Levivirales

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Levivirales
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Order:	Levivirales
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Levivirales is an order of viruses that are characterized by their small, single-stranded RNA genomes. These viruses primarily infect bacteria, particularly those in the family Enterobacteriaceae. The order Levivirales is notable for its simplicity and has been extensively studied as a model for understanding RNA virus biology.

Taxonomy

The order Levivirales is divided into two families:

• Leviviridae: This family includes the well-known bacteriophage MS2, which infects Escherichia coli. Members of this family have a linear, positive-sense single-stranded RNA genome.
• Alloleviviridae: This family includes bacteriophages such as Qβ, which also infect E. coli. These viruses have a similar genome organization to Leviviridae but differ in certain structural proteins.

Genome and Structure

Levivirales viruses have a simple genome organization, typically consisting of a single-stranded RNA molecule of about 3,500 to 4,200 nucleotides. The genome encodes a small number of proteins, usually including:

• A coat protein, which forms the capsid of the virus.
• A maturation protein, which is involved in the attachment and entry of the virus into the host cell.
• A replicase protein, which is an RNA-dependent RNA polymerase responsible for replicating the viral RNA.
• In some cases, a lysis protein, which helps in the release of progeny virions from the host cell.

The virions are non-enveloped and have an icosahedral symmetry with a diameter of about 25-30 nm.

Life Cycle

The life cycle of Levivirales viruses begins with the attachment of the virion to the host cell surface. The maturation protein binds to specific receptors on the bacterial surface, facilitating the entry of the viral RNA into the host cell. Once inside, the viral RNA is translated by the host's ribosomes to produce viral proteins. The replicase protein then synthesizes complementary RNA strands, which serve as templates for producing new viral genomes.

The assembly of new virions occurs in the cytoplasm, where coat proteins encapsulate the newly synthesized RNA genomes. The lysis protein, if present, disrupts the host cell membrane, allowing the release of progeny virions to infect new cells.

Research and Applications

Levivirales viruses, particularly MS2, have been used extensively in molecular biology research. They serve as models for studying RNA virus replication, translation, and evolution. The simplicity of their genome makes them ideal for genetic manipulation and understanding fundamental processes such as RNA folding and protein-RNA interactions.

In biotechnology, MS2 coat protein has been used in the development of RNA detection assays and as a tool for studying RNA-protein interactions.

Also see

• Bacteriophage
• RNA virus
• Viral replication
• Virus classification