend on the type of polymerase, however, since the removal of some types of DNA damage is slower from RNA-polymerase I transcribed ribosomal DNA than from a nuclear gene [160], The lower level of repair in the nucleolus could also reflect the influence of other transcription factors, such as the HMG-domain protein UBF, which bind to cisplatin-mod-ified DNA [145]. When HeLa cells were exposed to cisplatin at concentrations which did not seem to affect nuclear transcription, inhibition of rDNA gene expression was associated with the redistribution of UBF, along with other factors responsible for rRNA transcription [138], These observations indicate how cisplatin might exert a combination of effects. Transcription is stopped due to titration of essential factors by the platinum-DNA adducts, and the same proteins could shield the lesions from the repair activity. 

Very little is known about the processing of rRNA transcripts to generate mature rRNAs in the archaea, except that one of the enzymes involved recognizes a structure that consists of bulges that are staggered on opposite sides of the chain and that the processing cuts occur in the bulges [27,29,43]. 

In sulfothermophile transcripts, the 16S-23S rRNA spacers are short compared with those of other archaea and bacteria they lack tRNAs and, although they contribute to the processing stems of the 16S and 23S rRNAs, the stems are truncated and, at least for Tp. tenax, a single, bifurcated processing stem is generated for the two rRNAs [82], However, despite differences between the organization and structure of the sulfothermophile rRNA transcripts and those of the other archaea, secondary structural motifs can be discerned in the 16S rRNA leader sequences and in the 16S-23S rRNA spacer regions which are common and exclusive to the archaea [108],... 

Figure 29.24 Visualization of rRNA transcription and processing in eukaryotes.

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. 

Nucleolar organizing region (NOR) Area of the nucleolus where a great deal of rRNA transcription and synthesis occurs. 

A FIGURE 12-32 Electron micrograph of pre-rRNA transcription units from nucleolus of a frog oocyte. Each "feather" represents a pre-rRNA molecule associated with protein in a pre-ribonucleoprotein particle (pre-RNP) emerging from a transcription unit. Pre-rRNA transcription units are arranged in tandem, separated by nontranscribed spacer regions of nucleolar chromatin. [Courtesy of Y. Osheim and O. J. Miller, Jr.]... 

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