Ribosomes precursors

The nucleus is not capable of synthesizing proteins. All of the nuclear proteins therefore have to be imported—the histones with which DNA is associated in chromatin, and also the so-called non-histone proteins (DNA polymerases and RNA polymerases, auxiliary and structural proteins, transcription factors, and ribosomal proteins). Ribosomal RNA (rRNA) already associates with proteins in the nucleolus to form ribosome precursors. 

Processing of Ribosomal Precursor Leads to Three RNAs... 

A variety of powerful methods is available for fractionating short oligonucleotides. Work on nucleotide sequence analysis of small RNA molecules (n=80 or 120) and more recently viral RNA molecules (n= 3000) and even ribosomal precursor RNAs (up to 45 s) (Holley et al. 1965a Brownlee 1972 Maden et al. 1972) has stimulated the development of these procedures. They can also be used for synthetic oligonucleotides (e.g. Hachman and Khorana, 1969). The methods depend largely on chromatography and electrophoresis on filter papers, diethylaminoethyl(DEAE) paper, cellulose acetate, thin layers of cellulose or polyethyleneimine (PEI)-cellulose, or columns. 

Fig. 10.22. Demonstration by gel electrophoresis of heterogeneity in the 45 S ribosomal precursor RNA from mammalian cells. Three species, nominally 45, 46 and 47 S are resolved. The visualisation of the different peaks is enhanced by labelling at different times with (O) and ( ) (Tiollais et al. 1971).

Protein L 27 (tobacco ohioroplast olone L27-1 ribosome precursor reduced) 1124b, 4249... 

Normal cells. It is much more difficult to demonstrate free informosomes in the cells of adult organisms than in embryos since in adult cells there is intensive synthesis of rRNA and ribosomal precursors. Hoagland and Askonas (1963) first observed nucleoprotein particles containing RNA in rat liver cytoplasm and were able to use them to stimulate protein synthesis in a cell-free system. 

Warner, J. R., R. Soeiro, H. S. Birnboim, M. Girard, and J. E. Darnell. 1966. Rapidly labeled HeLa cell nuclear RNA. I. Identification by sedimentation of a heterogeneous fraction separated from ribosomal precursor RNA. J. Molec. Biol., 19 349-361. 

Table 1 lists proteins that we regard as molecular chaperones, together with their proposed roles. Two recently discovered examples are of especial interest because these chaperones are attached covalently to the molecules whose assembly they control, viz. the pro-sequence of subtilisin(12) and the ubiquitin sequence present at the aminoterminus of two ribosomal precursor proteins in yeast and other eukaryotes (13). The advantage of these cotranslational chaperones is that they can bind their ligands without requiring a specific recognition site in these ligands. 

Although the mt rRNA from N. crassa differs drastically from the rRNA species found in animal mitochondria (Table X) it is noteworthy that Kuriyama and Luck (1973) reported evidence of a common large molecular weight precursor (32 S) for both the mt rRNA s. The molecular weight of the precursor RNA was about 120 of the sum of the molecular weights of both mt rRNA s. The spacer region would, therefore, be considerably smaller than that of the nucleolar precursor of microsomal rRNA s which represents about 56 of the ribosomal precursor RNA (Quagliarotti et al., 1970). 

Studies on the species of ENA that are inhibited after poliovirus infection have led to the conclusion that the synthesis of 45s nucleolar ribosomal precursor ENA is primarily depressed (18). The synthesis of non-ribosomal REA was not affected at early times after infection (I8). The appearance of mREA in polysomes is also not depressed in infected cells for at least 1-3 hours after infection (9f 19> 20). The conversion of the 45S ribosomal ENA precursor to the 32S and 18S ENA is inhibited or slowed (I8). The methylation of the 458 ENA was not inhibited, suggesting tha.t the virus is not exerting its action at this level (I8). 

However, 5-fluoro-orotic and orotic acids are utilized differentially for the synthesis of cytoplasmic liver RNA. 5-Fluoro-orotate is incorporated preferentially into a fraction of non-ribosomal RNA which has several properties in common with messenger RNA [271]. Analysis of microsomal RNA showed Uttle or no incorporation of 5-fluoro-orotic acid into either 18 S or 28 S ribosomal RNA. The analogue is rapidly incorporated into 45 S ribosomal precursor RNA but its subsequent processing into mature 18 S and 28 S RNA is inhibited [272]. The analogue also greatly inhibits the incorporation of orotic acid into ribosomal RNA but has little effect on its incorporation into messenger RNA [273]. 

Ajuh, P.M., Heeney, P.A. and Maden, B.E.H. (1991) Xenopus borealis and Xenopus laevis 28S ribosomal DNA and the complete 40S ribosomal precursor RNA coding units of both species , Proceedings of the Royal Society of London B, 245, 65-71. 

Craig, N., and Perry, R. P., 1970, Aberrant intranucleolar maturation of ribosomal precursors in the absence of protein synthesis, J. Cell. Biol. 45 554. 

Ledinko, N., 1972, Nucleolar ribosomal precursor RNA and protein synthesis in HEK cultures infected with Adl2, Virology 49 79. 

Vaccinia virus infection inhibits the transport of newly synthesized ribosomal RNA (rRNA) to the cytoplasm (Becker and Joklik, 1964 Salzman et al., 1964). This inhibition was studied in HeLa cells and it was detected 3-4 hr after infection. In another study where L cells were used, inhibition of rRNA synthesis occurred more rapidly (Jefferts and Holowczak, 1971). These authors showed that the cleavage of the 45 S ribosomal precursor RNA to 32 S and 18 S was inhibited by 2 hr after infection and the transport of rRNA to the cytoplasm was curtailed by 3 hr after infection. The methylation of the 45 S RNA was reduced in infected cells but was not completely abolished. Furthermore, inhibition of ribosomal protein synthesis occurred before a decline in the synthesis of rRNA. The product responsible for these changes has not been determined however, it may... 

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