Microviridae
| Microviridae | |
|---|---|
| Virus classification | |
| Group: | Group II (ssDNA) |
| Order: | Unassigned |
| Family: | Microviridae |
| Subfamily and Genera | |
|
Subfamily: Gokushovirinae Subfamily: Unassigned |
|
Subfamily: Gokushovirinae
Subfamily: Unassigned
Microviridae is a family of bacteriophages with a single-stranded DNA genome. The name of this family is derived from the Greek micro, meaning small. This refers to the size of their genomes, which are among the smallest of the DNA viruses. Enterobacteria, intracellular parasitic bacteria, and spiroplasma serve as natural hosts. There are currently 12 species in this family, divided among 7 genera and one subfamily.
The virons are non-enveloped, round with an icosahedral symmetry (T = 1). They have a diameter between 25–27 nanometers and lack tails. Each viron has 60 copies each of the F, G, and J proteins and 12 copies of the H protein. They have 12 pentagonal trumpet-shaped pentomers (~7.1 nm wide × 3.8 nm high), each of which composed of 5 copies of G and one of the H protein.
Viruses in this family replicate their genomes via a rolling circle mechanism and encode dedicated RCR initiation proteins.
Although the majority of species in this family have lytic life cycles, a few may have temperate life cycles.
The genome sizes range from 4.5–6kb and is circular. It encodes 11 genes (in order: A, A*, B, C, K, D, E, J, F, G, and H), nine of which are essential. The nonessential genes are E and K. Several of the genes have overlapping reading frames. Protein A* is encoded within protein A. It lacks ~1/3 of the amino acids from the N terminal of the A protein and is encoded in the same frame as the A protein. It is translated from an internal start site within the messenger RNA. Gene E is encoded with gene D with a +1 frameshift. Gene K overlaps genes A, B, and C. The origin of replication lies within a 30 base sequence. The entire 30 base sequence is required for replication.
The major capsid protein (F) has 426 amino acids, the major spike protein (G) has 175 amino acids, the small DNA-binding protein (J) has 25–40 amino acids, and the DNA pilot protein (H) has 328 amino acids. The major folding motif of protein F is the eight-stranded antiparallel beta barrel common to many viral capsid proteins. The G protein is a tight beta barrel with its strands running radially outward. The G proteins occur in groups of five forming 12 spikes that enclose a hydrophilic channel. The highly basic J protein lacks any secondary structure and is situated in an interior cleft of the F protein. It has no acidic amino acid residues in the protein and the twelve basic residues are concentrated in two clusters in the N-terminus separated by a proline-rich region.
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