MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 12. VIRUSES

Viroids and Other Infectious Agents

Several molecular pathogens are known that resemble Viruses and apparently originated from the genomes of Bacteria and eukaryotes. Of particular importance among them are Viroids, which, despite their name, differ sharply from viruses.

Viroids are the smallest known disease agents; they are much smaller than the smallest viral genomes and lack a protein coat. Only plant viroids are known; they consist of a single-stranded RNA molecule that replicates autonomously in infected Cells. Viroids have been identified as the causative agents of dangerous diseases. One of them caused the death of millions of coconut palms in the Philippines over the past fifty years, while another damaged the commercial cultivation of chrysanthemums in the USA in the early 1950s.

The first viroid — potato spindle tuber viroid, or PSTV — was identified by Theodor Diener of the US Department of Agriculture in 1971 (Fig. 12-10). Potato tubers infected with PSTV have an elongated and distorted shape; deep cracks sometimes appear on them. PSTV is the largest known viroid; its RNA consists of 359 bases and forms either a closed ring or a hairpin-like Structure. In both cases, complementary Base Pairs are held together by Hydrogen Bonds, forming a double-stranded RNA similar to DNA. Under an Electron microscope, both forms of PSTV appear rod-shaped, with a length of 50 nm. Although it is the largest viroid, its size is only one-tenth that of The Genome of the smallest virus. Viroids are found exclusively in the nuclei of infected cells; they replicate similarly to viruses—that is, by synthesizing a complementary strand that Functions as a template. In doing so, viroids utilize the host Cell's enzyme systems.

Class="center">Fig. 12-10. Electron micrograph of the viroid (indicated by arrows) that causes potato spindle tuber disease, along with a single-stranded DNA of a bacteriophage. The difference in size between the viroid and the viral DNA is clearly visible.

Because viroids are localized in The Nucleus and presumably cannot function as mRNA, it is suggested that they cause disease by interfering with The regulation of host Gene Expression. Certain Proteins are present in infected plant cells in much greater quantities than in healthy ones. Although nucleotide sequences complementary to PSTV have not been detected in healthy plants, it is hypothesized that PSTV may have originated from alterations in the genome of certain potato species—its primary host.

Other molecular pathogens not classified as viroids are also found in living organisms. The existence of viroid-like structures composed of DNA has also been postulated in animals; these are called "subviral particles." Remarkably, certain protein fragments are capable of directing their own Replication in animal cells without the involvement of Nucleic Acids; such particles are called "Prions."

Viruses and Cancer

Viruses cause Cancer in many animal groups; Retroviruses, belonging to the group of enveloped positive-sense single-stranded RNA viruses, have recently become the subject of intense study as potential etiological agents of cancer. These viruses possess an enzyme called Reverse Transcriptase, which synthesizes complementary DNA from viral RNA. The double-stranded DNA fragment is then integrated into the host cell chromosome, causing malignant tumors or leukemias in mammals, birds, and reptiles.

While studying Rous Sarcoma virus (a retrovirus that causes tumors in chickens), scientists discovered oncogenes, or "cancer-causing" genes, which induce cancer by interfering with normal cellular regulation. Oncogenes are also present in healthy Human and Animal cells, but when altered by mutagenic agents (such as those found in cigarette smoke or other carcinogens), they lead to The formation of malignant tumors. Accumulating evidence suggests that The Development of the disease requires not just one, but multiple mutant oncogenes.

In addition to retroviruses, There is a group of DNA-containing Herpesviruses (Epstein-Barr viruses) that cause Two Types of human cancer. These same viruses are responsible for infectious mononucleosis. They can persist in cells without multiplying or producing any disease symptoms.

The Origin of Viruses

Due to their relative structural simplicity, one might mistakenly assume that viruses evolved from the earliest self-replicating structures that gave rise to the first cells. It is clear this is not the case, since viruses exist solely through their unique ability to reproduce inside a host cell. They compete with the nucleic acids of infected cells, redirecting their genetic and metabolic activity toward The production of new Viral Particles. Viruses must have arisen only after cells had evolved, once METABOLISM/28.html">The Genetic Code had already formed and stabilized.

To a large extent, viruses are akin to bacterial Chromosomes or Plasmids (in the case of double-stranded DNA viruses) or mRNA molecules (in single- or double-stranded RNA viruses) enclosed within a protein coat. The phenomenon of transformation, discussed earlier, suggests a plausible hypothesis for THE ORIGIN OF viruses. Fragments of DNA or RNAS accidentally penetrate cells by one means or another, where they can influence genetic processes. By replicating and passing from one cell to another, these fragments ensure their own survival. Enveloping themselves in a protective protein coat, they become viruses. It is believed that the largest and most complex Poxviruses originated from bacteria.

It is possible that viruses emerged through independent pathways about 3 billion years ago. Furthermore, recent studies indicate that existing viruses can exhibit surprisingly rapid responses to strong Selection pressures. It is quite likely that new types of viruses continue to evolve today from the genomes of bacteria and eukaryotes.