Microviridae
Encyclopedia
The Microviridae are a family of bacteriophage
s with a single stranded DNA genome. The name of this family is derived from the Greek
word 'micro' meaning small. This refers to the size of their genomes which are among the smallest of the DNA viruses.
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.
s (in order: A, A*, B, C, K, D, E, J, F, G and H), nine of which are essential. The non essential 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.
s, 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 outwards. 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.
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. 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 which causes host cell lysis by inhibiting the host translocase
MraY. This inhibitory activity is located within the N terminal 29 amino acids.Protein A is a single strand endonuclease
and is responsible for the initation of viral DNA replication. It catalyses cleavage and ligation of a phosphodiester bond between a G
and A nucleotide residue pair at the phi X origin. It may not be essential for phage viability but busrt sizs are reduced by 50% when it is mutated. Protein A* inhibits host DNA replication. Unlike protein A it is capable of cleaving the phi X viral DNA in the presence of single stranded binding protein of the host. Protein A*, like Protein A, may not be required for phage viability. Protein C increases the fidelity of the termination and reinitiation reactions and is required for the packagaing of the viral DNA in to the protein shell. 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.
ese for very small.
Subfamily Gokushovirinae:
Subfamily Microvirinae:
A putative third subfamily has been proposed - Alpavirinae - which infect the order Bacteroidales
. Additional work on these viruses seems indicated before subfamily status is granted.
Another virus has been isolate from the turkey gut with features similar to other microviruses but quite distinct from the known species.
1. Adsorbion 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. The preinitiation complex consists of the host cell protein rep
and viral A and C proteins. These associate with the procapsid forming a 50S complex.
7. Maturation of the virons in the host cytoplasm
8. Release from the host
This requires the viral lysozyme
which attacks the peptidoglycan
wall of the host.
episode The Vengeance Factor includes a reference to a microvirus that is genetically engineered to attack nerve
s of the parasympathetic system.
Genomes. Journal of Bacteriology, 118(3) p1134-1142
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
s with a single stranded DNA genome. The name of this family is derived from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
word 'micro' meaning small. This refers to the size of their genomes which are among the smallest of the DNA viruses.
Virology
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 x 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.
Genome
The genome sizes range from 4.5 - 6kb and is circular. It encodes 11 geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s (in order: A, A*, B, C, K, D, E, J, F, G and H), nine of which are essential. The non essential 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.
Molecular biology
The major capsid protein (F) has 426 amino acidAmino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s, 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 outwards. 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.
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. 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 which causes host cell lysis by inhibiting the host translocase
Translocase
Translocase is a general term for an enzyme that assists in moving another molecule, usually across a membrane.Translocases are most common secretion system in Gram positive bacteria.Examples include:...
MraY. This inhibitory activity is located within the N terminal 29 amino acids.Protein A is a single strand endonuclease
Endonuclease
Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain, in contrast to exonucleases, which cleave phosphodiester bonds at the end of a polynucleotide chain. Typically, a restriction site will be a palindromic sequence four to six nucleotides long. Most...
and is responsible for the initation of viral DNA replication. It catalyses cleavage and ligation of a phosphodiester bond between a G
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
and A nucleotide residue pair at the phi X origin. It may not be essential for phage viability but busrt sizs are reduced by 50% when it is mutated. Protein A* inhibits host DNA replication. Unlike protein A it is capable of cleaving the phi X viral DNA in the presence of single stranded binding protein of the host. Protein A*, like Protein A, may not be required for phage viability. Protein C increases the fidelity of the termination and reinitiation reactions and is required for the packagaing of the viral DNA in to the protein shell. 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.
Taxonomy
This family is divided into two subfamilies - subfamilies Gokushovirinae and Microvirinae. These groups differ in their hosts, genome structure and viron composition. The name Gokushovirinae is derived from the JapanJapan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese for very small.
Subfamily Gokushovirinae:
- Genus Bdellomicrovirus; type species: Bdellovibrio phage MAC1
- Genus Chlamydiamicrovirus; type species: Chlamydia phage 1
- Genus Spiromicrovirus; type species: Spiroplasma phage 4
Subfamily Microvirinae:
- Genus Microvirus; type species: Enterobacteria phage φX174
A putative third subfamily has been proposed - Alpavirinae - which infect the order Bacteroidales
Bacteroidales
The order Bacteroidales is composed of four families of environmental bacteria....
. Additional work on these viruses seems indicated before subfamily status is granted.
Another virus has been isolate from the turkey gut with features similar to other microviruses but quite distinct from the known species.
Life cycle
There are a number of steps in the life cycle1. Adsorbion 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
DNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
converts the single stranded DNA into double stranded DNA.
6. Late genes are now transcribed by the host's RNA polymerase
RNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
.
7. Synthesis of the new virons
Viral protein C binds to replication complex, inducing packaging of new viral positive stranded DNA into procapsids. The preinitiation complex consists of the host cell protein rep
Rep
Rep or REP may refer to:* Rep , a ribbed woven fabric made from various materials* rep, the Röntgen equivalent physical, a unit of exposure to radiation* REP, an x86 assembly language instruction...
and viral A and C proteins. These associate with the procapsid forming a 50S complex.
7. Maturation of the virons in the host cytoplasm
8. Release from the host
This requires the viral lysozyme
Lysozyme
Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, are glycoside hydrolases, enzymes that damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between...
which attacks the peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
wall of the host.
Popular culture
The Star TrekStar Trek
Star Trek is an American science fiction entertainment franchise created by Gene Roddenberry. The core of Star Trek is its six television series: The Original Series, The Animated Series, The Next Generation, Deep Space Nine, Voyager, and Enterprise...
episode The Vengeance Factor includes a reference to a microvirus that is genetically engineered to attack nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
s of the parasympathetic system.
Additional reading
Roykta, D.R. et al., 2006. Horizontal Gene Transfer and the Evolution of Microvirid ColiphageColiphage
A coliphage is a type of bacteriophage that infects Escherichia coli.Examples include Bacteriophage lambda and Leviviridae....
Genomes. Journal of Bacteriology, 118(3) p1134-1142
External links
- ViralZone: Microviridae
- List of sequenced genomes http://www.dpvweb.net/notes/showallgenusseqs.php?genus=Microvirus