RNA transfer synthesis

Protein and Nucleic Acid Synthesis 159 5.2.4. Ribosomal and Transfer RNA Synthesis... 

Vinca alkaloids interfere with transfer - RNA synthesis and therefore also with protein synthesis... 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

Transfer RNA (tRNA) serves as a carrier of amino acid residues for protein synthesis. Transfer RNA molecules also fold into a characteristic secondary structure (marginal figure). The amino acid is attached as an aminoacyl ester to the 3 -terminus of the tRNA. Aminoacyl-tRNAs are the substrates for protein biosynthesis. The tRNAs are the smallest RNAs (size range—23 to 30 kD) and contain 73 to 94 residues, a substantial number of which are methylated or otherwise unusually modified. Transfer RNA derives its name from its role as the carrier of amino acids during the process of protein synthesis (see Chapters 32 and 33). Each of the 20 amino acids of proteins has at least one unique tRNA species dedicated to chauffeuring its delivery to ribosomes for insertion into growing polypeptide chains, and some amino acids are served by several tRNAs. For example, five different tRNAs act in the transfer of leucine into... 

H Translation is the process by which mRNA directs protein synthesis. Each mRNA is divided into codons, ribonucleotide triplets that are recognized by small amino acid-carrying molecules of transfer RNA (tRNA), which deliver the appropriate amino acids needed for protein synthesis. 

All eukaryotic cells have four major classes of RNA ri-bosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). The first three are involved in protein synthesis, and snRNA is involved in mRNA splicing. As shown in Table 37-1, these various classes of RNA are different in their diversity, stability, and abundance in cells. 

Several different types of RNA, including ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA), are involved in protein synthesis. 

Figure 8.4 outlines the proeess of protein synthesis involving the ribosome, ruRNA, a series of aminoacyl transfer RNA (tRNA) moleeules (at least one for eaeh amino aeid)... 

As we have noted, the outcome of a virus infection is the synthesis of viral nucleic acid and viral protein coats. In effect, the virus takes over the biosynthetic machinery of the host and uses it for its own synthesis. A few enzymes needed for virus replication may be present in the virus particle and may be introduced into the cell during the infection process, but the host supplies everything else energy-generating system, ribosomes, amino-acid activating enzymes, transfer RNA (with a few exceptions), and all soluble factors. The virus genome codes for all new proteins. Such proteins would include the coat protein subunits (of which there are generally more than one kind) plus any new virus-specific enzymes. 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

The physiologist de Duve concentrated his efforts on a material link between the prebiotic phase of the primeval Earth and the state of development at which RNA (or a similar type of molecule) determined the further progress of the evolution process. In particular, this connecting link needed to have been able to transfer chemical energy, since without such a procedure, the RNA synthesis appears impossible. The molecular species which Christian de Duve favours for this important function is that of the thioesters. The exact reasoning as to why this is the case is discussed in detail in his book Vital Dust Life As a Cosmic Imperative (de Duve, 1996). 

This dilemma could be overcome by the hypercycle model hypercycles are in fact not theoretical concepts, but can be observed (in a simple form) in today s organisms, where an RNA virus transfers the information for an enzyme in the host cell, which is able to carry out the preferred synthesis of new virus RNA. This RNA synthesis is supported by host factors, and an RNA minus-strand is formed. The following RNA replication affords a plus-strand. The process corresponds to a double feedback loop and involves the enzyme coded by the RNA matrix and the information present in the matrix in the form of a nucleotide sequence. Both factors contribute to the replication of the matrix, so that there is second-order autocatalysis (Eigen et al., 1982). 

A primer is a very special sequence, which plays an important role in duplication, (i) In RNA, it is a short sequence that is paired with one strand of DNA and provides a free 3 -OH terminus at which a DNA polymerase starts synthesis of a deoxyribonucleotide chain, (ii) In DNA, it is another short sequence, which is complementary to a sequence of messenger RNA and allows reverse transcriptase to start copying the adjacent sequences of mRNA. (iii) In retroviruses, it is a cellular transfer RNA whose elongation initiates RNA-directed DNA synthesis by the DNA polymerase. 

Ribonucleic acid (RNA) Molecules including messenger RNA, transfer RNA, ribosomal RNA, or small RNA. RNA serves as a template for protein synthesis and other biochemical processes of the cell. The structure of RNA is similar to that of DNA except for the base thymidine being replaced by uracil. 

Transfer RNA (tRNA) RNA with a triplet nucleotide sequence that is complementary to the triplet nucleotide coding sequences of mRNA. tRNAs in protein synthesis bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by mRNA... 

A systematic replacement of any amino acid in the sequence for photoreac-tive analogues allows a photoaffinity scanning of the binding interface. Since solid-phase synthesis is limited in the length of the peptide, Schultz et al. developed a sophisticated method which makes it possible to incorporate unnatural amino acids into large peptide sequences. The photoreactive amino acid was linked to transfer RNA, which inserted the amino acid into the required position by in vivo translation [44]. 

Ribosome A granule of protein and RNA, synthesized in the nudeolus and found in the cytoplasm of cells. Ribosomes are the main sites of protein synthesis. Messenger RNA attaches to them and there receives molecules of transfer RNA bearing amino adds. [NIH]... 

L. Feng D. Tumbula-Hansen B. Min S. Namgoong J. Salazar O. Orellana D. Soil, Transfer RNA-Dependent Amidotransferases Key Enzyme for Asn-tRNA and GIn-tRNA Synthesis in Nature. In The Aminoacyl-tRNA Synthetases M. Ibba,... 

Transfer RNA (tRNA), which is the second most abundant type of RNA. Its function is to carry amino acids to the ribosome, where they will be linked together during protein synthesis. 

Nucleotides are needed for DNA and RNA synthesis (DNA replication and transcription) and for energy transfer. Nucleoside triphosphates (ATP and GTP) provide energy for reactions that would otherwise be extremely unfevorable in the cell. 

Hatfield GW, Gurman GA (1992) Codon pair utilization bias in bacteria, yeast and mammals. In Hatfield DL, Lee BJ, Pirtle RM (eds) Transfer RNA in protein synthesis. CRC Press, Boca Raton, chap 7... 

To say that RNA molecules are single-stranded molecules is not the same as saying that they have no higher-order structures, hi fact they have several. The formation of Watson-Crick complementary base pairs is a driving force for formation of higher-order structures. These include the stem-loop and hairpin secondary structures, as well as more complex tertiary structures. Of particular note, are the complex structures for transfer RNAs, tRNAs. Examples are provided in figure 12.5 (note that there are several nnnsnal bases in these structnres this is typical of tRNAs but not of RNA molecnles in general). These strnctures are intimately related to the function of these molecnles as adaptors in the process of protein synthesis, as developed in the next chapter. 

In protein synthesis, there are three classes of RNA to worry about ribosomal RNA, rRNA messenger RNA, mRNA and transfer RNA, tRNA. All three classes of RNA play key roles in the final stage of the process the biosynthesis of proteins. However, we are going to take this one step at a time. We turn attention first to the... 

RNA polymerase III catalyses the synthesis of small RNA molecules, such as the transfer RNAs (see below). 

Protein synthesis means translation into a peptide chain of a genetic message first copied (transcribed) into m-RNA (p. 274). Amino acid (AA) assembly occurs at the ribosome. Delivery of amino acids to m-RNA involves different transfer RNA molecules (t-RNA), each of which binds a specific AA. Each t-RNA bears an anticodon nucleobase triplet that is complementary to a particular m-RNA coding unit (codon, consisting of 3 nucleobases. 

---