MRNA-linked protein synthesis

Messenger RNA is only one of several classes of cellular RNA. Transfer RNAs serve as adapter molecules in protein synthesis covalently linked to an amino acid at one end, they pair with the mRNA in such a way that amino acids are joined to a growing polypeptide in the correct sequence. Ribosomal RNAs are components of ribosomes. There is also a wide variety of special-function RNAs, including some (called ribozymes) that have enzymatic activity. All the RNAs are considered in detail in Chapter 26. The diverse and often complex functions of these RNAs reflect a diversity of structure much richer than that observed in DNA molecules. 

The presence of RNA in the cytoplasm had been linked to protein synthesis by experiments done in the early 1940s. After the discovery of the double helix, the concept followed quickly that DNA was the master "blueprint" from which secondary blueprints or transcripts of RNA could be copied. The RNA copies, later identified as messenger RNA (mRNA), provided the genetic information for specifying protein sequence. The flow of information from DNA to RNA to proteins could be symbolized as in Eq. 26-1. 

Immediately after transcription, the 5 phosphate is removed, guanosyl transferase adds a G residue linked via a 5 -5 covalent bond, and this is methylated to form a 7-methylguanosine (m7G) cap (methylated in N-7 position of the base). The ribose residues of either the adjacent one or two nucleotides may also be methylated by methyl group addition to the 2 OH of the sugar. The cap protects the 5 end of the mRNA against ribonuclease degradation and also functions in the initiation of protein synthesis. 

Protein synthesis is called translation because information present as a nucleic acid sequence is translated into a different language, the sequence of amino acids in a protein. This complex process is mediated by the coordinated interplay of more than a hundred macromolecules, including mRNA, rRNAs, tRNAs, aminoacyl-tRNA synthetases, and protein factors. Given that proteins typically comprise from 100 to 1000 amino acids, the frequency at vchich an incorrect amino acid is incorporated in the course of protein synthesis must be less than 10 4. Transfer RNAs are the adaptors that make the link betvceen a nucleic acid and an amino acid. These molecules, single chains of about 80 nucleotides, have an L-shaped structure. 

Plasma leptin levels and leptin mRNA and protein levels in adipose tissues also increase. Most importantly, data indicate that leptin synthesis is also stimulated by either hyperglycemia or hyperlipidemia, which also increases tissue levels of UDP-A-acetyl-D-glucosamine in conscious rodents. These findings unveil an important biochemical link between the increased availability of certain nutrients and leptin expression [161]. 

Transfer RNA molecules (iRNAs), messenger RNA (mRNA) and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyhtRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes translate the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is well understood. Some distinctive features of protein synthesis in eukaryotes also are presented. 

As discussed in detail later, tRNA molecules adopt a well-defined three-dimensional architecture in solution that Is crucial in protein synthesis. Larger rRNA molecules also have locally well-defined three-dimensional structures, with more flexible links In between. Secondary and tertiary structures also have been recognized in mRNA, particularly near the ends of molecules. Clearly, then, RNA molecules are like proteins in that they have structured domains connected by less structured, flexible stretches. 

There are two steps in protein synthesis where polarity of information is important. The first is the relationship between the 50 to 30 directionality of mRNA, and the NH3+ to COO- terminal direction of protein synthesis. The utilization of tRNA as the adaptor is the second step where polarity of information is crucial. The tRNA has a bipolar function, it needs to correctly link each amino acid to the corresponding position encoded by the mRNA. Figure 26.1 shows an overview of how mRNA synthesis and protein translation share the same polarity. Moreover, similar to transcription, translation can also be broken down into three discrete components initiation, elongation, and termination. 

Soon after the structure of DNA was discovered, Francis Crick hypothesized that an adaptor molecule, such as the one shown in Figure 26.2, would be required for protein synthesis. It was discovered that tRNA serves as the adaptor molecule by linking the information stored in mRNA to the primary sequence of the polypeptide. The adaptor function of tRNA is mediated by base pairing between mRNA sequences called codons, and complementary sequences on tRNA called anticodons. For every codon on the mRNA, a single amino acid is delivered by the tRNA to the growing polypeptide chain. The term genetic code refers to the specific sequences in the mRNA codon that determine which tRNA molecule is going to have the complementary anticodon and, therefore, what amino acid is required at that position in the final polypeptide chain. 

In this section, we describe the three basic stages of protein synthesis initiation, elongation, and termination. These three processes are fairly similar between prokaryotes and eukaryotes, with the two exceptions being that more protein factors have been identified as necessary for eukaryotic protein synthesis, and that transcription and translation are physically linked in prokaryotes but not in eukaryotes. Note that the reactions will be schematized as a single ribosome transversing the mRNA, but as shown in Figure 26.3, translation actually occurs on polyribosomes. 

Another example of translational control in eukaryotes is the inhibition of yeast GCN4 protein synthesis by stem-loop structures present in the 50 end of the mRNA. GCN4 control, and an analogous situation in bacteria, links amino-acid biosynthesis to ribosome pausing in the 50 end of the mRNA. This mechanism was first described for the tryptophan operon in E. coli and it is often referred to as attenuation. Transcriptional and translational control of the tryptophan biosynthetic enzymes are described in Chapter 28. 

The process of protein synthesis is called translation. The genetic code words on the mRNA are decoded by tRNA. Each tRNA has an anticodon that is complementary to a codon on the mRNA. In addition the tRNA is covalently linked to its correct amino acid. Thus hydrogen bonding between codon and anticodon brings the correct amino acid to the site of protein s)mthesis. Translation also occurs in three stages called initiation, chain elongation, and termination. 

Prokaryotic initiation factors. In addition to the ribosomal proteins, the initiation factors IFl, IF2, and IF3, whose molecular masses are 9.5, 9.7, and 19.7 kDa, respectively, " are essential. They coordinate a sequence of reactions that begins with the dissociation of 70S ribosomes into their 30S and 50S subunits. Then, as is shown in Fig. 29-10, the mRNA, the initiator tRNA charged with formylmethionine, the three initiation factors, and the ribosomal subunits react to form 70S programmed ribosomes, which carry the bound mRNA and are ready to initiate protein synthesis. IF2 is a specialized G protein (Chapter 11), which binds and hydrolyzes GTP. It resembles the better known elongation factor EF-Tu (Section 2). The 172-residue IF3 consists of two compact a/p domains linked by a flexible sequence, which may exist as an a Its C-terminal domain binds to the central domain of the 16S RNA near nucleotides 819-859 (Fig. 

Fig. 12.19. The regions of eukaryotic mRNA. The wavy line indicates the polynucleotide chain of the mRNA and the As constituting the poly(A) tail. The 5 -cap consists of a guanosine residue linked at its 5 hydroxyl group to three phosphates, which are linked to the 5 -hydroxyl group of the next nucleotide in the RNA chain. The start and stop codons represent where protein synthesis is initiated and terminated from this mRNA.

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