Ribosomal maturation

The mature red blood cell cannot synthesize protein. Reticulocytes are active in protein synthesis. Once reticulocytes enter the circulation, they lose their intracellular organelles (ribosomes, mitochondria, etc) within about 24 hours, becoming young red blood cells and concomitandy losing their ability to synthesize protein. Extracts of rabbit reticulocytes (obtained by injecting rabbits with a chemical—phenylhydrazine—that causes a severe hemolytic anemia, so that the red cells are almost completely replaced by reticulocytes) are widely used as an in vitro system for synthesizing proteins. Endogenous mRNAs present in these reticulocytes are destroyed by use of a nuclease, whose activity can be inhibited by addition of Ca +. The system is then pro-... 

All enveloped human vimses acquire their phospholipid coating by budding through cellular membranes. The maturation and release of enveloped influenza particles is illustrated in Fig. 3.8. The capsid protein subunits are transported flom the ribosomes to the nucleus, where they combine with new viral RNA molecules and are assembled into the helical capsids. The haemagglutinin and neuraminidase proteins that project fiom the envelope of the normal particles migrate to the cytoplasmic membrane where they displace the normal cell membrane proteins. The assembled nucleocapsids finally pass out from the nucleus, and as they impinge on the altered cytoplasmic membrane they cause it to bulge and bud off completed enveloped particles flxm the cell. Vims particles are released in this way over a period of hours before the cell eventually dies. 

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability. 

The genetic map is shown and the flow of events of MS2 multiplication. The infecting RNA goes to the host ribosome, where it is translated into four (or more) proteins. The four proteins that have been recognized are maturation protein (A-protein present in... 

Protein biosynthesis by eukaryotic ribosomes, which are larger and more complex than those of bacteria, is very similar in its basic outline to that of bacteria. The major difference is in the initiation of translation, which involves recognition of the 5 -cap on the mature mRNA, mentioned earlier, by the small ribosomal subunit. 

Microscopic examination of the mature neutrophils reveals two striking features a single multilobed nucleus and a dense, granular appearance of the cytoplasm (see Fig. 1.1a). The nucleus typically comprises two to four segments, and within this organelle the chromatin is coarsely clumped. Until recently, this abnormal chromatin structure was taken as evidence that the nucleus was transcriptionally inactive however, it is now appreciated that the mature neutrophil does perform active transcription ( 7.3), although rates of biosynthesis are somewhat lower than those observed in cells such as monocytes. There is no detectable nucleolus, so there can be only limited synthesis of ribosomal RNA in these cells. 

One important function of DUBs is the processing of ubiquitin or ubiquitin-like proteins to their mature forms. Ubiquitin is expressed in cells as either linear poly-ubiquitin or N-terminally fused to certain ribosomal proteins [79, 80]. These gene products are processed by DUBs to separate the ubiquitin into monomers and expose the gly-gly motif at the G-terminus. Many DUBs process linear polyubiquitin or Ub-fusion proteins in vitro, but this processing appears to take place cotransla-tionally in vivo and is extremely rapid. This makes analysis difficult and leaves unanswered the question of which DUBs actually perform this function in vivo. Multiple DUBs may be able to perform this processing at a physiologically relevant level since DUB deletions rarely shows processing defects [81]. 

Bose S, Basu M, Banerjee AK (2004) Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells. J Virol 78 8146-8158 Bouche G, Caizergues-Ferrer M, Bugler B, Amalric F (1984) Interrelations between the maturation of a 100 kDa nucleolar protein and pre rRNA synthesis in CHO cells. Nucleic Acids Res 12 3025-3035 Bouche G, Gas N, Prats H, Baldin V, Tauber JP, Teissie J, Amalric F (1987) Basic fibroblast growth factor enters the nucleolus and stimulates the transcription of ribosomal genes in ABAE cells undergoing GO-Gl transition. Proc Natl Acad Sci U S A 84(19) 6770-6774. 

Figure 4 The biosynthesis of nisin A as a representative example of the posttranslational maturation process of lantibiotics. Following ribosomal synthesis, NisB dehydrates serine and threonine residues in the structural region of the prepeptide NisA. NisC subsequently catalyzes intramolecular addition of cysteine residues onto the dehydro amino acids in a stereo- and regioselective manner. Subsequent transport of the final product across the cell membrane by NisT and proteolytic cleavage of the leader sequence by NisP produces the mature lantibiotic. For the sequence of the leader peptide, see Figure 6. Adapted with permission from J. M. Willey W. A. van der Donk, Annu. Rev. Microbiol. 2007, 61, 477-501.

Figure 19.7 A diagram of a Graafian follicle. In the rim of cytosol around the nucleus are spherical mitochondria, ribosomes and the Golgi. A mature follicle is relatively large, about 1 cm in diameter.

Messenger RNAs (mRNAs) transfer genetic information from the cell nucleus to the cytoplasm. The primary transcripts are substantially modified while still in the nucleus (mRNA maturation see p.246). Since mRNAs have to be read codon by codon in the ribosome, they must not form a stable tertiary structure. This is ensured in part by the attachment of RNA-binding proteins, which prevent base pairing. Due to the varying amounts of information that they carry, the lengths of mRNAs also vary widely. Their lifespan is usually short, as they are quickly broken down after translation. 

Translation. Mature mRNA enters the cytoplasm, where it binds to ribosomes, which convert the RNA information into a peptide sequence. The ribosomes (see p. 250) consist of more than 100 proteins and several RNA molecules (rRNA see p. 82). rRNA plays a role as a ribosomal structural element and is also involved in the binding of mRNA to the ribosome and the formation of the peptide bond. 

Transcription is catalyzed by DNA-dependent RNA polymerases. These act in a similar way to DNA polymerases (see p. 240), except that they incorporate ribonucleotides instead of deoxyribonucleotides into the newly synthesized strand also, they do not require a primer. Eukaryotic cells contain at least three different types of RNA polymerase. RNA polymerase I synthesizes an RNA with a sedimentation coef cient (see p. 200) of 45 S, which serves as precursor for three ribosomal RNAs. The products of RNA polymerase II are hnRNAs, from which mRNAs later develop, as well as precursors for snRNAs. Finally, RNA polymerase III transcribes genes that code for tRNAs, 5S rRNA, and certain snRNAs. These precursors give rise to functional RNA molecules by a process called RNA maturation (see p. 246). Polymerases II and III are inhibited by a-amanitin, a toxin in the Amanita phalloides mushroom. 

Several reticula of the r-ER show an ordered arrangement and many ribosomes are attached to their membrane during the primary wall development stage. As maturation proceeds, the r-ER s then gradually decrease not only in number and length of reticula, but also in the number of ribosomes. The s-ER s, on the other hand, become largest after the S3 stage, and sometimes attach ribosomes at their terminals. 

Wilkinson DS, Pitot HC. Inhibition of ribosomal ribonucleic acid maturation in Novikoff hepatoma cells by 5-fluorouracil and 5-fluorouridine. J Biol Chem 1973 248 63-68. 

Although the anti-tumour activity of 5-FU is mainly because of the inhibition of the thymidylate synthase, it is actually multifactor. The incorporation of 5-FU, under F-desoxyuridine form, into various types of RNA generates cytotoxicity. This occurs through the inhibition of the methylation and of the maturation of the ribosomal RNA [86]. The 5-FU is also precursor, by metabolism, of a monofluoroalanine, which is itself toxic. 

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