Nucleic acid , virus, synthesis

Viral replication consists of several steps (Figure 49-1) (1) attachment of the vims to receptors on the host cell surface (2) entry of the virus through the host cell membrane (3) uncoating of viral nucleic acid (4) synthesis of early regulatory proteins, eg, nucleic acid polymerases (5) synthesis of new viral RNA or DNA (6) synthesis of late, structural proteins (7) assembly (maturation) of viral particles and (8) release from the cell. Antiviral agents can potentially target any of these steps. 

FIGURE 1.25 The virus life cycle. Viruses are mobile bits of genetic iuformatiou encapsulated in a protein coat. The genetic material can be either DNA or RNA. Once this genetic material gains entry to its host cell, it takes over the host machinery for macromolecular synthesis and subverts it to the synthesis of viral-specific nucleic acids and proteins. These virus components are then assembled into mature virus particles that are released from the cell. Often, this parasitic cycle of virus infection leads to cell death and disease. 

NUCLEOPROTEINS. Nucleoprotein conjugates have many roles in the storage and transmission of genetic information. Ribosomes are the sites of protein synthesis. Virus particles and even chromosomes are protein-nucleic acid complexes. 

Early steps in replication of the virus nucleic acid, in which the host cell biosynthetic machinery is altered as a prelude to virus nucleic acid synthesis. Virus-specific enzymes may be made ... 

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. 

A virus-specific RNA RNA polymerase is needed, since the cell RNA polymerase will generally not copy double-stranded RNA (and ribosomes are not able to translate double-stranded RNA either). A wide variety of modes of viral mRNA synthesis are outlined in Figure. By convention, the chemical sense of the mRNA is considered to be of the plus (+) configuration. The sense of the viral genome nucleic acid is then indicated by a plus if it is the same as the mRNA and a minus if it is of oppposite sense. If the virus has double-stranded DNA (ds DNA), then mRNA synthesis can proceed directly as in uninfected cells. However, if the virus has a singlestranded DNA (ss DNA), then it is first converted to ds DNA and the latter serves as the template for mRNA synthesis with the cell RNA polymerase. 

Virus infection obviously upsets the regulatory mechanisms of the host, since there is a marked overproduction of nucleic acid and protein in the infected cell. In some cases, virus infection causes a complete shutdown of host macromolecular synthesis while in other cases host synthesis proceeds concurrently with virus synthesis. In either case, the regulation of virus synthesis is under the control of the virus rather than the host. There are several elements of this control which are similar to the host regulatory mechanisms, but there are also some uniquely viral regulatory mechanisms. We discuss various regulatory mechanisms when we consider the individual viruses later in this chapter. 

It should already be clear from what has been stated that a great diversity of viruses exist. It should therefore not be surprising that there is also a great diversity in the manner by which virus multiplication occurs. Interestingly, many viruses have special features of their nucleic acid and protein synthesis processes that are not found in cells. In the present chapter, we are only able to present some of the major types of virus replication patterns, and must skip some of the interesting exceptional cases. 

The discovery of the base-paired, double-helical structure of deoxyribonucleic acid (DNA) provides the theoretic framework for determining how the information coded into DNA sequences is replicated and how these sequences direct the synthesis of ribonucleic acid (RNA) and proteins. Already clinical medicine has taken advantage of many of these discoveries, and the future promises much more. For example, the biochemistry of the nucleic acids is central to an understanding of virus-induced diseases, the immune re-sponse, the mechanism of action of drugs and antibiotics, and the spectrum of inherited diseases. 

Subsequently, similar experiments were done with viral nucleic acids. The pure viral nucleic acid, when added to cells, led to the synthesis of complete virus particles the protein coat was not required. This process is called transfection. More recently, DNA has been used in cell-free extracts to program the synthesis of RNA that functions as the template for the synthesis of proteins characteristic of the DNA... 

The Li+-induced inhibition of the production of the HSV virus may be related to its actions upon viral DNA polymerase production and activity. Li+ reduces both the synthesis of DNA polymerase in tissue culture and the activity of DNA polymerase in vitro, each by about 50%. It has been proposed that Li+ reduces the biosynthesis of viral polypeptides and nucleic acids, and hence inhibits viral DNA replication by competition with Mg2+, a cofactor of many enzymes [243]. However, the inhibitory effect of Li+ on HSV replication in tissue culture is not affected by Mg2+ levels. A more likely hypothesis is the alteration of the intracellular K+ levels, possibly modifying levels of the high-energy phosphate compounds by replacement of either Na+ or K+ in Na+/K+-ATPase [244]. In tissue culture, HSV replication has been shown to be affected by the... 

Bose S, Basu M, Banerjee AK (2004) Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells. J Virol 78 8146-8158 Bouche G, Caizergues-Ferrer M, Bugler B, Amalric F (1984) Interrelations between the maturation of a 100 kDa nucleolar protein and pre rRNA synthesis in CHO cells. Nucleic Acids Res 12 3025-3035 Bouche G, Gas N, Prats H, Baldin V, Tauber JP, Teissie J, Amalric F (1987) Basic fibroblast growth factor enters the nucleolus and stimulates the transcription of ribosomal genes in ABAE cells undergoing GO-Gl transition. Proc Natl Acad Sci U S A 84(19) 6770-6774. 

The use of antibiotics for the control of plant virus diseases( ) is of interest. Several antibiotics have been tested for inhibition of replication of viral nucleic acid and/or protein synthesis within the host cell. Chloramphenicol, cycloheximide, actinomycin D and others are the most used antibiotics and the disease caused by tobacco mosaic... 

Viruses are obligate intracellular parasites that use many of the host cell s biochemical mechanisms and products to sustain their viability. A mature virus (virion) can exist outside a host cell and still retain its infective properties. However, to reproduce, the virus must enter the host cell, take over the host cell s mechanisms for nucleic acid and protein synthesis, and direct the host cell to make new viral particles. 

Viral DNA polymerase is an important catalyst for the synthesis of viral nucleic acids. DNA polymerase inhibitors have already been encountered as antitumor agents. Ara-A (9.5, vidarabine) is a DNA polymerase inhibitor that has demonstrated activity against herpes simplex virus type I (HSV-1) infections, responsible for cold sores on... 

In our discussion of DNA and RNA synthesis up to this point, the role of the template strand has been reserved for DNA. However, some enzymes use an RNA template for nucleic acid synthesis. With the very important exception of viruses with an RNA genome, these enzymes play only a modest role in information pathways. RNA viruses are the source of most RNA-dependent polymerases characterized so far. 

RNA replicase isolated from Qj8-infected E. coli cells catalyzes the formation of an RNA complementary to the viral RNA, in a reaction equivalent to that catalyzed by DNA-dependent RNA polymerases. New RNA strand synthesis proceeds in the 5 —>3 direction by a chemical mechanism identical to that used in all other nucleic acid synthetic reactions that require a template. RNA replicase requires RNA as its template and will not function with DNA. It lacks a separate proofreading endonuclease activity and has an error rate similar to that of RNA polymerase. Unlike the DNA and RNA polymerases, RNA replicases are specific for the RNA of their own virus the RNAs of the host cell are generally not replicated. This explains how RNA viruses are preferentially replicated in the host cell, which contains many other types of RNA. 

A search for antimetabolites, i.e. analogues of essential metabolites that might displace the latter in vital processes, was proposed as a rational approach to the discovery of antibacterial agents, but it has had little success other than the achievements in the folic acid field (Section 1.06.6). Substances that resemble the components of nucleic acids have, however, had considerable success in the chemotherapy of cancer and of some virus diseases and in the suppression of the immune response. They may act by becoming incorporated in false nucleic acids or by blocking the synthesis of nucleic acids, nucleotides, nucleosides or of the pyrimidine and purine bases cytosine (88), thymine (89 R = Me), adenine (90) and guanine (91 X = CH). The simplest antimetabolites are analogues of these bases. 

The majority of antiviral drugs which are under clinical development today generally interrupt viral nucleic acid synthesis. These compounds often do not affect host cell metabolism and possess considerable selectivity against virus-induced enzymes. This article discusses agents exhibiting significant antiviral activity against viral infections in animal model systems. 

From the complementary duplex structure of DNA described in chapter 25, it is a short intuitive hop to a model for replication that satisfies the requirement for one round of DNA duplication for every cell division. In chapter 26, DNA Replication, Repair, and Recombination, key experiments demonstrating the semiconservative mode of replication in vivo are presented. This is followed by a detailed examination of the enzymology of replication, first for how it occurs in bacteria and then for how it occurs in animal cells. Also included in this chapter are select aspects of the metabolism of DNA repair and recombination. The novel process of DNA synthesis using RNA-directed DNA polymerases is also considered. First discovered as part of the mechanisms for the replication of nucleic acids in certain RNA viruses, this mode of DNA synthesis is now recognized as occurring in the cell for certain movable genetic segments and as the means whereby the ends of linear chromosomes in eukaryotes are synthesized. 

At the present time, there is no accepted chelating agent which can be used against common influenza viruses in humans. A virus has a core of either DNA or RNA and a protective coat of many identical protein units. All viruses are either rods or spheres, that is the protein coats are cylindrical shells having helical symmetry or spherical shells having icosahedral symmetry. Viruses reproduce inside living cells, where each viral nucleic acid directs the synthesis of about 1000 fresh viruses. These are then released and the host cell may die. 

Talib, S., and J.E. Hearst. 1983. Initiation of RNA synthesis in vitro by vesicular stomatitis virus Single internal initiation in the presence of aurintricarboxylic acid and vanadyl ribonucleoside complexes. Nucleic Acids Res 11 7031. 

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