Ribosome assembly

Nucleus The nucleus is separated from the cytosol by a double membrane, the nuclear envelope. The DNA is complexed with basic proteins (histones) to form chromatin fibers, the material from which chromosomes are made. A distinct RNA-rich region, the nucleolus, is the site of ribosome assembly. The nucleus is the repository of genetic information encoded in DNA and organized into chromosomes. During mitosis, the chromosomes are replicated and transmitted to the daughter cells. The genetic information of DNA is transcribed into RNA in the nucleus and passes into the cytosol where it is translated into protein by ribosomes. 

Ribosomal assembly status—ER membrane-restricted initiation... 

Jan, E., and Samow, P. (2002). Factorless ribosome assembly on the internal ribosome entry site of cricket paralysis virus. J. Mol. Biol. 324, 889—902. 

After attachment of amino acids to tRNA, the amino acids are assembled beginning with the amino terminus and proceeding in the direction of the carboxy terminus. The ribosome is the machinery that translates the mRNA into protein. The ribosome is a very complex protein that contains ribosomal RNA as a functional and structural component. The ribosome assembles around the mRNA, and the cap and other signals allow alignment of the mRNA into the correct position. The initial assembly of the mRNA into the ribosome requires association of the small ribosomal subunit with an initiator tRNA (Met or fMet). Small is a misstatement, because the small ribosomal subunit is a large, complex assembly of numerous smaller proteins—it s just smaller than the... 

Many polypeptides undergo covalent modification after (or sometimes during) their ribosomal assembly. The most commonly observed such PTMs are listed in Table 2.7. Such modifications generally influence either the biological activity or the structural stability of the polypeptide. The majority of therapeutic proteins bear some form of PTM. Although glycosylation represents the most common such modification, additional PTMs important in a biopharmaceutical context include carboxylation, hydroxylation, sulfation and amidation these PTMs are now considered further. 

The Xenopus transcription factor IIIA not only acts as an essential RNA polymerase transcription factor for the expression of the 5S rRNA gene, it also binds to the 5S rRNA to form a 7S ribonucleoprotein particle that stabilizes the RNA until it is required for ribosome assembly and facilitates nuclear export of the 5S rRNA. Indeed, it was originally shown to be the protein component associated with 5S rRNA in the 7S particle in Xenopus oocytes before it was recognized as a transcription factor. How, we may ask, can this protein not only recognize specific DNA sequences in the 5S rRNA gene upstream region, but also recognize different, but equally specific, sequences in 5S rRNA ... 

Eukaryotic ABC transport system Phosphotransferase system (PTS) Ion-coupled transport system Signal Transduction Two-component system Bacterial chemotaxis MAPK signaling pathway Second messenger signaling pathway Ligand-Receptor Interaction G-protein-coupled receptors Ion-channel-linked receptors Cytokine receptors Molecular Assembly Ribosome assembly Flagellar assembly Enzyme assembly... 

Euchromatin generally corresponds to looped 30-nm fibers. Heterochromatin is more highly condensed. Figure 1-1-14 shows an electron micrograph of an interphase nucleus containing euchromatin, heterochromatin, and a nucleolus. The nudeolus is a nuclear region spedalized for ribosome assembly (discussed in Chapter 3). 

Proteins are synthesized by the ribosomal assembly in living organisms, details of which have been the subject of excellent recent reviews. The ribosome is a marvelously complex system of three large RNA molecules and over 50 proteins. This assembly translates the information from mRNA into protein sequence with a fidelity of one mistake per each 10" elongation cycles. ... 

All ribosomes have two subunits, and each subunit contains several protein chains and one or more chains of RNA (ribosomal RNA, or rRNA). In the ribosome from E. coli, the smaller of the two subunits is known as the 30S subunit and the larger is referred to as the 50S subunit. (The unit S stands for Svedberg, a measure of how rapidly a particle sediments in a centrifuge.) The two subunits combine to form the active 70S ribosomal assembly. The special RNA molecules that are a part of the ribosome are quite distinct from messenger or transfer RNA molecules, and they play important roles in forming the overall ribosomal quaternary structure and in aligning mRNA and tRNA molecules during protein biosynthesis. 

The electron micrograph in Fig. 28-5 shows RNA polymerase complexes apparently moving along a DNA strand with ribosomes assembled on the RNA and... 

Woodson, S. A. (2008). RNA folding and ribosome assembly. Curr. Opitt. Chem. Biol. 12, 667-673. 

Genetic information is accessed by a process known as transcription, in which the double-stranded DNA splits and the genetic code is transcribed onto a single-strand messenger RNA(mRNA). The mRNA is comprised of the same bases as the DNA, arranged in the same sequence, but in a complementary fashion. The mRNA migrates out of the nucleus and into the cytoplasm, where it attaches to ribosomes. The ribosomes assemble amino acids to form protein molecules through a process known as translation. 

Cui Q, Case DA (2005) Low-resolution modeling of the ribosome assembly of the 30s subunit by molecular dynamics simulations, Abstracts of Papers of the American Chemical Society, 229 U779... 

Pestova, T.Y, I.B. Lomakin, and C.U. Hellen. 2004. Position of the CrPV IRES on the 40S subunit and factor dependence of IRES/80S ribosome assembly. EMBO Rep 5 906-913. 

Recently, Sulfolobus 238-I-58 RNA and total large subunit proteins (TP 50) have been found to recombine into a low-sedimenting (428), protein-deficient particle at low temperature (0-20°C) and in the absence of spermine [103]. The 428 particle, which contains about one half of the subunit proteins, appears to represent a genuine reconstitution intermediate as it is converted into an active 508 subunit following further incubation at 75-80 C in the presence of spermine [103]. Therefore, the correct initiation of Sulfolobus ribosome assembly is independent of both heat and spermine. The identification and the isolation of the low-temperature precursor particles has provided insight into the order in which the various ribosomal proteins are sequentially assembled in the nascent particle. 

Ribosomal assembly and intra nuclear transport Rsa4p... 

Speculations as to the cause of the reduction in j8 chain synthesis have been many. It seems that this reduction is caused either by a defective mechanism of ribosomal assembly of (structurally normal) f3 chains due to an abnormal mRNA or by a decreased amount of normal mRNA. 

Evidence has been presented for the presence of N -methylglutamine in ribo-somal protein L3 of Escherichia coli. Mutants of this organism,/irm-2, are defective in the methyltransferase activity that catalyzes this reaction. These mutants have impaired ribosomal assembly at low temperatures, suggesting a role for this modification in allowing bacteria to grow at suboptimal temperatures (Lhoest and Colson, 1981). 

Lhoest, J. Colson, C. (1981). Cold-sensitive ribosome assembly in an . colt mutant lacking a single methyl group in ribosomal protein L3. Eur. J. Biochem. 121, 33-37. 

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