Viruses, Viroids, Prions and Lichens

Why viruses are difficult to place among organisms

The five-kingdom system classifies cellular organisms. Viruses do not fit comfortably because they have no cellular structure, no ribosomes and no independent system for making proteins. Outside a suitable cell, an individual virus particle does not carry out ordinary cellular metabolism. Once it enters a compatible host, its genetic information directs the host's molecular machinery toward making new viral components. The particle is not a miniature bacterium: it is an acellular infectious entity.

Whether viruses should be called living depends partly on which features of life one emphasises. Viruses have genetic information and evolve through mutation and natural selection, yet cannot reproduce independently of cells. Their dependence on hosts is why they are often described as occupying the boundary of living and non-living systems. A virus does not always immediately kill its host cell; infections vary in duration and effects.

What is inside a virus?

A virus genome consists of DNA or RNA. The genome may be single-stranded or double-stranded depending on the virus group. The genetic material is surrounded by a protein coat called the capsid, assembled from protein subunits called capsomeres. Many viruses additionally possess a lipid-containing envelope acquired from a host membrane; not every virus has one.

Capsids can be helical, icosahedral or more elaborate. Their organisation protects genetic material and can assist its delivery into a host. A tobacco mosaic virus particle is a rigid rod with a helical protein arrangement surrounding an RNA genome. Many bacteriophages that infect bacteria have a complex structure with a head housing genetic material and tail components involved in attachment and delivery.

In the diagram, the long rod at left indicates a helical capsid protecting RNA. The bacteriophage at right shows a head housing its genome, with connecting structures and fibres that help the virus attach to its bacterial host. Not all phages have this exact shape. The drawing is simplified and does not represent relative sizes.

Some historical discoveries

Dmitri Ivanovsky showed in 1892 that an infectious agent associated with tobacco mosaic disease could pass through filters retaining bacteria. In 1898 Martinus Beijerinck demonstrated that filtered material could still cause infection and popularised the term virus. In 1935 Wendell Stanley obtained crystalline preparations of tobacco mosaic virus. These findings helped establish viruses as distinct from familiar cellular microbes.

Viral genomes and the diseases they can cause

Different viral groups carry different genomes. Tobacco mosaic virus has an RNA genome. Many bacteriophages have double-stranded DNA genomes; others use RNA or different DNA arrangements. Animal viruses may use DNA or RNA and can be single- or double-stranded. The complete viral replication cycle may contain both RNA and DNA intermediates in particular groups, but each virus has a defined genome type.

Influenza, mumps, herpes infections, smallpox and HIV-related disease are caused by viruses. Viral infection of plants can produce leaf mottling or mosaic patterns, curling, yellowing, reduced growth and other symptoms. The presence of a symptom alone cannot identify the responsible virus, since different pathogens or nutrient deficiencies can produce similar appearances.

Viroids: tiny infectious RNA

Theodor Diener identified viroids in 1971 while studying potato spindle tuber disease. A viroid does not have a capsid or its own protein-coding genes. It depends on host cellular enzymes and other processes for replication, and the effects of its RNA may disrupt plant growth and development. Although both viroids and viruses require hosts, the absence of a protein coat and of protein-coding capacity distinguishes classical viroids from viruses.

Prions: infectious protein conformations

A prion is not a cell or a virus and does not contain a DNA or RNA genome as its infectious principle. In certain tissues, misfolded prion proteins can induce normally folded related proteins to adopt the abnormal form. Over time this can damage nervous tissue and cause fatal neurodegenerative disease. Bovine spongiform encephalopathy affects cattle. A particular human condition, variant Creutzfeldt–Jakob disease, is linked to exposure to the BSE agent; other forms of Creutzfeldt–Jakob disease have different origins.

Lichens: an intimate association

A lichen is a close association between a fungal partner and one or more photosynthetic partners, often a green alga or cyanobacterium. The fungal member typically makes much of the visible lichen body, protects the photosynthetic cells and helps gather water and minerals. The photosynthetic partner produces organic compounds using light. In cyanobacteria-containing lichens, fixation of atmospheric nitrogen can also be important.

A lichen is not a single unusual plant. It combines organisms from different lineages living in a sustained partnership. Many lichens grow on rock, bark or soil and are sensitive to changes in air quality. Certain species decline in polluted environments, making lichen communities useful biological indicators of some pollution patterns. Sensitivity varies among species, so an individual lichen's presence or absence is not by itself a universal pollution test.

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