Subviral agents are infectious entities that resemble viruses but lack one or more viral components, such as a capsid or essential replication machinery. These agents include viroids, prions, and satellites, each possessing distinct structural and functional characteristics that influence their mode of infection and replication.
Viroids are the simplest subviral agents, consisting of circular, single-stranded RNA molecules without a protein coat. They exclusively infect plants, relying entirely on host cell machinery for replication. Viroids are replicated by host RNA polymerase through a rolling-circle mechanism, and their cleavage into functional units occurs via ribozyme activity. Notably, viroids do not encode proteins; instead, their structured RNA acts as small interfering RNAs (siRNAs) that interfere with host gene expression, often leading to disease. Pathogenic viroids include the Potato spindle tuber viroid (PSTVd), which affects potato and tomato plants, leading to stunted growth and reduced yield.
Prions are unique infectious agents composed solely of misfolded proteins, without any associated nucleic acids. The pathogenic prion protein, PrPSc, is an aberrantly folded form of the normal cellular protein PrPC. When PrPSc interacts with PrPC, it induces a conformational change, causing the normal protein to misfold and aggregate into amyloid fibrils. These aggregates accumulate in neural tissues, leading to neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs). TSEs are characterized by progressive brain degeneration, neuronal loss, and the formation of vacuoles in brain tissue. Prion diseases can be inherited, spontaneously arise or be acquired through exposure to contaminated food, surgical instruments, or transplants. Examples include scrapie in sheep, bovine spongiform encephalopathy (BSE) or “mad cow disease” in cattle, and Creutzfeldt–Jakob disease (CJD) in humans.
Satellites are subviral agents that possess either RNA or DNA enclosed in a capsid. However, they lack the genetic information necessary for independent replication and require co-infection with a helper virus. Satellite viruses rely on the helper virus for replication and often modify the disease symptoms caused by the helper virus. Hepatitis D virus (HDV) is a satellite RNA virus that depends on Hepatitis B (HBV) for its replication and packaging, exacerbating liver disease in co-infected individuals.
Subviral agents - viroids, prions, and satellites- resemble viruses but lack viral components.
Viroids consist of a single-stranded RNA, lack a capsid, and only infect plants.
They rely on the host's RNA polymerase to replicate and self-cleave to release RNA units.
These RNAs do not encode proteins but form stable secondary structures, acting as small interfering RNAs that silence plant genes.
Prions are infectious agents composed entirely of proteins and lacking nucleic acids.
PrPC is a normal protein in animals. When infectious PrPSc enters the cells, it misfolds PrPC into PrPSc, forming harmful amyloid aggregates that damage the neurons.
This leads to transmissible spongiform encephalopathies, marked by brain degeneration and vacuole formation.
Prion diseases can be inherited or acquired through infected meat or surgical instruments.
Examples include scrapie in sheep, mad cow disease in cattle, and Creutzfeldt–Jakob disease in humans.
Satellites are DNA or RNA enclosed in a capsid. They rely on helper viruses for replication.
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