BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 30. VIRUSES

Conclusion

Viruses are tightly packaged infectious Nucleic Acids surrounded by a protective coat. They contain either DNA or RNA, which can be single-stranded or double-stranded. The simplest viruses have as few as 3 genes, whereas the most complex ones contain about 250 genes. In most cases, viral coats consist of

A large number of protein subunits of one or a few types, since viruses contain a strictly limited amount of Genetic information. The two main assembly patterns are a cylindrical coat with helical Symmetry and a spherical coat with icosahedral symmetry. The tobacco mosaic virus particle (virion) is composed of 2,130 identical subunits arranged in a helix around a single-stranded RNA molecule; it forms via self-assembly, which proceeds through the attachment of protein discs to an RNA loop. The tomato bushy stunt virus forms an icosahedral coat because the domains comprising its coat protein are flexibly linked to one another. The assembly of phage T4 is a much more complex process involving approximately 50 Proteins. Three main assembly pathways lead to the independent Formation of the HEAD, the tail, and the tail fibers. The morphogenesis of phage T4 proceeds in a strictly defined order, involving both self-assembly and enzyme-mediated assembly processes. In addition, accessory (morphopoietic) proteins play a crucial role in the assembly of phage T4.

Uninfected Cells are incapable of synthesizing RNA using an RNA template. Consequently, a general property of Introduction/7.html">RNA-containing Viruses (except for RNA-containing tumor viruses) is the encoding of an RNA-dependent RNA polymerase. The RNA contained within the virions of certain viruses (such as phage Qβ) Functions directly as Messenger RNA upon infection. In other viruses (such as reoviruses), the genomic RNA must be transcribed immediately after infection. This METABOLISM/31.html">Transcription is catalyzed by an enzyme carried within the virion. RNA viruses employ various Mechanisms of Protein synthesis. Poliovirus proteins are produced via the Cleavage of a giant precursor polyprotein. Reoviruses contain a separate "chromosome" for each protein they synthesize, whereas vesicular stomatitis virus transcribes several monocistronic RNAs from a single genomic RNA.

Not all viruses destroy their host cells. The temperate phage can adopt the form of a dormant prophage when its DNA is integrated into the bacterial chromosome via reciprocal recombination. This prophage retains the capacity for Replication and lysis, which is manifested when it is excised from the host Cell DNA. Similarly, the DNA-containing tumor viruses SV-40 and polyomavirus either replicate and lyse their host cells or have their DNA integrated into the host cell genome. Retroviruses can only replicate by integrating with the cellular DNA. These viruses contain Reverse Transcriptase, an enzyme that catalyzes the synthesis of double-stranded DNA using genomic RNA as a template. Productive infection by retroviruses, unlike that by oncogenic DNA viruses, is non-lytic. Cells whose genomes harbor the DNA of SV-40, polyomavirus, or a retrovirus exhibit abnormal Structure/108.html">Surface Properties and growth control defects. Upon introduction into susceptible animals, they form tumors. These viruses serve as convenient models for studying the Molecular Basis of Cancer because they possess only about five genes. It has been demonstrated that cellular transformation is caused by a kinase encoded by the $src$ Gene of avian Sarcoma virus.

The resistance of animal cells to many viruses is significantly enhanced by interferon, a protein synthesized and secreted by virus-infected cells. Interferon upregulates three Enzymes that block Protein Synthesis by inhibiting an initiation factor and stimulating mRNA degradation.

Biological research represents, among other things, an exercise in the aesthetics of nature. In pursuing biology, we derive profound pleasure precisely from recognizing, over and over again, how economical, elegant, and purposeful are the mechanisms that arose by chance in the course of evolution and were preserved by Selection. A virologist may well feel like one of the happiest biologists, for they are able to study their chosen subject down to the detailed structure of its individual molecules. The virologist observes how a virus—this ultimate parasite—harnesses the most fundamental principles of cellular biology to sustain its existence...

David Baltimore, Nobel Lecture, 1976