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Microviridae is a family of with a single-stranded DNA genome. The name of this family is derived from the ancient Greek word (), 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 22 species in this family, divided among seven genera and two subfamilies.
Viruses in this family replicate their genomes via a rolling circle mechanism and encode dedicated RCR initiation proteins.Keegstra W, Baas PD, Jansz HS (1979) Bacteriophage phi X174 RF DNA replication in vivo. A study by electron microscopy" J Mol Biol 135(1) 69–89Fluit AC, Baas PD, Jansz HS (1986) Termination and reinitiation signals of bacteriophage phi X174 rolling circle DNA replication" Virology 154(2) 357–368
Although the majority of species in this family have lytic life cycles, a few may have temperate life cycles.
Assembly of the virion uses two scaffolding proteins, internal scaffolding protein B and external scaffolding protein D. The function of protein B seems to be to lower the amount of protein D needed by the virion for assembly.Chen M, Uchiyama A, Fane BA (2007) Eliminating the requirement of an essential gene product in an already very small virus: scaffolding protein B-free øX174, B-free" J Mol Biol 373(2) 308–314 Protein H is a multifunctional structural protein required for piloting the viral DNA into the host cell interior during the entry process. Protein E is a 91-amino acid membrane protein that causes host cell lysis by inhibiting the host translocase MraY.Zheng Y, Struck DK, Young R (2009) Purification and functional characterization of phiX174 lysis protein E" Biochemistry 48(22) 4999–5006 This inhibitory activity is located within the N terminal 29 amino acids.Buckley KJ, Hayashi M (1986) Lytic activity localized to membrane-spanning region of phi X174 E protein. Mol Gen Genet 204(1) 120–125 Protein A is a single strand endonucleaseEisenberg S (1980) Cleavage of phi X174 single-stranded DNA by gene A protein and formation of a tight protein-DNA complex" J Virol 35(2) 409–413 and is responsible for the initiation of viral DNA replication.van Mansfeld AD, Langeveld SA, Baas PD, Jansz HS, van der Marel GA, Veeneman GH, van Boom JH (1980) Recognition sequence of bacteriophage phi X174 gene A protein--an initiator of DNA replication. Nature 288(5791) 561–566 It catalyses cleavage and ligation of a phosphodiester bond between a guanine and adenosine nucleotide residue pair at the phi X origin.van Mansfeld AD, van Teeffelen HA, Baas PD, Jansz HS (1987) Two juxtaposed tyrosyl-OH groups participate in phi X174 gene A protein catalysed cleavage and ligation of DNA" Nucleic Acids Res 14(10) 4229–4238 It may not be essential for phage viability but burst sizes are reduced by 50% when it is mutated.Baas PD, Liewerink H, van Teeffelen HA, van Mansfeld AD, van Boom JH, Jansz HS (1987) Alteration of the ATG start codon of the A protein of bacteriophage phi X174 into an ATT codon yields a viable phage indicating that A protein is not essential for phi X174 reproduction" FEBS Lett 218(1) 119–125 Protein A* inhibits host DNA replication.Eisenberg S, Ascarelli R (1981) The A* protein of phi X174 is an inhibitor of DNA replication" Nucleic Acids Res 9(8) 1991–2002 Unlike protein A it is capable of cleaving the phi X viral DNA in the presence of single-stranded binding protein of the host.van Mansfeld AD, van Teeffelen HA, Fluit AC, Baas PD, Jansz HS (1986) Effect of SSB protein on cleavage of single-stranded DNA by phi X gene A protein and A* protein" Nucleic Acids Res 14(4) 1845–1861 Protein A*, like Protein A, may not be required for phage viability.Colasanti J, Denhardt DT (1987) Mechanism of replication of bacteriophage phi X174. XXII. Site-specific mutagenesis of the A* gene reveals that A* protein is not essential for phi X174 DNA replication" J Mol Biol 197(1) 47–54 Protein C increases the fidelity of the termination and reinitiation reactions and is required for the packagaging of the viral DNA in to the protein shell.Goetz GS, Englard S, Schmidt-Glenewinkel T, Aoyama A, Hayashi M, Hurwitz J (1988) Effect of phi X C protein on leading strand DNA synthesis in the phi X174 replication pathway" J Biol Chem 263(31) 16452–16460 Protein K has 56 amino acids and is found in the membrane of the host cell. It appears to be able to increase the burst size of the virus.Gillam S, Atkinson T, Markham A, Smith M (1985) Gene K of bacteriophage phi X174 codes for a protein that affects the burst size of phage production" J Virol 53(2) 708–709
A putative third grouping has been proposed—Alpavirinae—which infect the order Bacteroidales. This a group of viruses known only as prophages and additional work on these viruses seems indicated before subfamily status is granted.
A fourth clade has been proposed—Pichovirinae. This clade has a genome organisation that differs from the other members of this family. The name is derived from picho which means small in Occitan language.
Another virus has been isolate from the turkey gut with features similar to other microviruses but quite distinct from the known species.Zsak L, Day JM, Oakley BB, Seal BS (2011) The complete genome sequence and genetic analysis of ΦCA82 a novel uncultured microphage from the turkey gastrointestinal system" Virol J 8:331.
Members of this subfamily can be separated into three main clades according to genome sizes.Kodaira K, Nakano K, Okada S, Taketo A (1992) Nucleotide sequence of the genome of the bacteriophage alpha 3: interrelationship of the genome structure and the gene products with those of the phages, phi X174, G4 and phi K" Biochim Biophys Acta 1130(3) 277–288 Size variability within the groups occurs mainly as a result of insertions and deletions of the intergenic regions. Viruses are assigned according to their similarity to known lab based strains—the ΦX174-like clade, G4-like clade and the α3-like clade. The ΦX174-like clade of microviridae have the smallest and least variable genomes (5,386–5,387 bp); the G4-like clade varies in size from 5,486 to 5,487 bp; while the largest genome sized group is the α3-like clade with genomes ranging from 6,061–6259bp.
Members of the subfamily Gokushovirinae have only two structural proteins: capsid proteins F (Virus Protein 1) and DNA pilot protein H (Virus Protein 2) and do not use scaffolding proteins. They also possess 'mushroom-like' protrusions positioned at the three-fold axes of symmetry of their icosahedral capsids. These are formed by large insertion loops within the protein F of gokushoviruses and are absent in the microviruses. They lack both the external scaffolding protein D and the major spike protein G of the species in the genus Microvirus. The genomes of this group tend to be smaller—about 4.5 kb in length. This subfamily includes the genera Bdellomicrovirus, Chlamydiamicrovirus and Spiromicrovirus.
1. Adsorption to the host via specific receptor(s)
2. Movement of the viral DNA into the host cell
3. Conversion of the single strand form to a double-stranded intermediate
This is known as the replicative form I.
4. Transcription of early genes
5. Replication of the viral genome
Viral protein A cleaves replicative form I DNA strand at the origin of replication ( ori) and covalently attaches itself to the DNA, generating replicative form II molecule. Replication of the genome now begins via a rolling circle mechanism. The host's DNA polymerase converts the single-stranded DNA into double-stranded DNA.
6. Late genes are now transcribed by the host's RNA polymerase.
7. Synthesis of the new virons
Viral protein C binds to replication complex, inducing packaging of new viral positive-stranded DNA into procapsids.Aoyama A, Hayashi M (1986) Synthesis of bacteriophage phi X174 in vitro: mechanism of switch from DNA replication to DNA packaging" Cell 47(1) 99–106 The preinitiation complex consists of the host cell protein rep and viral A and C proteins.Hafenstein S and Fane BA (2002) X174 Genome-capsid interactions influence the biophysical properties of the virion: Evidence for a scaffolding-like function for the genome during the final stages of morphogenesis" J Virol 76(11) 5350–5356 These associate with the procapsid forming a 50S complex.
8. Maturation of the virons in the host cytoplasm
9. Release from the host
Cell lysis is mediated by the phiX174-encoded protein E, which inhibits the peptidoglycan synthesis leading to an eventual bursting of the infected cell.
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