RNA and Protein Biosynthesis Translation

The beginning of secondary product formation is often directly coupled with the synthesis of the corresponding enzymes. One example is the biosynthesis of flavonoids in cell cultures of Petroselinum hortense (Fig. 7). Here the regulatory mechanisms were extensively investigated with phenylalanine ammonia-lyase (PAL) (D 22.2.1) and chalcone synthase (D 22.3.3), the key enzymes of the biosynthetic chain. Experiments with inhibitors of transcription and translation showed that the increase of enzyme activity depends on RNA and protein biosynthesis. Labeling experiments demonstrated that it is caused by an accelerated rate of enzyme synthesis. 

Though still scanty and incomplete, the data indicate that the control of synthesis of enzymes participating in secondary biosynthesis is accomplished at the transcription level. Their formation takes place after a reduction in the rate of RNA and protein syntheses. The period of intensive secondary biosynthesis is marked by the continuous replenishment of specific synthases its discontinuation brings about a drop in the production rate. The "late" expression of synthesis of secondary metabolites after the termination of intensive growth is thus probably not the result of a post-translation control (23, 24). So far, we cannot eliminate a number of other control mechanisms at the level of activity of the enzymes formed, yet the rise in synthase levels in the period of active secondary biosynthesis (4, 30, 38) attests to our hypothesis. 

This inhibition is not immediate but requires several hoiors of active cellular RNA and protein synthesis before it fully develops. In the past year, interferon was shown to induce, in the treated cells, several new enzymatic activities which regulate protein biosynthesis and produce the translational inhibition observed. These biochemical mechanisms induced by interferon are reviewed here, and tentatively shown as three "pathways in Figure 1. 

Proteins and peptides are polymers of o-A. a., which are also occur in the free form in all living cells and body fluids. TTie twenty A. a. encoded in messenger RNA (see protein biosynthesis. Genetic code) occur in proteins and are known as proteogenic or proteino-genic A.a. (Table ). The occurrence of nonproteo-genic A. a. in proteins is due to Post-translational modification of proteins (see). Further information on individual A. a. can be found in the appropriate entries. 

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. 

Whereas DNA is mostly located in the nucleus of cells in higher organisms (with some also in mitochondria and in plant chloroplasts), RNA comes in three major and distinct forms, each of which plays a crucial role in protein biosynthesis in the cytoplasm. These are, respectively, ribosomal RNA (rRNA), which represents two-thirds of the mass of the ribosome, messenger RNA (mRNA), which encodes the information for the sequence of proteins, and transfer RNAs (tRNAs) which serve as adaptor molecules, allowing the 4-letter code of nucleic acids to be translated into the 20-letter code of proteins. These latter molecules contain a substantial number of modified bases, which are introduced enzymatically. 

The ribosome is where the message carried by the mRNA is translated into the amino sequence of a protein. How it occurs is described in the next section. One of its most noteworthy aspects was discovered only recently. It was formerly believed that the RNA part of the ribosome was a structural component and the protein part was the catalyst for protein biosynthesis. Present thinking tilts toward reversing these two functions by ascribing the structural role to the protein and the catalytic one to rRNA. RNAs that catalyze biological processes are called ribozymes. Catalysis by RNA is an important element in origins of life theories as outlined in the accompanying boxed essay RNA World. ... 

Deoxyribonucleic acid is the genetic material such that the information to make all the functional macromolecules of the cell is preserved in DNA (Sinden, 1994). Ribonucleic acids occur in three functionally different classes messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) (Simons and Grun-berg-Manago, 1997). Messenger RNA serves to carry the information encoded from DNA to the sites of protein synthesis in the cell where this information is translated into a polypeptide sequence. Ribosomal RNA is the component of ribosome which serves as the site of protein synthesis. Transfer RNA (tRNA) serves as a carrier of amino acid residues for protein synthesis. Amino acids are attached as aminoacyl esters to the 3 -termini of the tRNA to form aminoacyl-tRNA, which is the substrate for protein biosynthesis. 

Finally, novel nucleic acid catalysts have also been selected from random sequence pools (reviewed in Ref. 19). Joyce and co-workers have manipulated the function of the Group I self-splicing ribozyme, selecting variants that can utilize calcium or cleave DNA from partially randomized pools [20,21], Lorsch and Szostak [22] selected a polynucleotide kinase ribozyme from a completely random sequence pool that flanked a previously selected ATP binding site. Many of the novel ribozymes can catalyze reactions that are relevant to protein biosynthesis, bolstering arguments that translation may have arisen in a putative RNA world. For example, Lohse and Szostak [23] have selected ribozymes that can carry out an acyl transfer reaction, while Illangasekare et al. [24] have isolated a... 

Fig. 20.2. Simplified scheme describing the central dogma in molecular biology. DNA is replicated and passed from one generation to the next. For protein biosynthesis, DNA sequence is first transcribed into complementary messenger RNA (mRNA) sequence which, by means of the adapter molecule transfer RNA (tRNA), is translated into protein sequence. The translation follows the genetic code where a nucleotide triplet (e.g., AGC) codes for an amino acid (e.g., serine) [522]...

Soprano DR, Smith JF, and Goodman DS (1982) Effect of retinol status on retinol-binding protein biosynthesis rate and translatable messenger RNA level in rat liver. Journal of... 

Modification at the 5 and 3 end of the pre-RNA, as well as splicing of the primary transcript are the major modifications that are necessary to form the mature mRNA ready for translation at the ribosome. The 3 end modifications and splicing decide which information contained in the primary transcript is made available for protein biosynthesis. The information content of the processed mRNA can be specifically influenced by these processes. This has an important impact on the tissue- and cell-specific protein expression. 

Protein biosynthesis is directed by DNA through the agency of several types of ribonucleic acid called messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). There are two main stages in protein biosynthesis transcription and translation. 

Section 26.11 Three RNAs are involved in gene expression. In the transcription phase, a strand of messenger RNA (mRNA) is synthesized from a DNA template. The four bases A, G, C, and U, taken three at a time, generate 64 possible combinations called codons. These 64 codons comprise the genetic code and code for the 20 amino acids found in proteins plus start and stop signals. The mRNA sequence is translated into a prescribed protein sequence at the ribosomes. There, small polynucleotides called transfer RNA (tRNA), each of which contains an anticodon complementary to an mRNA codon, carries the correct amino acid for incorporation into the growing protein. Ribosomal RNA (rRNA) is the main constituent of ribosomes and appears to catalyze protein biosynthesis. 

Figure 3.13. The hydrogen bonds exhibited by the A-T and G-C base pairs as they occur in the structure of DNA. This is the type of base pairing responsible for accurate replication of DNA, for transcription of DNA to form RNA, and for translation of RNA to form protein, as discussed in Chapter 4 in relation to the energetics of protein biosynthesis. (Reproduced with permission from S. Arnott, S.D. Dover, and A.J. Wonacott, Acta Cryst. B25, 2196, 1969. Copyright 1969 lUCr journals.)...

Biosynthesis of protein can be seen in three steps replication, the production of the DNA for a daughter cell transcription, the conversion of the DNA into the equivalent sequence of RNA and translation, the conversion of the ribonucleic acid sequence into the specified protein sequence, that is,... 

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