Other Information-Processing RNAs

Other stable RNAs in the cell are involved in protein and RNA processing. Processing means changes in a protein after the synthesis of the peptide bond, or in an RNA after the synthesis of phosphodiester bonds during transcription. Some of these RNAs are catalytic. 

Transcription is the synthesis of RNA from a DNA template. The primary transcript may then be modified or processed to the final product. Eventually, the RNA product is degraded to nucleotides. While all of these reactions are potential sites for control of gene expression, most control of gene expression is transcriptional. This is an example of a general principle of biochemical control Pathways are controlled at the first committed step, and RNA synthesis is the first committed step of gene expression. 

Different RNAs are transcribed with different efficiencies, so that structural (transfer and ribosomal) RNAs are transcribed very efficiently. Each transcription cycle leads to a single molecule. In contrast, 

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This process of expression is so well established that it is called the central dogma of molecular biology. The accepted dogma, or principle, says that genetic information contained in DNA molecules is transferred to RNA molecules. The transferred information of RNA molecules is then expressed in the structure of synthesized proteins. In other... 

Enzymes in viruses We have stated that virus particles do not carry out metabolic processes. Outside of a host cell, a virus particle is metabolically inert. However, some viruses do contain enzymes which play roles in the infectious process. For instance, many viruses contain their own nucleic acid polymerases which transcribe the viral nucleic acid into messenger RNA once the infection process has begun. The retroviruses are RNA viruses which replicate inside the cell as DNA intermediates. These viruses possess an enzyme, an RNA-dependent DNA popo called reverse transcriptase, which transcribes the information in the incoming RNA into a DNA intermediate. It should be noted that reverse transcriptase is unique to the retroviruses and is not found in any other viruses or in cells. 

Information metabolism provides a way to store and retrieve the information that guides the development of cellular structure, communication, and regulation. Like other metabolic pathways, this process is highly regulated. Information is stored by the process of DNA replication and meiosis, in which we form our germ-line cells. These processes are limited to specific portions of the cell cycle. Information is retrieved by the transcription of DNA into RNA and the ultimate translation of the signals in the mRNA into protein. 

The translation of the mRNA into proteins is the final step in the biological flow of information (see Fig. 6.1). Similar to other macromolecular polymerizations, protein synthesis can be divided into initiation, chain elongation, and termination. Critical players in this process are the aminoacyl transfer RNAs (tRNAs). These molecules form the interface between the mRNA and the growing polypeptide. Activation of tRNA involves the addition of an amino acid to its acceptor stem, a reaction catalyzed by an aminoacyl-tRNA synthetase. Each aminoacyl-tRNA synthetase is highly specific for one amino acid and its corresponding tRNA molecule. The anticodon loop of each aminoacyl-tRNA interacts... 

There are other ways in which nucleic-acid-related compounds could be exploited as therapeutics. A new, emerging area concerns the application of RNA as a dmg. The discovery of catalytic RNA (ribozymes) by Cech and Altman was a fundamental advance in nucleic acid chemistry. According to traditional double helix dogma, RNA was a passive information-transmitting molecule. The identification of ribozymes enabled the conceptual advance that RNA can also act as a catalyst for the following biochemical processes ... 

The dynamic interaction between nucleic acids and proteins is another central theme of Part III. With the important exception of a few catalytic RNA molecules (discussed in Chapters 26 and 27), the processes that make up the pathways of cellular information flow are catalyzed and regulated by proteins. An understanding of these enzymes and other proteins can have practical as well as intellectual rewards, because they form the basis of recombinant DNA technology (introduced in Chapter 9). 

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