Subviruses

The smallest infectious agents known to researchers are termed subviral infectious agents, or subviruses. Scientists have identified at least six different strains: satellite viruses, virinos, viroids, virusoids, virogenes, and prions.

Members of one of the better understood strains, prions, range in size from considerably smaller than viruses, sometimes 100 times smaller, to almost as large as mitochondria and bacteria. Prions have been found to cause certain diseases and are implicated as the cause of others. Included in this list of diseases that prions seem to promote are scrapies and several similar degenerative brain diseases.

It has been theorized that prions may be radically different from any other known self-replicating entities. There is no evidence that prions contain any nucleic acids, DNA and/or RNA; instead, they appear to be little more than dots of protein. Even if they were found to contain nucleic acids, prions are so small that there is little chance they contain a nucleic acid any longer than 50 nucleotides. This is not large enough to encode a protein containing more than about 12 amino acids.

Despite indications to the contrary, it has even been suggested that prions may actually be conventional viruses, but this is quite unlikely. It appears equally unlikely that they will be found to represent a new category of protoorganismal material that reproduces in living cells, employing a technique that has yet to be elucidated. It has even been suggested that they may reproduce using a technique similar to that employed by viruses, without being viruses.

Some researchers have suggested that the mode of prion reproduction might involve fracture and continued growth, which would explain their small and uncertain molecular weights, their rod-like appearance, their varying lengths, and the unpredictability of which amino acid occurs terminally. The most recent work has shown that prions may be proteins produced somewhat abnormally by infected genes that somehow go awry.

Among the other subviruses are the viroids, minute rings of RNA that infect certain plants. Virusoids appear to be loops of RNA that occur inside regular viruses. Virinos, like viruses, need an outer coat of protein, which they are unable to make on their own, but which they induce host cells to manufacture. Virogenes are otherwise normal genes that generate infectious particles under certain circumstances. Satellite viruses are tiny pieces of RNA that make full-size viruses work for them. These tiny nucleic acids multiply inside viruses that are inside cells.

Comrehension check

1. Choose the right variant for the multiple-choice statements.

1. Viruses contain

1. nucleic acids b. a protein coat

3. DNA or RNA d. viral capsid

5. all of the above.

2. Viruses do not

1. Metabolize b. generate their own energy

c. replicate (or duplicate, or reproduce) without injecting cells

d. all of the above e. none of the above.

3. The information contained in the viral DNA or RNA is

1. inserted into its host’s cellular machinery

2. contained in the viral nucleic acids that are inserted into their host’s DNA

3. used to direct the host to produce more viruses

4. all of the above e. none of the above.

4. Each of the many different types of viruses “know” which cells to attack by identifying … on the potential host’s outer … .

1. receptor sites, protein coat b. nucleic acids, viral capsid

c. receptacles, bacteriophages d. all of the above

e. none of the above.